Coordination and Movement
| サイト: | Young Education |
| コース: | Animal Physiology |
| ブック: | Coordination and Movement |
| 印刷者: | 访客用户 |
| 日付: | 2026年 10月 5日(月曜日) 04:59 |
1. Nervous Systems
Learning outcomes
- I can identify the major components of a nervous system.
- I can explain how nerve cells transmit information.
- I can describe the functions of the brain, spinal cord, and nerves.
- I can explain how nervous systems coordinate responses.
- I can compare simple and complex nervous systems.
What Is a Nervous System?
Animals must constantly detect changes in their surroundings and inside their bodies.
They may need to:
- Detect light, sound, temperature, pressure, or chemicals.
- Locate food.
- Avoid predators.
- Maintain balance.
- Coordinate movement.
- Control internal organs.
- Respond rapidly to danger.
- Learn from previous experiences.
The nervous system is a communication and coordination system that detects information, processes it, and produces appropriate responses.
A simplified pathway is:
Stimulus → receptor → nervous system → effector → response
The nervous system is particularly important for rapid communication within an animal.
Major Components of the Nervous System
In humans and other vertebrates, the nervous system can be divided into two major parts:
- Central nervous system (CNS)
- Peripheral nervous system (PNS)
The central nervous system consists of:
- Brain
- Spinal cord
The peripheral nervous system consists mainly of:
- Nerves extending throughout the body.
Together, these structures allow information to travel between receptors, the central nervous system, and effectors.
The Central Nervous System
The central nervous system, or CNS, acts as a major processing and coordinating centre.
It consists of:
Brain + spinal cord
Information from sensory receptors can be sent to the CNS.
The CNS processes this information and can send signals to muscles or glands.
The Brain
The brain is an extremely complex organ containing billions of nerve cells and supporting cells.
Different regions perform different functions.
The brain is involved in:
- Processing sensory information.
- Coordinating movement.
- Memory.
- Learning.
- Emotions.
- Decision-making.
- Language.
- Maintaining balance.
- Controlling many automatic body functions.
The Cerebrum
The cerebrum is the largest region of the human brain.
It is involved in functions including:
- Conscious thought.
- Memory.
- Learning.
- Interpretation of sensory information.
- Voluntary movement.
- Language.
- Decision-making.
The outer region of the cerebrum, called the cerebral cortex, contains highly interconnected networks of neurons.
Different regions of the cortex are specialised for different functions, although they work together extensively.
The Cerebellum
The cerebellum is located toward the back and lower part of the brain.
It is especially important for:
- Coordination of movement.
- Balance.
- Posture.
- Fine control of muscular activity.
- Motor learning.
For example, catching a ball requires the nervous system to coordinate visual information with precise movements of the arms and hands.
The cerebellum contributes to this coordination.
The Brainstem
The brainstem connects the brain with the spinal cord.
It helps regulate several essential automatic functions, including aspects of:
- Breathing.
- Heart rate.
- Blood pressure.
- Swallowing.
- Sleep and wakefulness.
These functions continue without requiring conscious control.
This demonstrates that the nervous system coordinates both voluntary and involuntary activities.
The Spinal Cord
The spinal cord is a long bundle of nervous tissue extending from the brain through the vertebral column.
It performs two especially important functions.
Communication
It carries information between the brain and much of the body.
Coordination of Reflexes
It can coordinate some rapid responses without waiting for conscious processing by the brain.
The vertebral column surrounds and protects the spinal cord.
The Peripheral Nervous System
The peripheral nervous system, or PNS, connects the central nervous system with the rest of the body.
It includes nerves that carry information:
Toward the CNS
and
Away from the CNS
The PNS connects the brain and spinal cord with:
- Sensory receptors.
- Muscles.
- Glands.
- Internal organs.
Without the peripheral nervous system, the CNS would have no efficient way to receive information from or control most of the body.
What Is a Nerve?
A nerve is a bundle of nerve fibres, or axons, in the peripheral nervous system.
A useful comparison is an electrical cable.
A cable contains many individual wires.
Similarly, a nerve can contain many individual axons carrying signals.
A nerve and a neuron are therefore not the same thing.
A neuron is an individual cell.
A nerve contains many nerve fibres from neurons.
Neurons
The specialised cells that transmit information in the nervous system are called neurons.
A typical neuron contains:
- Cell body
- Dendrites
- Axon
- Axon terminals
The structure of a neuron is closely related to its function.
Dendrites
Dendrites are branching structures that receive signals from other cells.
Their branching shape provides a large surface area for connections.
A single neuron may receive information from many other neurons.
The Cell Body
The cell body contains:
- Nucleus.
- Cytoplasm.
- Organelles required to maintain the cell.
The cell body integrates many of the signals arriving at the neuron.
The Axon
The axon is a long extension that carries electrical signals away from the cell body.
Some axons are extremely long.
For example, neurons controlling muscles in the foot may have axons extending much of the length of the leg.
This allows rapid communication over relatively large distances.
Myelin
Many axons are surrounded by a fatty insulating material called the myelin sheath.
Myelin:
- Electrically insulates the axon.
- Helps signals travel more rapidly.
- Protects and supports the nerve fibre.
Small gaps between sections of myelin are called nodes of Ranvier.
In myelinated neurons, electrical activity effectively moves rapidly from node to node, greatly increasing conduction speed.
How Neurons Transmit Information
Neurons transmit information using both electrical and chemical signals.
Along an axon, information travels as electrical changes in the neuron's membrane.
These rapid signals are called action potentials or nerve impulses.
At connections between neurons, information is usually transmitted chemically.
Therefore:
Within a neuron → mainly electrical signalling
Between many neurons → chemical signalling across synapses
The Nerve Impulse
At rest, there is an electrical difference across a neuron's cell membrane.
This results from unequal distributions of ions inside and outside the cell.
When a neuron is sufficiently stimulated, ion channels in the membrane open and the electrical condition of the membrane changes rapidly.
This produces an action potential.
The action potential travels along the axon.
The signal does not simply consist of electricity flowing through the neuron like current through a copper wire.
It results from controlled movements of ions across the neuron's membrane.
Synapses
Neurons usually do not directly touch one another.
The small junction between one neuron and another cell is called a synapse.
At many synapses, a tiny gap called the synaptic cleft separates the cells.
Neurotransmitters
When an action potential reaches an axon terminal:
- Chemical messengers called neurotransmitters are released.
- Neurotransmitters move across the synaptic cleft.
- They bind to receptors on the next cell.
- This changes the activity of the receiving cell.
The sequence can be simplified as:
Electrical signal → chemical signal → electrical response
Synapses allow neurons to form enormous communication networks.
Types of Neurons
Three useful functional categories are:
- Sensory neurons
- Relay neurons
- Motor neurons
Sensory Neurons
Sensory neurons carry information from receptors toward the central nervous system.
Receptors can detect stimuli such as:
- Light.
- Sound.
- Pressure.
- Temperature.
- Chemicals.
- Tissue damage.
For example:
Heat receptor in skin → sensory neuron → CNS
Relay Neurons
Relay neurons, also called interneurons, are located mainly within the central nervous system.
They connect neurons and help process information.
A relay neuron may receive signals from several neurons and pass information to other neurons.
Complex nervous systems contain enormous networks of interconnected relay neurons.
Motor Neurons
Motor neurons carry signals from the central nervous system toward effectors.
Effectors include:
- Muscles.
- Glands.
For example:
CNS → motor neuron → arm muscle contracts
Receptors and Effectors
A nervous response usually begins with a receptor.
A receptor detects a stimulus.
The response is carried out by an effector.
An effector may be:
- A muscle that contracts.
- A gland that releases a substance.
A useful general pathway is:
Stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response
Worked Example: Catching a Ball
Imagine someone throws a ball toward you.
Stimulus
Light reflected from the moving ball enters your eyes.
Receptors
Photoreceptors in the retina detect the light.
Sensory Information
Signals travel toward the brain.
Processing
The brain interprets:
- Direction.
- Speed.
- Position.
Motor Output
Signals travel through motor pathways to skeletal muscles.
Effectors
Muscles in the arms and hands contract.
Response
Your hands move into position and catch the ball.
This entire process requires rapid coordination between sensory and motor systems.
Reflex Actions
Some responses must occur extremely quickly.
A reflex is a rapid, automatic response to a stimulus.
For example, touching a dangerously hot surface can trigger rapid withdrawal of the hand.
The Reflex Arc
A simplified reflex pathway is:
Stimulus
↓
Receptor
↓
Sensory neuron
↓
Relay neuron in spinal cord
↓
Motor neuron
↓
Effector
↓
Response
The spinal cord can coordinate the withdrawal response before conscious awareness of the pain is fully processed by the brain.
This reduces reaction time.
Worked Example: Touching a Hot Object
Suppose your finger touches a hot pan.
Step 1
Temperature and pain receptors are stimulated.
Step 2
Sensory neurons carry impulses toward the spinal cord.
Step 3
Neurons in the spinal cord process the information.
Step 4
Motor neurons carry impulses toward arm muscles.
Step 5
Muscles contract.
Step 6
Your hand moves away.
Information also travels to the brain, allowing you to become consciously aware of the pain and what happened.
Voluntary and Involuntary Responses
Nervous systems coordinate both voluntary and involuntary responses.
Voluntary Responses
Usually involve conscious control.
Examples include:
- Writing.
- Walking toward a door.
- Throwing a ball.
- Playing a musical instrument.
Involuntary Responses
Occur without deliberate conscious control.
Examples include:
- Changes in heart rate.
- Changes in pupil size.
- Digestive movements.
- Many reflexes.
Both require coordinated nervous-system activity.
The Autonomic Nervous System
Many internal organs are regulated by the autonomic nervous system.
This system helps control functions such as:
- Heart rate.
- Digestion.
- Pupil diameter.
- Activity of certain glands.
- Diameter of some blood vessels.
These processes usually occur without conscious control.
This allows the nervous system to continuously adjust internal conditions while conscious attention is focused elsewhere.
Nervous Coordination and Homeostasis
The nervous system plays an important role in homeostasis.
For example, when body temperature changes, receptors detect the change.
Information is processed in the brain, particularly in the hypothalamus.
Responses can then be coordinated.
If the body becomes too hot, responses may include:
- Increased sweating.
- Changes in blood flow near the skin.
If the body becomes too cold, responses can include:
- Shivering.
- Changes in blood flow near the skin.
The nervous system therefore connects:
Detection → coordination → response
Simple Nervous Systems
Not all animals possess brains and nervous systems as complex as those of vertebrates.
Some relatively simple animals have a nerve net.
A nerve net consists of interconnected neurons distributed through the body rather than concentrated into a large central brain.
Cnidarians such as hydra and jellyfish have nerve nets.
These systems can coordinate:
- Movement.
- Feeding.
- Responses to touch.
- Contractions of the body.
A complex brain is not required for every type of coordinated behaviour.
Increasing Centralisation
Many animals show greater centralisation of nervous tissue.
Instead of neurons being distributed relatively evenly, large numbers become concentrated into:
- Nerve cords.
- Ganglia.
- Brains.
A ganglion is a cluster of neuron cell bodies.
This concentration of nervous tissue allows more complex processing of information.
Nervous Systems in Invertebrates
Many invertebrates have highly capable nervous systems.
For example, insects typically have:
- A brain.
- A ventral nerve cord.
- Several ganglia.
- Sophisticated sensory organs.
These systems allow behaviours such as:
- Flight.
- Navigation.
- Communication.
- Feeding.
- Courtship.
- Escape responses.
Complex behaviour does not require a vertebrate-style nervous system.
Vertebrate Nervous Systems
Vertebrates generally have a highly centralised nervous system consisting of:
- Brain.
- Spinal cord.
- Extensive peripheral nerves.
The brain contains specialised regions and enormous networks of interconnected neurons.
This allows sophisticated:
- Sensory processing.
- Motor control.
- Learning.
- Memory.
- Behaviour.
Comparing Simple and Complex Nervous Systems
| Simple nervous system | More complex centralised nervous system |
|---|---|
| May contain a nerve net | Contains specialised processing centres |
| Limited centralisation | Strong centralisation |
| Relatively simple processing | Extensive information processing |
| Coordinates basic responses | Can coordinate highly complex responses |
| Example: hydra | Example: vertebrates |
However, "simple" does not mean ineffective.
A nervous system evolves according to the requirements of an animal's lifestyle and environment.
