Coordination and Movement
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.