Coordination and Movement
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.