Nervous System Complexity and Lifestyle
Nervous systems are closely related to how animals interact with their environments.
A mobile predator, for example, may need to:
- Detect moving prey.
- Determine distance.
- Coordinate rapid movement.
- Maintain balance.
- Remember locations.
- Select between different behaviours.
These tasks require substantial information processing.
A relatively sedentary animal may face different sensory and coordination demands.
Nervous systems therefore reflect both evolutionary history and lifestyle.
Reaction Time
Reaction time is the time between detecting a stimulus and producing a response.
Reaction time depends on several processes:
Stimulus detection → nerve transmission → processing → motor transmission → muscle response
For example, catching a falling ruler requires:
- Visual receptors to detect movement.
- Sensory information to reach the CNS.
- The CNS to process the information.
- Motor signals to reach hand muscles.
- Muscles to contract.
Reaction time can be investigated experimentally using a simple ruler-drop test.
Why Nervous Responses Are Fast
Nervous-system communication can occur rapidly because:
- Electrical signals travel quickly along axons.
- Myelin increases conduction speed in many neurons.
- Synapses allow organised communication between cells.
- Neural pathways can connect receptors directly with appropriate processing centres.
- Reflex pathways can produce responses without waiting for conscious decision-making.
This makes nervous control particularly suitable for responses that must occur within fractions of a second.
Nervous and Hormonal Communication
Animals also communicate internally using hormones.
The nervous and endocrine systems both coordinate body functions, but they operate differently.
| Nervous communication | Hormonal communication |
|---|---|
| Uses neurons | Uses hormones |
| Signals travel along specific pathways | Hormones travel mainly through blood |
| Usually rapid | Often slower |
| Responses may be short-lived | Responses can be longer-lasting |
| Can target very specific cells | Hormones can circulate widely but affect cells with appropriate receptors |
The two systems frequently work together.
For example, the brain can stimulate hormonal responses during stressful situations.
Worked Example: Predator Detection
Imagine a deer detects a predator.
Its nervous system must rapidly:
- Detect the predator using sensory receptors.
- Transmit information to the CNS.
- Process the threat.
- Coordinate muscle activity.
- Adjust heart rate and breathing.
- Produce escape behaviour.
The nervous system therefore integrates information from many sources and coordinates multiple organs at the same time.
This ability can directly affect survival.
Worked Example: Damage to the Spinal Cord
Suppose the spinal cord is severely damaged.
The brain itself may still function normally, but communication between the brain and parts of the body can be disrupted.
Depending on the location and severity of the damage, this can interfere with:
- Sensory information reaching the brain.
- Motor signals reaching muscles.
- Some autonomic functions.
This demonstrates the spinal cord's critical role as a communication pathway between the brain and body.
Common Mistakes
Saying the Brain Is the Entire Nervous System
The nervous system includes the brain, spinal cord, nerves, neurons, and associated structures.
Confusing a Neuron With a Nerve
A neuron is an individual cell.
A peripheral nerve contains many nerve fibres.
Saying Nerve Impulses Are Simply Electricity Flowing Through Wires
Nerve impulses result from controlled movements of ions across neuronal membranes.
Saying Neurons Always Touch
Most neurons communicate across small junctions called synapses.
Thinking All Nervous Communication Is Electrical
Signals travel electrically along neurons, but communication across many synapses involves chemical neurotransmitters.
Thinking Reflexes Do Not Involve the CNS
Many reflexes involve the spinal cord, which is part of the central nervous system.
Thinking Reflexes Never Reach the Brain
A spinal reflex can begin before conscious processing, but information can still travel to the brain.
Assuming Simple Animals Have No Nervous System
Many simple animals have nerve nets or other forms of nervous organisation.
Assuming More Complex Always Means "Better"
Different nervous systems are adapted to different lifestyles and environments.
Check Your Understanding
1. What is the main function of a nervous system?
2. Name the two major divisions of the vertebrate nervous system.
3. What structures make up the central nervous system?
4. Describe two functions of the brain.
5. Explain two functions of the spinal cord.
6. What is the difference between a neuron and a nerve?
7. Describe the functions of dendrites and an axon.
8. Explain how myelin affects nerve transmission.
9. What happens at a synapse?
10. Compare sensory, relay, and motor neurons.
11. Write the pathway from a stimulus to a response using the terms receptor, CNS, sensory neuron, motor neuron, and effector.
12. Explain why withdrawal reflexes can protect an animal from injury.
13. Compare a nerve net with a centralised nervous system.
14. Explain how an animal's lifestyle might influence the complexity of its nervous system.
15. A person can feel a sharp object touching their foot but cannot voluntarily move the foot. What does this suggest about which nervous pathways may still be functioning and which may be disrupted?
Key Terms
- Nervous system – communication and coordination system that detects information, processes it, and coordinates responses.
- Central nervous system (CNS) – brain and spinal cord.
- Peripheral nervous system (PNS) – nerves connecting the CNS with the rest of the body.
- Brain – major processing and coordinating organ of the nervous system.
- Spinal cord – nervous tissue carrying information between the brain and body and coordinating many reflexes.
- Neuron – specialised cell that transmits information.
- Nerve – bundle of nerve fibres in the peripheral nervous system.
- Dendrite – branching part of a neuron specialised for receiving signals.
- Axon – long neuronal extension that carries electrical signals away from the cell body.
- Myelin sheath – insulating layer around many axons that increases conduction speed.
- Action potential – rapid electrical change that travels along a neuron's membrane.
- Synapse – junction through which a neuron communicates with another cell.
- Neurotransmitter – chemical messenger released at many synapses.
- Sensory neuron – neuron carrying information from receptors toward the CNS.
- Relay neuron – neuron within the CNS that connects and processes information between other neurons.
- Motor neuron – neuron carrying signals toward effectors.
- Receptor – specialised cell or structure that detects a stimulus.
- Effector – muscle or gland that produces a response.
- Reflex – rapid, automatic response to a stimulus.
- Reflex arc – neural pathway involved in producing a reflex.
- Nerve net – distributed network of neurons found in some relatively simple animals.
- Ganglion – cluster of neuron cell bodies.
- Centralisation – evolutionary concentration of nervous tissue into specialised processing regions.
- Reaction time – time between detecting a stimulus and producing a response.
Key Takeaways
- The nervous system allows animals to detect information, process it, and coordinate responses.
- Vertebrate nervous systems contain the central nervous system and peripheral nervous system.
- The CNS consists of the brain and spinal cord.
- The peripheral nervous system connects the CNS with receptors, muscles, glands, and internal organs.
- Neurons are specialised cells that transmit information.
- Dendrites receive signals, while axons carry signals away from the cell body.
- Myelin can greatly increase the speed of nerve transmission.
- Information travels along neurons through electrical changes in the cell membrane.
- Neurons communicate across many synapses using neurotransmitters.
- Sensory neurons carry information toward the CNS.
- Relay neurons process and connect information within the CNS.
- Motor neurons carry signals toward effectors.
- A basic nervous pathway is stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response.
- Reflexes provide rapid, automatic responses that can protect an animal from harm.
- The spinal cord both carries information and coordinates many reflexes.
- The brain contains specialised regions responsible for different but interconnected functions.
- Some animals possess relatively simple nerve nets, while others have highly centralised nervous systems.
- Nervous-system organisation is related to an animal's lifestyle, behaviour, sensory needs, and environment.
- Nervous communication is particularly useful for rapid and precisely targeted responses.
2. Sensory Receptors
Learning outcomes
- I can define sensory receptors and explain their function.
- I can identify different types of sensory receptors.
- I can explain how receptors detect environmental changes.
- I can describe how sensory information is processed.
- I can explain how sensory systems contribute to survival.
What Are Sensory Receptors?
Animals are constantly exposed to changes in both their external environment and their internal environment.
These changes are called stimuli.
Examples include:
- Light becoming brighter.
- A sudden sound.
- An increase in temperature.
- Pressure against the skin.
- The smell of food.
- Changes in blood carbon dioxide concentration.
- Changes in body position.
To respond to these changes, an animal must first detect them.
A sensory receptor is a specialised cell, group of cells, or nerve ending that detects a particular type of stimulus and converts information about that stimulus into signals that can be transmitted through the nervous system.
This conversion of stimulus energy into an electrical signal is called sensory transduction.
A general pathway is:
Stimulus → sensory receptor → sensory neuron → central nervous system → processing → response
Receptors Detect Specific Types of Stimuli
Sensory receptors are specialised.
Different receptors respond particularly well to different forms of energy or chemical change.
Major categories include:
- Photoreceptors – detect light.
- Mechanoreceptors – detect mechanical forces.
- Chemoreceptors – detect chemicals.
- Thermoreceptors – detect temperature.
- Nociceptors – detect potentially damaging stimuli.
- Proprioceptors – provide information about body position and movement.
This specialisation allows the nervous system to distinguish between different types of information.
Photoreceptors
Photoreceptors detect light.
In vertebrates, photoreceptors are found in the retina at the back of the eye.
Humans have two major types:
- Rod cells.
- Cone cells.
Rod Cells
Rods are highly sensitive to light.
They are especially useful in:
- Dim conditions.
- Night vision.
- Detecting changes in brightness.
- Peripheral vision.
Rods do not provide detailed colour information.
This is one reason colours become difficult to distinguish under very low light.
Cone Cells
Cones function best in brighter light.
They are important for:
- Colour vision.
- Fine visual detail.
- High visual acuity.
Humans normally have three types of cone cells with different sensitivities to wavelengths of visible light.
The brain compares information from these receptors to produce our perception of colour.
From Light to Vision
Seeing an object requires much more than light simply entering the eye.
A simplified sequence is:
Light from object
↓
Light enters eye
↓
Image forms on retina
↓
Photoreceptors detect light
↓
Electrical signals are produced
↓
Signals travel through the optic nerve
↓
Brain processes the information
↓
Visual perception
The eye detects light, but much of what we experience as vision results from processing within the brain.
Mechanoreceptors
Mechanoreceptors respond to mechanical forces.
These can include:
- Pressure.
- Touch.
- Stretch.
- Vibration.
- Sound waves.
- Movement.
Mechanoreceptors occur in many parts of the body.
Touch and Pressure Receptors
The skin contains several types of sensory nerve endings that respond to mechanical changes.
These allow us to detect:
- Light touch.
- Pressure.
- Vibration.
- Stretching of the skin.
Different regions of the body have different densities of receptors.
For example, the fingertips contain many touch receptors and can distinguish fine details much better than many areas of the back.
Mechanoreceptors and Hearing
Sound consists of vibrations travelling through a medium.
In the ear, sound vibrations eventually cause movement of specialised sensory structures in the cochlea.
Hair cells within the cochlea act as mechanoreceptors.
A simplified sequence is:
Sound wave → vibration of ear structures → movement within cochlea → hair cells stimulated → nerve signals → brain
The brain interprets these signals as sound.
Balance and Movement
Mechanoreceptors are also important for maintaining balance.
The inner ear contains structures that detect:
- Rotation of the head.
- Linear acceleration.
- Head position relative to gravity.
These signals are sent to the brain, where they are combined with information from:
- Eyes.
- Muscles.
- Joints.
This allows the nervous system to coordinate posture and balance.
Chemoreceptors
Chemoreceptors detect chemicals or changes in chemical concentration.
They are involved in senses such as:
- Taste.
- Smell.
Chemoreceptors also monitor internal conditions.
For example, some receptors detect changes related to:
- Carbon dioxide concentration.
- Oxygen concentration.
- Blood pH.
Smell
The sense of smell depends on olfactory receptors.
Chemical molecules in the air enter the nasal cavity and interact with receptors in specialised tissue.
Different molecules activate different combinations of receptors.
Signals travel to the brain, where the pattern is interpreted as a particular smell.
This allows animals to detect chemical information in their environment.
Taste
Taste receptors are found mainly in structures called taste buds.
They respond to dissolved substances.
Human taste perception includes sensations associated with:
- Sweet.
- Sour.
- Salty.
- Bitter.
- Umami.
Taste and smell work together closely.
This is why food can appear to have much less flavour when the nose is blocked.
Internal Chemoreceptors
Chemoreceptors are not limited to the traditional senses.
Some monitor the chemical composition of the body's internal fluids.
For example, changes in blood carbon dioxide and pH can be detected by chemoreceptors associated with the brainstem and major blood vessels.
