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In the last lesson you followed the full nervous pathway from stimulus to response, passing through the central nervous system where a decision is made. But some responses are so important for survival that there is no time to think about them. If a hot object touches your hand, waiting for your brain to consider the situation could mean a serious burn. The body's answer is the reflex action — a rapid, automatic response that does not involve conscious thought. This lesson explains how reflex actions work through a reflex arc, and looks closely at the synapse, the tiny gap where one neurone passes its signal to the next. It is part of Topic B3 of OCR Gateway Science A and builds directly on the neurones you met in the previous lesson.
By the end of this lesson you should be able to describe what a reflex action is and why it is useful, describe the reflex arc in the correct order, explain how a synapse transmits an impulse from one neurone to the next, and compare reflex responses with voluntary (conscious) responses.
A reflex action is a rapid, automatic response to a stimulus that does not involve conscious thought. Reflexes happen the same way every time, without you having to decide to do them — and often you become aware of the stimulus only after the response has already happened.
Most reflexes are protective: they help keep the body safe from harm. Familiar examples include:
Because these responses are automatic and do not wait for the brain to make a decision, they are extremely fast — and that speed is exactly what protects the body from damage.
Exam Tip: Two words earn marks when describing a reflex: rapid (or automatic) and protective. If a question asks "why are reflex actions important?", answer that they are fast and protect the body from harm because they do not involve conscious thought.
The pathway that a reflex impulse follows is called a reflex arc. It is just a special version of the coordination pathway from the last lesson, but with one crucial feature: the impulse usually passes through the spinal cord rather than the conscious part of the brain, which is why the response is so quick.
The reflex arc runs:
stimulus → receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector → response
flowchart LR
A["Stimulus<br/>(e.g. hot object)"] --> B["Receptor<br/>(in the skin)"]
B --> C["Sensory neurone"]
C --> D["Relay neurone<br/>(in the spinal cord / CNS)"]
D --> E["Motor neurone"]
E --> F["Effector<br/>(muscle)"]
F --> G["Response<br/>(hand pulled away)"]
Notice the order of the three neurones — sensory, then relay, then motor. The relay neurone sits inside the CNS (the spinal cord, for many reflexes) and connects the incoming sensory neurone straight to the outgoing motor neurone. Between each pair of neurones there is a synapse (covered in detail below).
The diagram below shows a reflex arc for the withdrawal reflex, with the impulse passing through the spinal cord.
Key: 1 receptor (in the skin) · 2 sensory neurone (impulse in, blue) · 3 relay neurone (in the spinal cord) · 4 motor neurone (impulse out, green) · 5 effector (muscle). The small yellow circles mark the synapses between neurones.
In an ordinary voluntary action, the impulse travels up to the conscious parts of the brain, which weigh up the situation and decide what to do. That decision-making takes time. In a reflex action, the impulse takes a short-cut: it passes through a relay neurone in the spinal cord (or in the unconscious parts of the brain for some reflexes) and goes straight back out to the effector, without waiting for the conscious brain to get involved.
This short-cut has one enormous advantage — speed. By skipping the decision-making stage, the response happens in a fraction of a second, fast enough to protect the body from a burn, a falling object or a foreign body in the eye.
It is worth being clear that the impulse does still often reach the brain — which is why you feel the pain shortly afterwards — but the protective movement has already happened by then. The reflex acts first; conscious awareness follows.
Exam Tip: A reflex arc is described as passing through the spinal cord (or unconscious brain) and not involving the conscious part of the brain. Saying the impulse "does not go to the brain at all" is risky — the safe phrasing is that it does not require the conscious decision-making part of the brain, which is what makes it fast.
Neurones do not actually touch one another. Where one neurone meets the next there is a tiny gap called a synapse. The electrical impulse cannot jump across this gap on its own, so the signal is carried across by a chemical. Understanding this hand-over is a favourite exam topic.
Here is what happens at a synapse, in order:
The synapse explains two important features of the nervous system. First, it ensures the impulse travels in one direction only, because the neurotransmitter is released on one side and the receptors are on the other. Second, it is the point at which many drugs and medicines act — they can boost or block the neurotransmitter, which is how some painkillers and other medicines affect the nervous system.
Exam Tip: The classic synapse question asks you to describe how the impulse crosses the gap. The four mark-earning steps are: neurotransmitter is released → it diffuses across the gap → it binds to receptors on the next neurone → a new impulse is triggered. The key verb is diffuses — make sure it appears.
It is important to be able to compare an automatic reflex response with a conscious voluntary response. The table below sets them side by side.
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