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How does the nervous system detect stimuli and produce responses, including reflex actions?

The structure and function of the nervous system, the pathway from stimulus to response, the role of the synapse, the reflex arc and reflex actions, and the required practical on reaction time.

A focused answer to AQA GCSE Biology 4.5.2, covering the structure of the nervous system, the stimulus to response pathway, the synapse, reflex arcs and reflex actions, and the reaction time required practical.

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  1. What this dot point is asking
  2. Structure of the nervous system
  3. Synapses
  4. Reflex actions and the reflex arc
  5. Required practical: reaction time
  6. Try this

What this dot point is asking

AQA wants you to describe the structure of the nervous system, explain the pathway from stimulus to response through neurones and synapses, describe a reflex arc and why reflexes are fast and automatic, and understand the reaction-time required practical.

Structure of the nervous system

The basic pathway from a stimulus to a response is: stimulus, then receptor, then coordinator (CNS), then effector, then response. Receptors are cells that detect stimuli such as light, sound, touch, chemicals or temperature. Effectors are muscles, which contract, or glands, which secrete hormones. This organisation lets the body respond appropriately to a wide range of changes.

Synapses

The chemical transmission across a synapse is slightly slower than the electrical impulse along a neurone, which is one reason responses are not instant. AQA wants you to be clear that the signal is electrical along a neurone but chemical across a synapse.

Reflex actions and the reflex arc

The reflex arc pathway, in order:

  1. A receptor detects the stimulus.
  2. A sensory neurone carries the impulse to the spinal cord.
  3. A relay neurone in the spinal cord passes it on (across synapses).
  4. A motor neurone carries the impulse to the effector.
  5. The effector (a muscle) brings about the response.

Reflexes are fast because the impulse does not have to travel up to the conscious brain and back first; it passes through the short reflex arc in the spinal cord. This speed is what makes reflexes protective.

Required practical: reaction time

You can measure reaction time using the ruler drop test (catching a falling ruler and converting the catch distance to a time) and investigate how a factor such as caffeine or practice affects it. Control variables (the same hand, the same starting position) and repeat to get a reliable mean.

Try this

Q1. Name the two parts of the central nervous system. [1 mark]

  • Cue. The brain and the spinal cord.

Q2. Explain why a reflex action is faster than a voluntary action. [2 marks]

  • Cue. It does not involve the conscious brain; the impulse passes straight through the reflex arc.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksDescribe the pathway taken by a nervous impulse during a reflex action when a person touches a hot object and pulls their hand away. Name the structures involved in the correct order.
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A 4-mark describe question rewards the reflex arc in the correct order.

A receptor in the skin detects the stimulus (heat or pain). A sensory neurone carries the electrical impulse from the receptor to the spinal cord. In the spinal cord the impulse passes across a synapse to a relay neurone. The relay neurone passes the impulse, across another synapse, to a motor neurone, which carries it to the effector. The effector is a muscle in the arm, which contracts to pull the hand away.

Markers reward the correct order: receptor, sensory neurone, relay neurone, motor neurone, effector (muscle), and mentioning the synapse.

AQA 20214 marksA student investigated the effect of caffeine on reaction time using a ruler drop test. Describe how the student could carry out a valid investigation, and explain why repeating the test improves the results.
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A 4-mark required-practical question rewards a clear method, controls and the reason for repeats.

One person drops a ruler between the open finger and thumb of the test subject, who catches it as quickly as possible; the distance the ruler falls is measured and converted to a reaction time. The subject does the test with and without caffeine. Control variables such as the starting position of the ruler, the hand used and the person dropping it, so the test is fair. Drinking water (without caffeine) acts as a control.

Repeating the test several times and taking a mean reduces the effect of random errors and anomalies, making the results more reliable.

Markers reward a workable method, at least one controlled variable, a control with no caffeine, and repeats improving reliability.

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