If carbon dioxide increases significantly:
Chemoreceptors detect change → CNS receives information → breathing is adjusted
This contributes to homeostasis.
Thermoreceptors
Thermoreceptors detect changes in temperature.
They are found in places including:
- Skin.
- Hypothalamus in the brain.
Skin thermoreceptors provide information about external temperature.
Internal thermoreceptors help monitor the temperature of the body itself.
Thermoreceptors and Homeostasis
Suppose body temperature rises.
Temperature-sensitive receptors detect the change.
Information is processed by the nervous system.
Responses can include:
- Increased sweating.
- Increased blood flow near the skin.
If body temperature falls:
- Shivering may begin.
- Blood flow near the skin may decrease.
This creates a regulatory pathway:
Temperature change → receptors → CNS → effectors → corrective response
Sensory receptors therefore contribute directly to homeostasis.
Nociceptors
Nociceptors are sensory receptors that respond to stimuli associated with actual or potential tissue damage.
These can include:
- Extreme heat.
- Extreme cold.
- Strong mechanical pressure.
- Chemicals released by damaged tissue.
Their activity contributes to the sensation of pain.
Pain is unpleasant, but it has an important protective function.
It can cause an animal to:
- Withdraw from danger.
- Protect an injured body part.
- Avoid repeating harmful behaviour.
Proprioceptors
Close your eyes and raise one arm.
Even though you cannot see your arm, you still have a good idea where it is.
This ability depends partly on proprioception.
Proprioceptors are sensory receptors associated with structures such as:
- Muscles.
- Tendons.
- Joints.
They provide information about:
- Muscle length.
- Muscle tension.
- Joint position.
- Movement.
This information allows the nervous system to continuously monitor the position of different body parts.
Proprioception and Movement
Imagine walking down a staircase.
Your nervous system must constantly determine:
- Where your feet are.
- Whether your knees are bent.
- How much your muscles are stretched.
- Whether your body is balanced.
Much of this information is provided automatically by proprioceptors.
Without proprioception, smooth coordinated movement would be extremely difficult.
Receptor Specialisation
Why have different receptors?
Different stimuli involve different forms of energy or chemical change.
For example:
- Light consists of electromagnetic radiation.
- Sound involves mechanical vibrations.
- Odours involve chemical molecules.
- Temperature involves thermal conditions.
A single receptor type would not detect all of these equally effectively.
Specialised receptors allow animals to collect many different types of information simultaneously.
Sensory Transduction
A receptor must convert a stimulus into a form the nervous system can transmit.
This process is called sensory transduction.
For example:
Light → photoreceptor → electrical change
Pressure → mechanoreceptor → electrical change
Chemical molecule → chemoreceptor → electrical change
Temperature change → thermoreceptor → electrical change
The nervous system can then transmit information using patterns of action potentials.
Stimulus Strength
Sensory systems must also provide information about the strength of a stimulus.
A stronger stimulus does not normally produce a "bigger" individual action potential.
Instead, stimulus intensity can be represented partly by:
- Increased frequency of action potentials.
- Activation of additional sensory receptors.
Therefore, the nervous system can distinguish between:
- Dim and bright light.
- Gentle and strong pressure.
- Quiet and loud sounds.
Sensory Neurons
After a receptor detects a stimulus, information is commonly transmitted toward the CNS by sensory neurons.
A simplified pathway is:
Receptor → sensory neuron → spinal cord and/or brain
Sensory neurons therefore connect detection with processing.
The receptor detects the change.
The sensory neuron carries information about it.
Sensory Information Is Processed
Detecting a stimulus is only the beginning.
The central nervous system must interpret the incoming information.
For example, imagine touching an object.
Your brain may need to determine:
- Where you were touched.
- How strong the pressure was.
- Whether the object is moving.
- Whether it is hot or cold.
- Whether it may cause injury.
Information from several receptor types can be combined to produce a useful interpretation.
The Brain Constructs Perception
Sensory receptors provide information, but our conscious experience is produced through processing by the nervous system.
Consider vision.
The retina detects patterns of light.
The brain processes information about:
- Colour.
- Shape.
- Movement.
- Depth.
- Position.
- Previous experience.
Together, this produces visual perception.
This explains why sensory perception is more complex than simply detecting a stimulus.
Sensory Adaptation
Have you ever entered a room and noticed a strong smell, only to notice it much less a few minutes later?
This is an example of sensory adaptation.
Sensory adaptation is a reduction in sensitivity to a constant stimulus over time.
Examples can include:
- Becoming less aware of clothing touching your skin.
- Becoming less aware of a constant smell.
- Adjusting to changes in light level.
Adaptation allows the nervous system to focus attention on new or changing information.
Sensory Systems and Survival
Sensory systems provide animals with information needed for survival.
They help animals:
- Locate food.
- Detect predators.
- Find mates.
- Navigate.
- Communicate.
- Avoid harmful environments.
- Maintain homeostasis.
Different species have evolved sensory systems suited to their environments and lifestyles.
Predator Detection
Imagine a grazing animal detects movement nearby.
Photoreceptors detect the visual change.
The information travels to the brain.
The brain integrates this with other information, such as:
- Sound.
- Smell.
- Previous experience.
If the movement is identified as a predator, the nervous system can rapidly coordinate escape.
The sequence might be:
Movement → photoreceptors → sensory neurons → brain → motor neurons → muscles → escape
A rapid sensory response can make the difference between survival and capture.
Finding Food
Sensory systems also help animals locate food.
Different animals may rely heavily on different senses.
Examples include:
- Vision for detecting prey.
- Smell for locating food at a distance.
- Taste for evaluating potential food.
- Touch for locating food in dark environments.
- Hearing for detecting moving prey.
The sensory system most useful to an animal depends strongly on its habitat and feeding strategy.
Sensory Adaptations in Nocturnal Animals
Animals active at night often have sensory adaptations that allow them to function under low-light conditions.
These may include:
- Eyes adapted for greater light sensitivity.
- Large numbers of rod photoreceptors.
- Highly sensitive hearing.
- Strong sense of smell.
- Specialised touch receptors.
An animal does not necessarily rely on only one sensory system.
Combining several types of information can provide a more accurate picture of the environment.
Echolocation
Bats and some other animals can use echolocation.
The animal produces sound.
The sound travels through the environment and reflects from objects.
Returning echoes are detected by auditory receptors.
The nervous system analyses characteristics of the returning echoes.
This can provide information about:
- Distance.
- Direction.
- Size.
- Movement.
Echolocation demonstrates how sensory systems can become highly specialised for particular lifestyles.
Sensory Systems in Aquatic Animals
Water transmits many stimuli differently from air.
Aquatic animals therefore possess specialised sensory adaptations.
Fish, for example, possess a lateral line system.
The lateral line contains mechanoreceptors that detect water movement and vibrations.
This can help fish:
- Detect nearby movement.
- Avoid obstacles.
- Detect predators.
- Locate prey.
- Coordinate schooling behaviour.
Sensory Systems and Communication
Sensory receptors also allow animals to receive signals from other members of their species.
Communication signals can include:
- Visual displays.
- Sounds.
- Chemical signals.
- Touch.
- Vibrations.
For communication to work, the receiver must have receptors capable of detecting the signal.
For example:
Sound-producing animal → sound waves → auditory receptors of another animal → nervous processing
Communication therefore depends on both signal production and sensory detection.
Sensory Systems and Homeostasis
Sensory receptors do not only monitor the outside world.
Internal receptors continually monitor conditions within the body.
These can detect changes associated with:
- Temperature.
- Blood pressure.
- Carbon dioxide concentration.
- Oxygen concentration.
- Muscle stretch.
- Water balance.
This information allows the nervous system to coordinate corrective responses.
Therefore, sensory receptors contribute to both:
survival in the external environment + regulation of the internal environment
Worked Example: Touching a Hot Surface
Imagine accidentally touching a hot metal surface.
Detection
Thermoreceptors and nociceptors detect potentially damaging conditions.
Transmission
Sensory neurons carry signals toward the spinal cord.
Processing
A reflex pathway can rapidly activate motor neurons.
Response
Arm muscles contract and withdraw the hand.
Further Processing
Information also reaches the brain, producing conscious awareness of heat and pain.
The complete pathway can be simplified as:
Hot surface → receptors → sensory neuron → CNS → motor neuron → muscle → withdrawal
Worked Example: Smelling Food
Suppose you walk past a bakery.
Chemical molecules from food enter your nose.
Step 1
The molecules interact with olfactory receptors.
Step 2
Receptor cells generate electrical signals.
Step 3
Signals travel toward the brain.
Step 4
The brain processes the pattern of signals.
Step 5
The smell is recognised, potentially using stored memories.
The response might include:
- Increased attention.
- Salivation.
- Movement toward the food.
This demonstrates how sensory information can influence both behaviour and physiological responses.
Worked Example: Maintaining Balance
Imagine standing on one leg.
The nervous system receives information from:
- Eyes.
- Vestibular receptors in the inner ear.
- Proprioceptors in muscles and joints.
- Pressure receptors associated with contact between the foot and ground.
The brain combines these signals.
Motor commands are continually adjusted.
Muscles make small corrections to maintain posture.
Balance is therefore produced by the integration of several sensory systems, rather than by one receptor alone.
Comparing Receptor Types
| Receptor | Main stimulus detected | Example |
|---|---|---|
| Photoreceptor | Light | Rods and cones in retina |
| Mechanoreceptor | Pressure, vibration or movement | Touch receptors, inner-ear hair cells |
| Chemoreceptor | Chemicals | Taste and smell receptors |
| Thermoreceptor | Temperature | Receptors in skin |
| Nociceptor | Potential tissue damage | Pain-sensitive nerve endings |
| Proprioceptor | Body position and movement | Muscle and tendon receptors |
This classification is useful, but biological sensory systems can be complex and some receptors respond to more than one aspect of a stimulus.
Sensory Information Is Integrated
The brain rarely relies on one piece of sensory information alone.
Imagine crossing a busy street.
You might use:
- Vision to detect vehicles.
- Hearing to detect engines or horns.
- Proprioception to coordinate walking.
- Balance receptors to maintain posture.
- Touch receptors to detect the ground beneath your feet.
The nervous system combines this information to create an appropriate response.
This process is called sensory integration.
What Happens If Receptors Are Damaged?
Damage to sensory receptors can reduce or eliminate particular types of sensory information.
For example:
- Damage to retinal photoreceptors can affect vision.
- Damage to cochlear hair cells can cause hearing loss.
- Damage to sensory nerves can reduce touch or pain sensation.
The consequences demonstrate how important sensory information is for safe interaction with the environment.
Loss of pain sensation, for example, can be dangerous because injuries may occur without being detected.
Receptor Density and Sensitivity
Sensory receptors are not distributed equally throughout the body.
Some areas contain many more receptors than others.
The fingertips and lips, for example, have high densities of touch receptors.
This allows fine discrimination between nearby stimuli.
This can be investigated using a two-point discrimination test.
Two nearby points are touched against the skin.
If the points stimulate sufficiently separate sensory fields, they are perceived as two individual contacts.
Areas with high receptor density can usually distinguish points that are closer together.
Common Mistakes
Saying Sensory Receptors Produce the Response
Receptors detect stimuli. Effectors such as muscles and glands produce responses.
Saying All Receptors Detect the Same Stimulus
Different receptors are specialised for different forms of information.
Confusing Receptors With Sensory Neurons
A receptor detects a stimulus. Sensory neurons carry information toward the central nervous system. In some sensory systems, the receptor itself is a specialised neuron; in others, a separate receptor cell communicates with a neuron.
Saying Eyes "See" Everything Directly
Photoreceptors detect light. The brain performs extensive processing that produces visual perception.
Saying Pain Has No Useful Function
Pain can provide important information about actual or potential tissue damage and can trigger protective behaviour.
Thinking Sensory Adaptation Means the Stimulus Has Disappeared
The stimulus may still be present. The sensory system has simply become less responsive to a constant stimulus.
Thinking All Animals Sense the World in the Same Way
Different species have sensory systems adapted to different environments and lifestyles.
