🧠 Coordination & Response: How the Body Senses and Reacts
Explore how organisms detect changes in their environment and respond through nervous and hormonal coordination, from receptors to effectors.
Learning Goals
By the end of this lesson, you will be able to:
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Describe the pathway of a nervous response from stimulus to effector
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Explain the roles of receptors, neurones, the central nervous system, and effectors
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Compare nervous and hormonal coordination in terms of speed and duration
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Calculate reaction times and reflex speeds using simple measurements
Before You Begin
Before this lesson, you should know:
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Recognise that living organisms respond to stimuli as a characteristic of life
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Understand that cells can be specialised for particular functions
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Be able to rearrange and substitute values into the equation speed=timedistance
Notation & Symbols
Key notation and symbols used in this topic:
CNS
Central nervous system — comprises the brain and spinal cord, the coordination centre
PNS
Peripheral nervous system — the network of neurones connecting the CNS to the rest of the body
v=td
Speed of an impulse: distance travelled divided by time taken, in m s−1
ms
Millisecond — one thousandth of a second, 1ms=10−3s
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Indicates the direction of signal flow along a response pathway
The Nervous System: Coordination and Response
The nervous system is the body's rapid communication network, detecting stimuli from the environment and coordinating appropriate responses by transmitting electrical impulses along specialised cells called neurones.
Stimulus: Any change in the internal or external environment that is detected by a receptor — for example, a sharp increase in light intensity detected by the eye.
Receptor: Specialised cells or organs that detect a specific type of stimulus and convert it into an electrical nerve impulse. Different receptors are sensitive to different stimuli (e.g., photoreceptors for light, thermoreceptors for temperature).
Nerve Impulse: An electrical signal that travels along the membrane of a neurone. Impulses are transmitted at high speed, allowing near-instantaneous responses.
Effector: A muscle or gland that carries out the response. Muscles contract to produce movement; glands secrete hormones or other substances.
Reflex Arc: The pathway of a nerve impulse from receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector. Reflex arcs bypass the brain, producing rapid, involuntary responses.
Example
When you accidentally touch a hot surface, thermoreceptors and pain receptors in your skin detect the heat stimulus. An impulse travels along sensory neurones to the spinal cord, where relay neurones connect to motor neurones that signal the bicep muscle (effector) to contract, withdrawing your hand — all before conscious awareness in the brain.
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A schematic of the reflex arc showing the pathway from receptor through sensory neurone, relay neurone (spinal cord), and motor neurone to the effector, with a dashed line indicating the delayed signal to the brain.
Worked Example
Basic
A person touches a hot plate and quickly pulls their hand away. Identify the components of this reflex arc in the correct order.
Worked Example
Intermediate
Explain why a reflex action is faster than a conscious (voluntary) response, and state why this speed is biologically important.
Worked Example
Advanced
A nerve impulse travels at 100m/s. If a sensory pathway from the foot to the spinal cord is 1.2m long and the motor pathway back to the leg muscle is 1.0m, and each of the 2 synapses introduces a delay of 0.5ms, estimate the total time for a reflex withdrawal.
Why It Works
Understanding the reasoning behind the rule:
The speed of a reflex arises from both the physical structure of the pathway and the nature of impulse transmission. We can reason through why the arrangement minimises response time.
Your Turn
Try this one yourself — use the hint if you get stuck:
A reflex pathway has a total neurone length of 1.5m and impulses travel at 80m/s. There are 2 synapses, each with a delay of 0.6ms. Calculate the total reflex time in milliseconds.
Watch Out For...
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Confusing the nervous system and endocrine system as doing the same job in the same way.
Remember the nervous system uses fast electrical impulses along neurones for short-lived responses, while the endocrine system uses slower chemical hormones carried in the blood for longer-lasting effects.
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Thinking impulses travel across the synapse electrically.
At a synapse, the impulse triggers release of chemical neurotransmitters that diffuse across the gap and generate a new impulse in the next neurone — the transmission is chemical, not electrical.
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Describing a reflex arc as passing through the conscious brain.
A reflex arc goes sensory neurone → relay neurone (in the spinal cord) → motor neurone, bypassing conscious brain processing so the response is rapid and protective.
In the Real World
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Athletic sprint starts depend on minimising reaction time; training reduces the delay between the starting signal (stimulus) and muscle contraction (response).
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The pupil reflex, controlled automatically, protects the retina from bright light — a real example of a coordinated response involving receptor, coordinator and effector.
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Thermoregulation combines nervous and hormonal control to keep core body temperature near 37∘C, illustrating coordination between organ systems.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
Which sequence correctly describes a reflex arc?
MCQ
Q2.
Which statement best distinguishes hormonal from nervous control?
MCQ
Q3.
Define the term 'stimulus' and give one example.
A stimulus is a change in the internal or external environment that is detected by a receptor. Example: a bright light detected by the eye, or a loud sound detected by the ear.
Q4.
Explain why synaptic transmission makes nervous coordination one-directional.
Neurotransmitter is released only from the presynaptic neurone and receptors are only on the postsynaptic membrane, so impulses can only cross the synapse in one direction, ensuring signals travel the correct way.
Q5.
A reaction time experiment gives a mean of 0.22 s over 5 trials. If total time was summed, what was the total?
Mean =5total, so total =0.22×5=1.10 s.
Extension
Advanced
A harder challenge that connects to the next skill:
A ruler-drop experiment measures reaction time. A ruler falls a distance d=0.12 m before being caught. Using d=21gt2 with g=9.8m/s2, calculate the reaction time t.
Key Takeaways
1.Coordination links a stimulus to a response via receptor → coordinator → effector.
2.The nervous system gives fast, short-lived responses using electrical impulses; the endocrine system gives slower, longer-lasting responses using hormones.
3.Reflex arcs bypass conscious thought for rapid, protective responses.
4.Synaptic transmission is chemical and one-directional, ensuring impulses travel the correct way.
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Ready to Practice?
Now that you've learned Coordination & Response, put it to the test with a quick quiz!