Check Your Understanding
1. Define a sensory receptor.
2. What is a stimulus?
3. Explain what is meant by sensory transduction.
4. Name five major types of sensory receptors and identify the stimulus detected by each.
5. Compare rods and cones.
6. Explain how mechanoreceptors contribute to hearing.
7. Give two examples of chemoreceptors.
8. Explain how thermoreceptors contribute to homeostasis.
9. What is the function of nociceptors?
10. Explain the importance of proprioceptors.
11. Describe the pathway of sensory information from a receptor to the CNS.
12. What is sensory adaptation, and why might it be useful?
13. Explain how several sensory systems work together to maintain balance.
14. Give two examples of specialised sensory adaptations in animals.
15. An animal is active almost entirely at night and has highly sensitive hearing, many rod photoreceptors, and a strong sense of smell. Explain how these features could increase its chances of survival.
Key Terms
- Sensory receptor – specialised cell, group of cells, or nerve ending that detects a stimulus.
- Stimulus – detectable change in the internal or external environment.
- Sensory transduction – conversion of stimulus information into electrical signals used by the nervous system.
- Photoreceptor – receptor specialised for detecting light.
- Mechanoreceptor – receptor responding to mechanical forces such as pressure, vibration, or movement.
- Chemoreceptor – receptor responding to chemicals or changes in chemical concentration.
- Thermoreceptor – receptor responding to temperature.
- Nociceptor – receptor responding to potentially damaging stimuli.
- Proprioceptor – receptor providing information about body position and movement.
- Rod – retinal photoreceptor specialised for high sensitivity in low light.
- Cone – retinal photoreceptor important for colour vision and fine detail.
- Retina – light-sensitive tissue at the back of the eye.
- Olfactory receptor – chemoreceptor involved in smell.
- Sensory neuron – neuron carrying sensory information toward the CNS.
- Sensory adaptation – reduction in responsiveness to a constant stimulus.
- Sensory integration – combination and processing of information from different sensory sources.
- Proprioception – awareness of body position and movement.
- Echolocation – use of emitted sounds and returning echoes to obtain information about surroundings.
- Lateral line – mechanosensory system in fish and some aquatic vertebrates that detects water movement.
Key Takeaways
- Sensory receptors allow animals to detect changes in their internal and external environments.
- A stimulus is a detectable environmental or internal change.
- Receptors convert stimulus information into signals through sensory transduction.
- Photoreceptors detect light.
- Mechanoreceptors detect mechanical forces such as touch, vibration, and movement.
- Chemoreceptors detect chemical substances or changes in chemical conditions.
- Thermoreceptors detect temperature.
- Nociceptors detect potentially damaging stimuli.
- Proprioceptors provide information about body position and movement.
- Sensory information is transmitted toward the central nervous system for processing.
- The brain combines information from multiple receptors to produce useful perceptions and responses.
- Sensory adaptation reduces responsiveness to some constant stimuli.
- Internal sensory receptors contribute to homeostasis.
- Sensory systems help animals find food, avoid predators, navigate, communicate, and reproduce.
- Different species possess sensory adaptations suited to their particular habitats and lifestyles.
- Sensory systems are most effective when detection, nervous-system processing, and coordinated responses work together.
3. Muscles and Movement
Learning outcomes
- I can explain how muscles produce movement.
- I can describe the interaction between muscles and bones.
- I can distinguish between different types of muscle tissue.
- I can explain the role of energy in muscle contraction.
- I can analyze how movement supports survival and behavior.
How Do Animals Produce Movement?
Movement is one of the most obvious characteristics of animals. Animals move their entire bodies to find food, escape predators, locate mates, and explore their environments. They also produce smaller movements involved in breathing, digestion, circulation, and communication.
In vertebrates, most visible body movement results from cooperation between three major systems:
- The nervous system sends signals that control muscles.
- The muscular system produces force by contracting.
- The skeletal system provides support and structures that muscles can pull against.
Together, the muscles and skeleton form the musculoskeletal system.
A useful sequence is:
Nervous signal → muscle contraction → force on bone → movement at joint
Muscles Produce Force by Contracting
Muscles produce movement through contraction.
When a muscle contracts, it develops tension and can shorten, pulling on structures attached to it.
An important principle is:
Muscles pull; they do not actively push.
This means that moving a joint in opposite directions usually requires different muscles.
For example, bending and straightening the elbow involve different muscles working together.
Muscles and Bones
Most skeletal muscles are attached to bones by strong connective tissues called tendons.
When a skeletal muscle contracts:
- The muscle produces tension.
- The tendon transmits the force.
- The force pulls on a bone.
- The bone moves around a joint.
The skeleton therefore does more than support the body. It provides a mechanical framework against which muscles can produce movement.
Tendons and Ligaments
Tendons and ligaments are often confused.
A tendon connects:
Muscle → bone
A ligament connects:
Bone → bone
Ligaments help stabilise joints, while tendons transfer muscular force to the skeleton.
Joints
A joint is a location where two or more bones meet.
Some joints permit little or no movement, while others allow considerable movement.
Examples of movable joints include:
- Shoulder.
- Elbow.
- Wrist.
- Hip.
- Knee.
- Ankle.
Many freely movable joints are called synovial joints.
They contain structures that reduce friction and allow controlled movement.
Bones Act as Levers
Bones can act as levers.
A lever is a rigid structure that rotates around a pivot.
In the body:
- Bone acts as the lever.
- Joint acts as the pivot.
- Muscle provides the force.
- Body part or external object provides resistance.
For example, when the biceps contracts to lift the forearm, the forearm bones rotate around the elbow joint.
This arrangement converts muscle contraction into useful movement.
Antagonistic Muscle Pairs
Because muscles pull rather than push, many skeletal muscles work in antagonistic pairs.
An antagonistic pair consists of muscles that produce opposite movements.
The biceps and triceps are a familiar example.
Bending the Arm
When the elbow bends:
- Biceps contracts.
- Biceps shortens and produces tension.
- Triceps relaxes sufficiently to allow movement.
- Forearm moves upward.
This movement is called flexion.
The biceps acts as a flexor at the elbow.
Straightening the Arm
When the elbow straightens:
- Triceps contracts.
- Triceps produces force on the forearm.
- Biceps relaxes sufficiently to allow movement.
- Forearm moves downward or away from the upper arm.
This movement is called extension.
The triceps acts as an extensor at the elbow.
Therefore:
Flexion → biceps contracts
Extension → triceps contracts
In real movements, muscle control can be more complex, and opposing muscles may sometimes contract simultaneously to stabilise a joint.
Muscle Tissue
Animals contain different types of muscle tissue specialised for different functions.
Humans have three major types:
- Skeletal muscle.
- Smooth muscle.
- Cardiac muscle.
Although all three types can contract, their structures, locations, and functions differ.
Skeletal Muscle
Skeletal muscle is usually attached to bones.
It produces movements such as:
- Walking.
- Running.
- Jumping.
- Writing.
- Chewing.
- Moving the eyes.
- Maintaining posture.
Skeletal muscle is usually described as voluntary muscle because many of its movements can be consciously controlled.
However, skeletal muscles are also involved in automatic responses such as reflexes.
Under a microscope, skeletal muscle has a striped or striated appearance.
Smooth Muscle
Smooth muscle is found in the walls of many internal organs.
Examples include:
- Digestive tract.
- Blood vessels.
- Bladder.
- Airways.
- Uterus.
Smooth muscle usually operates without conscious control and is therefore described as involuntary.
For example, smooth muscle in the digestive tract produces waves of contraction called peristalsis, which help move food through the digestive system.
Cardiac Muscle
Cardiac muscle is found only in the heart.
It contracts rhythmically throughout life to pump blood around the body.
Cardiac muscle is:
- Involuntary.
- Striated.
- Highly resistant to fatigue under normal conditions.
- Specialised for repeated rhythmic contraction.
Cardiac muscle cells are connected in ways that allow coordinated contraction of heart tissue.
Comparing Muscle Types
| Feature | Skeletal Muscle | Smooth Muscle | Cardiac Muscle |
|---|---|---|---|
| Main location | Attached to skeleton | Internal organs | Heart |
| Control | Mostly voluntary | Involuntary | Involuntary |
| Appearance | Striated | Non-striated | Striated |
| Typical role | Body movement | Movement within organs | Pumping blood |
| Contraction | Can be rapid and powerful | Usually slower | Rhythmic |
Each muscle type is adapted to its particular function.
Structure of Skeletal Muscle
A skeletal muscle is organised into increasingly smaller structures.
A simplified organisation is:
Whole muscle → muscle bundles → muscle fibres → myofibrils → contractile proteins
A muscle fibre is actually a specialised muscle cell.
Inside the fibres are many myofibrils, which contain the protein structures responsible for contraction.
Actin and Myosin
Two important proteins involved in muscle contraction are:
- Actin
- Myosin
These proteins are arranged in repeating units called sarcomeres.
During contraction, actin and myosin interact so that the protein filaments slide past one another.
This is known as the sliding filament mechanism.
The filaments themselves do not simply become shorter.
Instead, their arrangement changes as they slide relative to one another, shortening the sarcomere.
Many sarcomeres shortening together causes the muscle fibre to shorten.
How Muscle Contraction Occurs
A skeletal muscle does not contract spontaneously during normal movement. It receives instructions from the nervous system.
A simplified sequence is:
Motor neuron stimulated
↓
Signal reaches muscle fibre
↓
Calcium ions become available inside the muscle fibre
↓
Actin and myosin interact
↓
Myosin pulls actin
↓
Sarcomeres shorten
↓
Muscle develops tension and may shorten
This connects the nervous system directly with movement.
The Neuromuscular Junction
The connection between a motor neuron and a skeletal muscle fibre is called a neuromuscular junction.
It is a specialised type of synapse.
When a nerve impulse reaches the end of the motor neuron:
- A neurotransmitter is released.
- The neurotransmitter crosses the small synaptic gap.
- It binds to receptors on the muscle cell.
- Electrical activity spreads across the muscle membrane.
- Processes inside the fibre trigger contraction.
The neurotransmitter used at vertebrate skeletal neuromuscular junctions is acetylcholine.
Muscles Need Energy
Muscle contraction requires energy.
The immediate usable energy is supplied by ATP.
ATP is needed for several processes involved in contraction, including the repeated interactions between actin and myosin.
Muscles therefore require a continuous supply of ATP during activity.
Where Does the ATP Come From?
Muscle cells regenerate ATP using energy released from nutrients.
During aerobic respiration:
glucose + oxygen → carbon dioxide + water + energy transferred
Some of this transferred energy is used to produce ATP.
Muscles therefore depend on several organ systems.
The respiratory system supplies oxygen.
The digestive system supplies nutrients.
The circulatory system transports oxygen and nutrients to muscle cells.
Muscle cells use these materials to release energy through cellular respiration.
ATP and the Sliding Filament Mechanism
ATP plays a direct role in interactions between actin and myosin.
Myosin structures repeatedly:
- Attach to actin.
- Produce a pulling movement.
- Detach.
- Reset their position.
ATP is required for this cycle to continue.
Without sufficient ATP, normal muscle contraction and relaxation cannot continue.
Muscular movement therefore represents a conversion of:
Chemical energy → mechanical work + thermal energy
Muscles and Oxygen
During sustained activity, muscles require large amounts of ATP.
This increases the rate of cellular respiration.
As a result:
- Breathing rate can increase.
- Breathing depth can increase.
- Heart rate can increase.
- Blood flow to active muscles can increase.
These responses help deliver additional oxygen and nutrients while removing carbon dioxide and other metabolic products.
Anaerobic Energy Supply
Sometimes the demand for ATP becomes greater than can be supported immediately by aerobic pathways alone.
Muscle cells can also regenerate ATP through processes that do not depend directly on oxygen.
During intense exercise, anaerobic glycolysis can contribute significantly.
In humans, this can result in increased production of lactate.
Anaerobic pathways can provide ATP relatively quickly but cannot support high-intensity activity indefinitely.
Muscle Fatigue
During prolonged or intense activity, the ability of muscles to maintain the same force may decrease.
This is called muscle fatigue.
Fatigue is complex and can involve factors such as:
- Changes in energy supply.
- Changes in ion concentrations.
- Accumulation of metabolic products.
- Changes in nervous-system activation.
It is therefore an oversimplification to say that muscle fatigue is caused only by "lactic acid."
Movement and Survival
Movement has enormous survival value.
Animals use movement to:
- Escape predators.
- Capture prey.
- Find food.
- Locate water.
- Find shelter.
- Find mates.
- Defend territory.
- Care for offspring.
- Migrate.
- Communicate.
The muscular system therefore affects how an animal interacts with virtually every part of its environment.
Movement and Feeding
Animals often need coordinated muscular movement to obtain food.
A predator may need to:
- Detect prey.
- Approach it.
- Accelerate rapidly.
- Change direction.
- Capture it.
- Manipulate and consume it.
Each stage requires cooperation between:
sensory systems + nervous system + muscles + skeleton
Movement is therefore closely connected with both feeding strategies and nervous coordination.
Movement and Predator Avoidance
For prey animals, rapid movement can be essential for survival.
Different species use strategies such as:
- Rapid acceleration.
- High maximum speed.
- Sudden changes of direction.
- Jumping.
- Climbing.
- Swimming.
- Flying.
The most useful movement depends on the animal's body structure, habitat, and predators.
Movement and Migration
Some animals travel enormous distances during seasonal migrations.
Examples include:
- Birds.
- Whales.
- Salmon.
- Caribou.
- Insects.
Migration may allow animals to:
- Reach breeding areas.
- Find food.
- Avoid harsh weather.
- Reach suitable temperatures.
- Access seasonal resources.
Muscles must therefore sometimes support prolonged activity rather than short bursts of speed.
Different Muscles for Different Lifestyles
Muscular systems reflect an animal's lifestyle.
For example:
Cheetah
Powerful limb muscles help produce rapid acceleration and high running speed.
Migratory Bird
Flight muscles can support prolonged muscular activity.
Fish
Muscles along the body produce waves of contraction that move the animal through water.
Snake
Coordinated muscle contractions interact with the skeleton and ground to produce locomotion without limbs.
There is no single "best" muscular system. Different arrangements are suited to different environments and behaviours.
Muscles Do More Than Produce Locomotion
Muscles are involved in many processes besides moving from one location to another.
Examples include:
- Breathing.
- Pumping blood.
- Moving food through the digestive tract.
- Maintaining posture.
- Producing facial expressions.
- Speaking.
- Shivering.
- Controlling body openings.
Movement occurs at many levels within an animal's body.
Muscles and Breathing
Breathing depends on skeletal muscles.
The diaphragm and intercostal muscles change the volume of the thoracic cavity.
During inhalation, contraction of these muscles increases thoracic volume, helping air enter the lungs.
Muscular activity is therefore essential for maintaining gas exchange.
Muscles and Digestion
Smooth muscle in the digestive system produces coordinated contractions.
One important process is peristalsis.
During peristalsis, waves of smooth muscle contraction move material through structures such as:
- Oesophagus.
- Stomach.
- Intestines.
This demonstrates that movement inside the body is just as important as external locomotion.
Muscles and Thermoregulation
Muscle activity can also help regulate body temperature.
When humans become cold, skeletal muscles may contract rapidly and repeatedly.
This is shivering.
Shivering increases metabolic activity and transfers more energy as heat.
Muscles therefore contribute to homeostasis as well as movement.
Movement Requires Coordination
Effective movement requires more than strong muscles.
The nervous system must control:
- Which muscles contract.
- When they contract.
- How strongly they contract.
- How long they contract.
- Which opposing muscles relax or stabilise a joint.
Sensory receptors provide continuous information about:
- Body position.
- Muscle stretch.
- Balance.
- External conditions.
The brain and spinal cord use this information to adjust movement.
Proprioception and Movement
Proprioceptors provide information about the position and movement of body parts.
Imagine walking upstairs without looking directly at your feet.
Your nervous system continually receives information about:
- Joint angles.
- Muscle length.
- Muscle tension.
- Body position.
This allows muscular contractions to be adjusted from moment to moment.
Movement is therefore a continuous feedback process rather than a simple sequence of fixed commands.
Worked Example: Lifting a Backpack
Suppose you lift a backpack from the floor.
Several processes occur.
Nervous Control
Motor neurons send signals to skeletal muscles.
Muscle Contraction
Actin and myosin interact, producing tension.
Force Transfer
Tendons transfer muscular force to bones.
Joint Movement
Bones rotate around joints.
Energy Supply
ATP provides energy for muscle contraction.
Feedback
Sensory receptors provide information about the backpack's weight and the positions of your limbs.
The nervous system adjusts muscle activity accordingly.
A seemingly simple action therefore involves several interacting body systems.
Worked Example: Running from a Predator
Consider an animal detecting a predator.
Sensory receptors detect the threat.
The nervous system processes the information.
Motor neurons activate appropriate skeletal muscles.
Muscles contract using ATP.
Bones move around joints.
The animal accelerates away.
At the same time:
- Heart rate increases.
- Breathing increases.
- More oxygen and nutrients are delivered to muscles.
This demonstrates how several organ systems cooperate to produce survival behaviour.
Worked Example: Holding an Object Still
Muscles do not only produce visible movement.
Suppose you hold a heavy book still with your elbow bent.
The position of the book may not change, but your muscles are still producing force.
Muscle contraction in which tension is produced without a major change in muscle length is called an isometric contraction.
This is important for:
- Maintaining posture.
- Holding objects.
- Stabilising joints.
Therefore, muscular contraction does not always result in obvious movement.
Exercise and Muscular Adaptation
Regular physical activity can produce adaptations in muscle and supporting systems.
Depending on the type of training, adaptations may include:
- Increased muscle size.
- Increased strength.
- Improved endurance.
- Increased blood supply to active tissues.
- Changes in energy-producing capacity.
- Improved coordination of movement.
The body can therefore adapt to repeated physical demands.
Common Mistakes
Saying Muscles Push Bones
Muscles generate pulling forces through contraction. Opposite movements generally require other muscles.
Confusing Tendons and Ligaments
Tendons connect muscle to bone.
Ligaments connect bone to bone.
Saying the Biceps and Triceps Always Work Independently
They form an antagonistic pair and their activities are coordinated. They may also contract together when joint stability is required.
Saying All Muscle Is Voluntary
Smooth and cardiac muscle are involuntary. Skeletal muscle is mostly under voluntary control but can also participate in automatic responses.
Saying Muscles "Make Energy"
Muscles do not create energy. They transform energy supplied through biochemical processes into mechanical work and thermal energy.
Saying Actin and Myosin Become Shorter
The sliding filament mechanism mainly involves actin and myosin filaments sliding past one another, shortening the sarcomere.
Saying Muscles Only Need Energy During Exercise
Muscles require ATP whenever they are active, including for posture, breathing, heart function, and internal movement.
Saying Lactate Alone Causes Muscle Fatigue
Muscle fatigue has several causes and cannot be explained by lactate alone.
Check Your Understanding
1. Explain how skeletal muscles produce movement of bones.
2. Why are many muscles arranged in antagonistic pairs?
3. Describe what happens to the biceps and triceps when the elbow bends.
4. Explain the difference between a tendon and a ligament.
5. Compare skeletal, smooth, and cardiac muscle.
6. Where is smooth muscle found? Give two examples.
7. Explain the roles of actin and myosin in muscle contraction.
8. Why is ATP necessary for muscle contraction?
9. Explain how the nervous system controls skeletal muscle.
10. What happens at a neuromuscular junction?
11. Explain why breathing and circulation increase during vigorous exercise.
12. Give three examples of movement that increase an animal's chances of survival.
13. Explain how sensory receptors contribute to coordinated movement.
14. Why can holding a heavy object still require considerable muscular effort even though the object is not moving?
15. A desert predator can sprint rapidly but cannot maintain its maximum speed for very long. Explain how muscle function and energy supply could account for this pattern.
Key Terms
- Muscle – tissue specialised to produce force through contraction.
- Contraction – development of tension by muscle tissue, sometimes accompanied by shortening.
- Skeletal muscle – mostly voluntary, striated muscle commonly attached to bones.
- Smooth muscle – involuntary muscle found in many internal organs.
- Cardiac muscle – specialised involuntary muscle forming the heart.
- Tendon – connective tissue attaching muscle to bone.
- Ligament – connective tissue connecting bone to bone.
- Joint – location where two or more bones meet.
- Antagonistic pair – muscles that produce opposing movements.
- Flexion – movement that decreases the angle at a joint.
- Extension – movement that increases the angle at a joint.
- Muscle fibre – specialised muscle cell.
- Myofibril – contractile structure within a muscle fibre.
- Sarcomere – repeating contractile unit of skeletal and cardiac muscle.
- Actin – protein forming thin filaments involved in muscle contraction.
- Myosin – motor protein forming thick filaments and interacting with actin during contraction.
- Sliding filament mechanism – process in which actin and myosin filaments slide relative to one another during contraction.
- Neuromuscular junction – specialised synapse between a motor neuron and skeletal muscle fibre.
- ATP – molecule providing immediately usable energy for cellular processes, including muscle contraction.
- Peristalsis – wave-like smooth muscle contractions that move material through the digestive tract.
- Proprioception – sensory awareness of body position and movement.
- Isometric contraction – muscle contraction producing tension without substantial shortening of the muscle.
Key Takeaways
- Muscles produce force through contraction.
- Skeletal muscles usually move bones by pulling on them through tendons.
- Bones act as levers and joints act as pivots during many movements.
- Because muscles pull rather than push, many skeletal muscles work in antagonistic pairs.
- The biceps and triceps produce opposing movements at the elbow.
- Tendons connect muscle to bone, while ligaments connect bone to bone.
- Animals have skeletal, smooth, and cardiac muscle.
- Skeletal muscle produces most movement of the skeleton.
- Smooth muscle moves materials through many internal organs.
- Cardiac muscle produces the contractions that pump blood.
- Skeletal muscle contains actin and myosin arranged in repeating sarcomeres.
- Muscle contraction occurs through the sliding filament mechanism.
- Motor neurons control skeletal muscle through neuromuscular junctions.
- ATP is required for muscle contraction and must continually be regenerated.
- Muscles depend on the respiratory, circulatory, and digestive systems for the materials needed to sustain activity.
- Movement helps animals obtain food, escape predators, reproduce, migrate, communicate, and interact with their environments.
- Muscles are also essential for breathing, digestion, circulation, posture, and thermoregulation.
- Effective movement requires continuous coordination between the nervous system, sensory receptors, muscles, and skeleton.
4. Skeletons and Support Systems
Learning outcomes
- I can compare hydrostatic, exoskeleton, and endoskeleton support systems.
- I can explain the functions of skeletons.
- I can describe how skeletons support movement and protection.
- I can identify adaptations of skeletal systems in different animals.
- I can explain the relationship between support and locomotion.
Why Do Animals Need Support Systems?
Animals need some form of structural support to maintain their shape and move effectively.
Without a support system, muscles would have little to pull against and many animals would be unable to maintain a stable body shape.
Support systems can perform several important functions:
- Support the body.
- Maintain body shape.
- Protect delicate organs.
- Provide attachment points for muscles.
- Allow movement.
- Support locomotion.
- In some animals, store minerals.
- Provide protection against predators or physical damage.
Animals have evolved several different solutions to the problem of support.
Three major types are:
- Hydrostatic skeletons
- Exoskeletons
- Endoskeletons
Each system has advantages and limitations and is suited to particular body forms and lifestyles.
What Is a Skeleton?
A skeleton is a structural support system that helps maintain body shape and allows forces produced by muscles to create controlled movement.
The word "skeleton" does not necessarily mean bones.
An earthworm, for example, has no bones but still has a support system that allows it to move effectively.
The three major systems work differently:
Hydrostatic skeleton → fluid provides support
Exoskeleton → rigid structure outside the body provides support
Endoskeleton → internal framework provides support
Hydrostatic Skeletons
A hydrostatic skeleton uses fluid contained within the body to provide support.
The fluid is largely incompressible, meaning that it does not easily decrease in volume when pressure is applied.
Muscles surrounding the fluid-filled space can change the animal's shape and produce movement.
Hydrostatic skeletons occur in animals such as:
- Earthworms.
- Many other worms.
- Sea anemones.
- Jellyfish.
Hydrostatic support also contributes to movement in structures such as the arms of octopuses.
How Does a Hydrostatic Skeleton Work?
Imagine a water-filled balloon.
The water inside cannot easily be compressed.
If one part of the balloon is squeezed, pressure is transferred through the fluid and the shape changes.
A hydrostatic skeleton uses a similar principle.
Muscles contract around a fluid-filled body cavity.
Because the fluid resists compression, muscular contractions can change the shape of the body and generate movement.
This allows the fluid to act as something for the muscles to push or pull against indirectly.
Earthworm Movement
Earthworms provide an excellent example of hydrostatic support.
They have two important sets of muscles:
- Circular muscles.
- Longitudinal muscles.
When circular muscles contract, a body segment becomes:
- Longer.
- Thinner.
When longitudinal muscles contract, the segment becomes:
- Shorter.
- Thicker.
Coordinated waves of contraction travel along the body.
Small bristles called setae help grip the ground.
The sequence can be simplified as:
Muscles contract → body shape changes → parts of body grip surface → animal moves forward
Advantages of Hydrostatic Skeletons
Hydrostatic skeletons have several advantages.
They are:
- Flexible.
- Lightweight.
- Capable of producing a wide range of body shapes.
- Useful for burrowing and squeezing through narrow spaces.
- Able to support soft-bodied animals without heavy rigid structures.
Flexibility can be particularly useful in aquatic environments and underground habitats.
Limitations of Hydrostatic Skeletons
Hydrostatic skeletons also have limitations.
They generally:
- Provide less rigid protection than hard skeletons.
- Depend on maintaining suitable internal fluid pressure.
- Provide fewer rigid attachment points for powerful muscles.
- May provide less support for large terrestrial animals.
For many small or soft-bodied animals, however, these limitations are not major disadvantages.
The effectiveness of a support system depends on the animal's environment and lifestyle.
Exoskeletons
An exoskeleton is a hard supporting structure located on the outside of the body.
Arthropods such as insects, spiders, crabs, and lobsters have exoskeletons.
Arthropod exoskeletons contain a material called chitin, often combined with other substances that change its strength and flexibility.
Functions of an Exoskeleton
An exoskeleton can perform several functions at once.
It can:
- Support the body.
- Maintain body shape.
- Protect internal organs.
- Provide attachment surfaces for muscles.
- Form joints for movement.
- Reduce water loss.
- Provide protection against predators and physical damage.
This combination has been highly successful. Arthropods are among the most diverse groups of animals on Earth.
Exoskeletons and Water Conservation
An important advantage of the arthropod exoskeleton is its ability to reduce water loss.
The outer surface can act as a barrier that limits evaporation.
This is especially important for terrestrial arthropods such as insects.
Without a protective outer covering, a small animal with a high surface-area-to-volume ratio could lose water rapidly.
The exoskeleton therefore contributes to both:
Support + water conservation
Exoskeletons and Movement
A rigid outer covering might seem as though it would prevent movement.
However, arthropod exoskeletons contain joints.
Flexible membranes between rigid sections allow body parts to move relative to one another.
Muscles attach to the inner surface of the exoskeleton.
When muscles contract, they move different sections of the exoskeleton around joints.
This allows movements such as:
- Walking.
- Running.
- Jumping.
- Swimming.
- Flying.
- Grasping.
A Major Problem: Growth
A rigid exoskeleton cannot simply expand continuously as an animal grows.
Arthropods therefore periodically shed their old exoskeleton.
This process is called molting, or ecdysis.
During a molt:
- A new exoskeleton develops beneath the old one.
- The old exoskeleton splits.
- The animal emerges.
- The new exoskeleton expands.
- The new covering eventually hardens.
The Risks of Molting
Molting allows growth, but it creates a period of vulnerability.
Immediately after leaving the old exoskeleton, the new exoskeleton may be relatively soft.
During this period, the animal may be:
- More vulnerable to predators.
- Less protected from physical damage.
- Less able to move normally.
- More vulnerable to water loss.
This is an important trade-off associated with having a rigid external skeleton.
Endoskeletons
An endoskeleton is an internal supporting framework.
Vertebrates have endoskeletons made primarily from:
- Bone.
- Cartilage.
Examples include:
- Fish.
- Amphibians.
- Reptiles.
- Birds.
- Mammals.
Some invertebrates, such as echinoderms, also have internal skeletal structures.
Functions of the Vertebrate Endoskeleton
The vertebrate skeleton performs several functions.
It:
- Supports the body.
- Maintains body shape.
- Protects organs.
- Provides attachment points for muscles.
- Allows movement around joints.
- Stores minerals.
- Contains bone marrow involved in blood-cell production.
The skeleton is therefore an active biological system, not simply a rigid framework.
Protection
Different parts of the vertebrate skeleton protect important organs.
For example:
Skull → protects brain
Vertebral column → surrounds and protects spinal cord
Rib cage → protects heart and lungs
Pelvis → helps protect organs in the lower abdomen and pelvic region
The shape of individual bones often reflects both their mechanical and protective functions.
Endoskeletons and Movement
Skeletal muscles are attached to bones by tendons.
When muscles contract, they pull on bones.
Bones then move around joints.
A simplified system is:
Muscle → tendon → bone → joint → movement
Bones can function as levers, while joints act as pivots.
This allows relatively small changes in muscle length to produce useful movements of limbs.
Antagonistic Muscles and the Skeleton
Because muscles pull rather than push, skeletal movement often requires antagonistic muscle pairs.
At the elbow:
Biceps contracts → forearm flexes
Triceps contracts → forearm extends
The endoskeleton therefore provides the rigid structures against which muscles can generate controlled forces.
Endoskeletons Grow with the Animal
Unlike an arthropod exoskeleton, a vertebrate endoskeleton can grow as the animal grows.
Bones are living tissues.
During growth, bone can:
- Increase in length.
- Increase in thickness.
- Change shape.
- Repair damage.
- Remodel in response to mechanical forces.
This means vertebrates do not need to shed their entire skeleton in order to grow.
Comparing the Three Support Systems
| Feature | Hydrostatic Skeleton | Exoskeleton | Endoskeleton |
|---|---|---|---|
| Main support | Pressurised fluid | Rigid external covering | Internal framework |
| Example | Earthworm | Insect | Human |
| Flexibility | High | Limited by rigid sections and joints | Moderate to high at joints |
| Protection | Usually limited | Strong external protection | Strong protection of selected organs |
| Muscle interaction | Muscles act against fluid | Muscles attach internally to exoskeleton | Muscles attach to bones |
| Growth | Can expand with body | Requires molting in arthropods | Grows with body |
| Water conservation | Usually limited | Can strongly reduce water loss | Skin provides main external barrier |
| Large body support | Usually limited on land | Size can be mechanically constrained | Can support large terrestrial animals |
No system is universally superior. Each represents a different solution to the problems of support, protection, and movement.
Support and Locomotion
Locomotion means movement of an animal from one place to another.
A support system makes locomotion more effective because it provides something against which muscles can generate forces.
The general principle is:
Muscle contraction + support system + interaction with environment → locomotion
For example:
- Earthworm muscles act against fluid.
- Insect muscles act against an exoskeleton.
- Human muscles pull on an endoskeleton.
Different structures, but the same fundamental requirement: muscular force must be translated into useful movement.
Support Systems and the Environment
The effectiveness of a support system depends partly on the environment.
Water provides buoyancy, which helps support an animal's weight.
This means aquatic animals may require less structural support against gravity than similarly sized terrestrial animals.
On land, gravity places greater demands on structures such as:
- Limbs.
- Vertebral columns.
- Joints.
- Muscles.
This helps explain why support structures differ between aquatic and terrestrial animals.
Adaptations for Running
Animals specialised for running often have skeletal adaptations that improve speed or efficiency.
These may include:
- Long limbs.
- Reduced mass toward the ends of limbs.
- Flexible spinal columns.
- Specialised joints.
- Elongated lower limb bones.
In a cheetah, for example, a flexible spine contributes to increased stride length during high-speed running.
Adaptations for Flying
Flight places very different demands on the skeleton.
Bird skeletons have several adaptations related to flight.
These can include:
- Lightweight bones.
- Fusion of some bones for strength.
- Modified forelimbs forming wings.
- Large surfaces for attachment of flight muscles.
- A streamlined body form.
Many flying birds possess a large keel on the sternum.
Powerful flight muscles attach to this structure.
The skeleton therefore provides both lightweight support and strong muscle attachment.
Adaptations for Swimming
Aquatic vertebrates also show specialised skeletal adaptations.
Fish typically have:
- Flexible vertebral columns.
- Fins supported by skeletal structures.
- Streamlined bodies.
- Muscles arranged to produce side-to-side movement.
The skeleton and muscles work together to push water backward.
The resulting forces move the fish forward.
Adaptations for Burrowing
Burrowing animals face yet another challenge.
They must move through soil or sediment.
Useful adaptations may include:
- Flexible bodies.
- Powerful digging limbs.
- Strong claws.
- Compact body shapes.
- Hydrostatic support in some worms.
An earthworm's hydrostatic skeleton is especially effective because the body can lengthen, shorten, and change shape while moving through narrow spaces.
Adaptations for Jumping
Animals specialised for jumping may have:
- Long hind limbs.
- Powerful muscles.
- Strong joints.
- Elastic structures that store and release energy.
Frogs, for example, possess elongated hind limbs that help generate large forces against the ground.
This demonstrates how skeletal proportions can be closely related to locomotion.
Protection Versus Mobility
Support systems often involve trade-offs.
A very thick, rigid skeleton might provide excellent protection but could:
- Increase body mass.
- Reduce flexibility.
- Require more energy to move.
A highly flexible support system might improve manoeuvrability but provide less physical protection.
Evolution therefore often produces compromises between:
Protection + support + flexibility + mass + energy cost
Different animals show different combinations depending on their environments and lifestyles.
Worked Example: Earthworm
Consider an earthworm moving through soil.
It has no rigid bones.
Circular muscles contract, causing sections of the body to lengthen.
Longitudinal muscles contract, causing sections to shorten.
Fluid inside the body resists compression.
Setae grip the soil.
The combination produces forward movement.
Therefore:
Muscles + fluid pressure + grip → locomotion
This is a hydrostatic skeleton in action.
Worked Example: Grasshopper
A grasshopper has an exoskeleton.
Its muscles attach to the inner surfaces of this external skeleton.
When leg muscles contract:
- Forces are transferred to rigid exoskeletal sections.
- Sections rotate around joints.
- The hind legs push strongly against the ground.
The ground exerts a force on the grasshopper, accelerating it upward and forward.
The exoskeleton therefore provides both support and a mechanical system for movement.
Worked Example: Human Arm
A human arm uses an endoskeleton.
When the biceps contracts:
- The muscle produces tension.
- The tendon transfers the force to the forearm.
- The forearm bones act as levers.
- The elbow acts as a pivot.
- The forearm moves.
Therefore:
Muscle contraction → tendon force → bone movement → movement at joint
The skeleton does not produce the force itself. The muscles provide the force, while the skeleton provides the mechanical framework.
Worked Example: Moving from Water to Land
Imagine an animal lineage moving from a mainly aquatic environment to a terrestrial environment.
In water, buoyancy supports much of the animal's weight.
On land, this support disappears.
Gravity places greater demands on:
- Limbs.
- Joints.
- Vertebral column.
- Muscles.
A successful terrestrial animal therefore requires structures capable of supporting its body against gravity while still permitting movement.
This illustrates how environment can influence the evolution of support systems.
Body Size and Support
As an animal becomes larger, its mass increases rapidly.
Its support system must withstand increasing forces caused by gravity and movement.
Large terrestrial vertebrates often have:
- Thick limb bones.
- Strong joints.
- Large muscle attachment areas.
- Limb positions that efficiently support body mass.
An elephant, for example, has thick, relatively column-like limbs that support its large mass.
A tiny insect can use a very different structural arrangement.
Skeletons Can Have Additional Functions
Support systems can perform functions beyond support and locomotion.
For example, vertebrate bones can:
- Store calcium and phosphate.
- Contain bone marrow.
- Produce blood cells.
- Protect organs.
- Store energy in yellow bone marrow.
An exoskeleton can also:
- Reduce water loss.
- Provide camouflage.
- Carry sensory structures.
- Provide defensive structures such as spines.
A skeleton is therefore often a multifunctional system.
Support Systems and Evolution
Hydrostatic skeletons, exoskeletons, and endoskeletons represent different evolutionary solutions to similar problems.
All must allow an animal to:
- Maintain body shape.
- Resist external forces.
- Produce controlled movement.
- Protect important structures.
- Function effectively in its environment.
The details differ because animals have different:
- Body sizes.
- Habitats.
- Feeding strategies.
- Predators.
- Methods of locomotion.
- Evolutionary histories.
Common Mistakes
Thinking Every Skeleton Is Made of Bone
Hydrostatic skeletons use fluid, and arthropod exoskeletons are largely composed of materials including chitin.
Thinking an Exoskeleton Is Just Protective Armour
An exoskeleton also provides support and attachment surfaces for muscles and plays an important role in movement.
Saying Exoskeleton Muscles Are Outside the Skeleton
In arthropods, muscles are located inside the exoskeleton and attach to its internal surfaces.
Saying Hydrostatic Skeletons Have No Structural Support
The pressurised fluid provides the resistance against which muscles act.
Saying Endoskeletons Do Not Protect the Body
Although they are internal, structures such as the skull, vertebral column, and rib cage provide substantial protection.
Thinking Skeletons Produce Movement by Themselves
Muscles produce force. Skeletons provide structures against which those forces act.
Saying Exoskeletons Cannot Move
Exoskeletons contain joints and flexible regions that allow movement.
Thinking an Exoskeleton Grows Continuously
In arthropods, the rigid exoskeleton must periodically be shed during molting.
Assuming One Skeleton Type Is Always Better
Each system has advantages and limitations. Its effectiveness depends on the animal's body form, size, environment, and lifestyle.
Check Your Understanding
1. Give four important functions of an animal skeleton.
2. Define a hydrostatic skeleton.
3. Explain how an earthworm uses its hydrostatic skeleton to move.
4. Give two advantages and two limitations of hydrostatic skeletons.
5. Define an exoskeleton and name two groups of animals that possess one.
6. Explain how muscles produce movement in an animal with an exoskeleton.
7. Why must arthropods molt?
8. Explain one disadvantage of molting.
9. Define an endoskeleton.
10. Explain how an endoskeleton works with muscles to produce movement.
11. Compare the growth of an exoskeleton with the growth of a vertebrate endoskeleton.
12. Explain why an exoskeleton can be useful for a small terrestrial animal.
13. Describe two skeletal adaptations for flight, running, swimming, or jumping.
14. Explain why large terrestrial animals require particularly strong support structures.
15. An unknown animal has a soft flexible body, no rigid skeleton, and muscles surrounding a fluid-filled cavity. Identify its type of support system and explain how it could use that system for locomotion.
Key Terms
- Skeleton – structural system that supports an animal and helps muscles produce controlled movement.
- Support system – structures or mechanisms that maintain body shape and resist forces.
- Hydrostatic skeleton – support system in which muscles act against pressurised, relatively incompressible fluid.
- Exoskeleton – rigid supporting structure located outside the body.
- Endoskeleton – internal supporting framework.
- Chitin – structural material forming an important part of arthropod exoskeletons.
- Molting – shedding of an old exoskeleton to allow growth.
- Ecdysis – another term for molting.
- Bone – strong living connective tissue forming much of the vertebrate skeleton.
- Cartilage – flexible supportive connective tissue.
- Joint – location where skeletal structures meet and, in many cases, permit movement.
- Tendon – connective tissue attaching muscle to skeletal structures.
- Lever – rigid structure that rotates around a pivot when a force is applied.
- Locomotion – movement of an organism from one location to another.
- Buoyancy – upward force exerted by a fluid that helps support objects immersed in it.
- Setae – small bristles that help organisms such as earthworms grip surfaces.
- Antagonistic muscles – muscles that produce opposing movements.
- Keel – projection of the sternum in many birds that provides attachment for major flight muscles.
Key Takeaways
- Animals require support systems to maintain shape, protect organs, and produce effective movement.
- The three major support systems are hydrostatic skeletons, exoskeletons, and endoskeletons.
- Hydrostatic skeletons use pressurised fluid as a supporting structure.
- Earthworms use circular and longitudinal muscles acting against fluid to produce locomotion.
- Hydrostatic skeletons provide excellent flexibility but relatively little rigid protection.
- Exoskeletons form rigid structures around the outside of the body.
- Arthropod exoskeletons provide support, protection, muscle attachment, and reduced water loss.
- Joints allow animals with exoskeletons to move despite their rigid outer covering.
- Arthropods must periodically molt because their rigid exoskeleton cannot continuously expand.
- Endoskeletons are internal support structures that can grow with the animal.
- Vertebrate endoskeletons consist largely of bone and cartilage.
- Bones protect important organs and provide attachment points for muscles.
- Muscles produce force while skeletons provide the structures against which those forces act.
- Bones and rigid exoskeletal sections can function as levers during movement.
- Aquatic animals benefit from buoyancy, while terrestrial animals require greater structural support against gravity.
- Skeletal adaptations can support specialised forms of locomotion such as running, flying, swimming, jumping, and burrowing.
- Support systems involve trade-offs between strength, protection, flexibility, mass, and energy cost.
- The structure of an animal's support system is closely related to its size, environment, lifestyle, and method of locomotion.
5. Coordination of Responses
Learning outcomes
- I can explain how animals coordinate responses to stimuli.
- I can describe the pathway of a simple reflex action.
- I can explain how different organ systems work together during responses.
- I can distinguish between voluntary and involuntary responses.
- I can analyze how coordinated responses improve survival.
Why Do Animals Need Coordinated Responses?
Animals live in environments that are constantly changing. To survive, they must detect these changes and produce appropriate responses.
A change that can be detected by an organism is called a stimulus.
Examples include:
- A sudden loud sound.
- A predator approaching.
- The smell of food.
- A change in temperature.
- Bright light.
- Pain from touching a hot surface.
- Changes in carbon dioxide concentration inside the body.
- Loss of balance.
Simply detecting a stimulus is not enough. Different parts of the body must work together to produce an appropriate response.
This process is called coordination.
A general response pathway is:
Stimulus → receptor → coordinator → effector → response
In animals, coordination commonly involves the nervous system, the endocrine system, or both.
The Stimulus-Response Pathway
Most responses begin when a receptor detects a stimulus.
The information must then be communicated to a coordinating system, which determines an appropriate response.
Signals are then sent to effectors.
An effector is a structure that produces a response.
The main effectors in animals are:
- Muscles.
- Glands.
For example:
Bright light → photoreceptors → nervous system → muscles of iris → pupil becomes smaller
Or:
High body temperature → thermoreceptors → nervous system → sweat glands → increased sweating
The same basic pattern can therefore produce many different responses.
Receptors
Sensory receptors are specialised cells or structures that detect changes in the internal or external environment.
Different receptors detect different stimuli.
Examples include:
- Photoreceptors detect light.
- Mechanoreceptors detect pressure, vibration, or movement.
- Chemoreceptors detect chemicals.
- Thermoreceptors detect temperature.
- Nociceptors detect potentially damaging stimuli.
- Proprioceptors detect body position and movement.
The receptors provide the nervous system with information about what is happening.
The Coordinator
After a stimulus has been detected, information is sent to a coordinator.
In nervous responses, the main coordinating structures are the:
- Brain.
- Spinal cord.
Together, these form the central nervous system, or CNS.
The CNS receives information, processes it, and sends appropriate signals toward effectors.
A coordinator does not necessarily produce the response itself.
Instead, it determines or organises what other structures should do.
Effectors
Effectors carry out responses.
Two important types are:
Muscles
Muscles respond by contracting.
Examples include:
- Pulling your hand away from something hot.
- Running away from danger.
- Shivering when cold.
- Changing pupil diameter.
Glands
Glands respond by releasing substances.
Examples include:
- Sweat glands producing sweat.
- Salivary glands releasing saliva.
- Endocrine glands releasing hormones.
Therefore:
Muscle → contraction
Gland → secretion
The Nervous Response Pathway
A more detailed nervous-system pathway is:
Stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response
Each part has a particular function.
Receptor: detects the stimulus.
Sensory neuron: carries information toward the CNS.
CNS: processes and coordinates information.
Motor neuron: carries signals toward an effector.
Effector: carries out the response.
Worked Example: Catching a Falling Object
Imagine someone drops a ruler and you attempt to catch it.
Stimulus
The ruler begins to fall.
Receptor
Photoreceptors in the eyes detect movement.
Sensory information
Information travels toward the brain.
Coordination
The brain processes the visual information and determines an appropriate movement.
Motor signals
Motor neurons carry signals toward muscles in the hand and arm.
Effector
Skeletal muscles contract.
Response
The fingers close around the ruler.
This entire process can occur in a fraction of a second.
Reflex Actions
Some situations require an especially rapid response.
A reflex action is a rapid, automatic response to a stimulus.
Examples include:
- Withdrawing from a hot object.
- Blinking when something approaches the eye.
- The knee-jerk reflex.
- Changes in pupil diameter.
Reflexes can protect the body and help maintain normal body functions.
The Reflex Arc
The nervous pathway involved in producing a reflex is called a reflex arc.
A simple reflex arc can be represented as:
Stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector → response
The relay neuron is located within the central nervous system.
For many withdrawal reflexes, much of the initial coordination occurs in the spinal cord.
This allows a response to begin rapidly.
Worked Example: Touching a Hot Pan
Suppose your finger touches a very hot pan.
Detection
Receptors in the skin detect potentially damaging conditions.
Sensory transmission
A sensory neuron carries impulses toward the spinal cord.
Coordination
Within the spinal cord, information passes through neurons involved in the reflex pathway.
Motor transmission
A motor neuron carries impulses toward muscles in the arm.
Response
The muscles contract and pull the hand away.
The pathway can be summarised as:
Heat → receptor → sensory neuron → spinal cord → motor neuron → muscle → withdrawal
Does the Brain Know About a Reflex?
A common misconception is that the brain is completely uninvolved in a spinal reflex.
The initial withdrawal response can begin through circuits in the spinal cord without waiting for conscious processing by the brain.
However, information also travels to the brain.
This allows you to become consciously aware of:
- Pain.
- Heat.
- Where the stimulus occurred.
- What caused the problem.
Therefore, you may begin withdrawing your hand before you consciously recognise exactly what has happened.
Why Are Reflexes Useful?
Reflexes can reduce the time required to respond to potentially dangerous situations.
Imagine touching a sharp object.
If the nervous system required a long period of conscious decision-making before responding, more tissue damage might occur.
A rapid reflex can:
- Reduce injury.
- Protect delicate structures.
- Help maintain posture.
- Maintain balance.
- Regulate internal conditions.
Reflexes therefore have important survival value.
Voluntary Responses
A voluntary response is a response that involves conscious control or decision-making.
Examples include:
- Choosing to pick up a book.
- Kicking a football.
- Writing your name.
- Walking toward a food source.
- Choosing to hide from a predator.
Voluntary responses usually involve extensive processing by the brain.
Sensory information may be combined with:
- Memory.
- Previous experience.
- Current goals.
- Information from other senses.
The animal can then select an appropriate behaviour.
Involuntary Responses
An involuntary response occurs without deliberate conscious control.
Examples include:
- Heart-rate changes.
- Pupil responses.
- Digestive movements.
- Sweating.
- Changes in blood-vessel diameter.
- Many reflexes.
These responses are often important for maintaining stable internal conditions.
An animal does not need to consciously decide to increase its heart rate during exercise.
The body's control systems automatically coordinate the necessary changes.
Voluntary and Involuntary Responses Compared
| Voluntary Response | Involuntary Response |
|---|---|
| Usually involves conscious control | Does not require conscious control |
| Often involves decision-making | Often automatic |
| Commonly controls skeletal muscles | Can control muscles or glands |
| Can often be deliberately started or stopped | Usually regulated automatically |
| Example: throwing a ball | Example: pupil constriction |
The distinction is useful, although real biological responses can involve combinations of voluntary and involuntary processes.
Coordination Requires Multiple Organ Systems
Many responses require several organ systems to work together.
Consider running away from danger.
The response involves much more than the leg muscles.
Systems involved can include:
- Nervous system.
- Muscular system.
- Skeletal system.
- Respiratory system.
- Circulatory system.
- Endocrine system.
Coordination allows these systems to respond as parts of one functioning organism.
Worked Example: Escaping a Predator
Imagine a grazing animal suddenly detects a predator.
Sensory System
Eyes, ears, or chemical receptors detect the predator.
Nervous System
Sensory information reaches the CNS.
The brain processes the threat.
Muscular System
Motor neurons stimulate skeletal muscles.
Muscles contract to produce rapid movement.
Skeletal System
Bones and joints provide support and allow muscular forces to produce locomotion.
Respiratory System
Breathing rate and depth increase.
More oxygen enters the body.
Circulatory System
Heart rate increases.
Blood transports oxygen and nutrients toward active muscles.
Endocrine System
Hormones can help prepare the body for intense activity.
The result is a coordinated escape response.
The Fight-or-Flight Response
When an animal encounters a serious threat, several physiological changes may occur together.
This is often called the fight-or-flight response.
The sympathetic nervous system and hormones such as adrenaline contribute to this response.
Changes can include:
- Increased heart rate.
- Increased breathing rate.
- Increased blood flow toward skeletal muscles.
- Increased availability of glucose.
- Changes in pupil diameter.
- Reduced activity of some processes that are less immediately important.
These changes prepare the animal for rapid action.
Why Increase Heart Rate?
Active muscles require ATP.
ATP must continually be regenerated.
During sustained aerobic activity, muscle cells require increased supplies of:
- Oxygen.
- Glucose and other fuels.
Increasing heart rate helps increase blood flow.
Blood delivers useful substances to muscles and removes substances such as carbon dioxide.
Therefore:
Threat → increased muscular activity → increased energy demand → increased oxygen and fuel demand → increased circulation
Why Increase Breathing?
During intense activity, muscles increase their rate of energy transfer.
More oxygen may therefore be required for aerobic respiration.
Breathing becomes:
- Faster.
- Often deeper.
This increases ventilation of the lungs.
The respiratory and circulatory systems therefore cooperate to support muscular activity.
Nervous and Endocrine Coordination
Animals possess two major systems for coordinating responses:
- Nervous system.
- Endocrine system.
The nervous system uses electrical signals along neurons and chemical communication at synapses.
The endocrine system uses chemical messengers called hormones that are transported through body fluids such as blood.
Comparing Nervous and Hormonal Responses
| Nervous Coordination | Hormonal Coordination |
|---|---|
| Uses neurons | Uses hormones |
| Signals travel along specific pathways | Hormones are carried through blood |
| Usually rapid | Often slower |
| Responses can be very precisely targeted | Hormones may circulate throughout the body |
| Effects may be relatively short-lived | Effects can be longer-lasting |
These systems are not independent.
They frequently interact.
The brain can influence hormone release, while hormones can influence nervous-system activity.
Coordination and Homeostasis
Coordination is also essential for homeostasis.
Homeostasis is the regulation of relatively stable internal conditions.
Variables that animals may regulate include:
- Body temperature.
- Blood glucose concentration.
- Water balance.
- Carbon dioxide concentration.
- Blood pressure.
A general homeostatic pathway is:
Change → receptor → control centre → effector → corrective response
Worked Example: Body Temperature Rises
Suppose a mammal becomes too warm.
Stimulus
Body temperature increases.
Receptors
Thermoreceptors detect the change.
Coordinator
The nervous system processes the information.
Effectors
Sweat glands become more active.
Blood vessels near the skin can widen.
Response
Heat transfer from the body increases.
Body temperature moves back toward its normal range.
This is an example of coordinated regulation.
Coordinating Movement
Even ordinary movement requires remarkable coordination.
Imagine walking across an uneven surface.
The nervous system receives information from:
- Eyes.
- Inner ear.
- Touch receptors.
- Proprioceptors in muscles and joints.
The brain processes this information and continuously adjusts muscular activity.
Different muscles:
- Contract.
- Relax.
- Stabilise joints.
- Adjust posture.
These changes happen continuously as conditions change.
Feedback During Movement
Movement is not simply a command sent once from the brain to the muscles.
Sensory information continuously returns to the CNS.
For example, proprioceptors provide information about:
- Muscle length.
- Muscle tension.
- Joint position.
- Movement.
This creates a feedback system.
Motor command → movement → sensory feedback → adjustment → improved movement
This allows animals to adapt movement while it is occurring.
Coordinating Balance
Maintaining balance requires information from several sensory systems.
Important sources include:
- Eyes.
- Vestibular system of the inner ear.
- Proprioceptors.
- Touch and pressure receptors.
The brain combines these signals.
Motor commands then adjust skeletal muscles to maintain posture.
If one source of information is unavailable, other systems may partially compensate.
For example, standing on one leg is usually more difficult with the eyes closed.
Coordinated Feeding Responses
Finding and eating food also involves multiple systems.
Imagine smelling food.
Chemoreceptors detect molecules in the air.
The brain processes the information.
Possible responses include:
- Turning toward the food.
- Moving toward it.
- Increased salivation.
- Changes in digestive activity.
Some responses are voluntary, while others are involuntary.
One stimulus can therefore produce several coordinated responses at the same time.
Coordinated Responses in Different Animals
Different animals have nervous systems of different complexity, but all require some method of coordinating responses.
A jellyfish can coordinate swimming movements through a relatively simple nerve net.
An insect can integrate information from:
- Compound eyes.
- Antennae.
- Touch receptors.
- Chemical receptors.
A vertebrate can process enormous amounts of sensory information through a highly centralised brain and spinal cord.
Different systems can therefore solve similar problems in different ways.
Coordination and Predator Avoidance
Avoiding predators demonstrates the survival value of coordination particularly clearly.
An animal must:
- Detect the predator.
- Determine where it is.
- Assess the threat.
- Select a response.
- Coordinate muscles.
- Adjust breathing and circulation.
- Monitor movement.
- Change direction if necessary.
A failure at any stage could reduce the animal's chance of escape.
Coordination and Capturing Prey
Predators also depend on coordinated responses.
A predator may need to:
- Detect prey.
- Judge distance.
- Track movement.
- Approach without being detected.
- Accelerate rapidly.
- Coordinate limbs or jaws.
- Adjust movement as the prey changes direction.
Sensory information must be continuously processed while muscular responses are adjusted.
This is an example of sensory-motor coordination.
Coordination and Communication
Animals respond not only to physical conditions but also to signals produced by other animals.
Communication signals can include:
- Sounds.
- Visual displays.
- Chemical signals.
- Touch.
- Vibrations.
For communication to work:
Signal → receptor → nervous processing → behavioural or physiological response
For example, hearing a warning call may cause an animal to immediately stop feeding and seek shelter.
Coordination Improves Survival
Effective coordination allows animals to respond appropriately rather than randomly.
Coordinated responses can help animals:
- Escape predators.
- Capture prey.
- Locate food.
- Maintain balance.
- Avoid harmful conditions.
- Find mates.
- Protect offspring.
- Defend territories.
- Maintain homeostasis.
Natural selection therefore strongly favours sensory and response systems that improve an animal's ability to interact successfully with its environment.
Reaction Time
Reaction time is the time between detection of a stimulus and the beginning of a response.
Reaction time depends on several processes:
Detection → transmission → processing → motor transmission → response
Reaction time can be affected by factors such as:
- Type of stimulus.
- Complexity of the required response.
- Attention.
- Practice.
- Fatigue.
A simple ruler-drop experiment can be used to investigate reaction time.
Worked Example: Crossing a Road
Imagine an animal—or a human—crossing a road.
The nervous system must process:
- Visual information about moving vehicles.
- Sounds from the environment.
- Body position.
- Balance.
- Previous experience.
The brain uses this information to make decisions.
Motor neurons then control the muscles required for movement.
Meanwhile, involuntary systems regulate:
- Heart rate.
- Breathing.
- Blood pressure.
A single behaviour can therefore involve both voluntary and involuntary coordination.
Worked Example: Stepping on a Sharp Object
Suppose you step barefoot on something sharp.
Several responses may occur.
Immediate Reflex
Sensory receptors detect tissue damage.
A reflex pathway can cause rapid withdrawal of the foot.
Postural Adjustment
Muscles in the other leg contract to maintain balance.
Conscious Awareness
Information reaches the brain and produces pain perception.
Voluntary Response
You may choose to sit down and inspect the foot.
Physiological Response
Heart rate or stress responses may change.
A single stimulus can therefore trigger several coordinated responses involving different pathways.
Why Coordination Is More Than a Simple Reflex
A reflex is an excellent example of coordination, but animal behaviour is often much more complex.
Animals may combine:
- Immediate reflexes.
- Learned behaviours.
- Memory.
- Sensory information.
- Hormonal responses.
- Voluntary decisions.
- Automatic physiological adjustments.
The nervous and endocrine systems integrate these processes so that the animal responds as a coordinated whole.
Common Mistakes
Saying a Receptor Produces the Response
A receptor detects the stimulus. The effector carries out the response.
Confusing Sensory and Motor Neurons
Sensory neurons carry information toward the CNS.
Motor neurons carry signals toward effectors.
Saying All Reflexes Are Controlled by the Brain
Many simple reflexes can be coordinated initially through the spinal cord, although information also reaches the brain.
Saying Reflexes Are Voluntary
Reflexes are automatic and do not require conscious decision-making.
Thinking Involuntary Means Unimportant
Many essential processes, including heart-rate regulation and digestive movement, are involuntary.
Thinking One Organ System Produces an Escape Response
Escaping danger can involve the nervous, muscular, skeletal, respiratory, circulatory, and endocrine systems.
Saying Hormones and Nerves Work Completely Separately
Nervous and endocrine coordination interact extensively.
Thinking Every Response Is Either Completely Voluntary or Completely Involuntary
Many behaviours involve both. Running may be voluntary, while accompanying changes in heart rate and breathing are largely involuntary.
Check Your Understanding
1. Define a stimulus.
2. What is the function of a receptor?
3. What is an effector? Give two examples.
4. Write the complete nervous pathway from a stimulus to a response.
5. Define a reflex action.
6. Describe the pathway of a withdrawal reflex.
7. Explain why withdrawal reflexes can occur before conscious awareness of pain.
8. Compare voluntary and involuntary responses.
9. Give three examples of involuntary responses.
10. Explain how the nervous, muscular, and skeletal systems cooperate when an animal runs.
11. Why might heart rate and breathing rate increase during an escape response?
12. Compare nervous and hormonal coordination.
13. Explain how sensory feedback helps coordinate movement.
14. Describe how a coordinated response could help a prey animal survive an encounter with a predator.
15. A rabbit detects a fox and begins running. Its heart rate increases, breathing becomes faster, and its leg muscles contract rapidly. Explain how different organ systems cooperate to produce this response.
Key Terms
- Stimulus – detectable change in an organism's internal or external environment.
- Response – change in activity resulting from a stimulus.
- Coordination – organisation of different processes so that an organism produces an appropriate response.
- Receptor – specialised cell or structure that detects a stimulus.
- Coordinator – structure or system that processes information and organises a response.
- Effector – muscle or gland that produces a response.
- Sensory neuron – neuron carrying information from receptors toward the CNS.
- Motor neuron – neuron carrying signals from the CNS toward effectors.
- Relay neuron – neuron within the CNS that connects and processes information between other neurons.
- Central nervous system (CNS) – brain and spinal cord.
- Reflex – rapid, automatic response to a stimulus.
- Reflex arc – nervous pathway involved in a reflex.
- Voluntary response – response involving conscious control or decision-making.
- Involuntary response – response that does not require conscious control.
- Hormone – chemical messenger transported through the body to target cells.
- Endocrine system – system of glands that produces hormones.
- Homeostasis – regulation of relatively stable internal conditions.
- Proprioceptor – receptor providing information about body position and movement.
- Reaction time – time between detecting a stimulus and beginning a response.
- Sensory-motor coordination – integration of sensory information with controlled muscular responses.
Key Takeaways
- Animals must detect, process, and respond to stimuli in order to interact successfully with their environments.
- A basic response pathway is stimulus → receptor → coordinator → effector → response.
- Sensory receptors detect changes in the internal or external environment.
- Sensory neurons carry information toward the central nervous system.
- The brain and spinal cord process and coordinate nervous information.
- Motor neurons carry signals toward effectors.
- Muscles and glands are the major types of effectors.
- A reflex is a rapid, automatic response to a stimulus.
- A simple reflex arc involves receptors, sensory neurons, neurons within the CNS, motor neurons, and effectors.
- Spinal reflexes can begin before conscious processing of the stimulus is complete.
- Voluntary responses generally involve conscious control and decision-making.
- Involuntary responses occur automatically and include many reflex and homeostatic responses.
- Nervous and endocrine systems frequently work together to coordinate behaviour and physiology.
- Complex responses can involve the nervous, muscular, skeletal, respiratory, circulatory, and endocrine systems simultaneously.
- Sensory feedback allows movements to be continuously adjusted.
- Coordinated responses help animals avoid danger, capture food, maintain balance, communicate, reproduce, and maintain homeostasis.
- Effective coordination allows an animal to respond as an integrated organism rather than as a collection of independent organs.