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A single-celled organism such as an Amoeba looks after itself inside one cell, sensing its surroundings and reacting without any help. You are built from trillions of cells, and those cells cannot each go their own way. They have to be coordinated, so that the whole body reacts in a sensible, joined-up manner. This lesson opens Topic B3 (Organism-level systems) of your OCR Gateway Combined Science course by studying the body's fast coordinating system: the nervous system. You will trace the pathway that links a change in the surroundings to a response, meet the central and peripheral parts of the system and the three kinds of neurone, and then see how some of the most important responses of all — reflexes — happen automatically, and how one neurone hands its signal to the next across a synapse.
By the end of this lesson you should be able to describe the stimulus → receptor → coordinator → effector → response pathway, name and describe the three types of neurone, describe a reflex arc and explain why reflexes are so fast, and explain how an impulse crosses a synapse.
This lesson mainly develops AO1 (recalling the structure of neurones and the reflex arc), with AO2 when you apply the pathway to unfamiliar stimulus–response examples and AO3 when you interpret and compare results from the reaction-time investigation.
Living things have to detect and react to changes around them in order to stay alive. A change that an organism can detect and respond to is called a stimulus (plural stimuli) — a bright light, a loud sound, a rise in temperature or the smell of food are all examples.
In a large animal, the cell that detects a stimulus is often a long way from the cell that has to respond. Touch something hot and the receptor cells are in your fingertip, but the muscle that yanks your hand back is in your arm. Something must carry the information quickly from one to the other and make sure the right response happens. Detecting stimuli and producing a coordinated response is called coordination, and in animals it is shared between the nervous system and the endocrine (hormonal) system.
The nervous system lets the body:
Exam Tip: A stimulus is the change, not the object. "A hot pan" is not a stimulus; the rise in temperature your skin detects is. Read these questions carefully, because OCR often asks you to name the stimulus precisely.
Every nervous response, however simple or complicated, follows the same five stages. Learning this order is the most useful single thing in the lesson, because so many B3 questions are built on it.
flowchart LR
A["Stimulus<br/>(a change)"] --> B["Receptor<br/>(detects the change)"]
B --> C["Coordinator<br/>(CNS: brain / spinal cord)"]
C --> D["Effector<br/>(muscle or gland)"]
D --> E["Response<br/>(action taken)"]
Information travels along this pathway carried by neurones (nerve cells) as tiny electrical impulses. The receptor turns the energy of the stimulus into an electrical impulse, and effectors turn that impulse back into an action.
Exam Tip: An effector is "a muscle or a gland". Many students write only "a muscle". A question about sweating, releasing a hormone or producing saliva needs the answer gland.
The human nervous system has two parts.
The central nervous system (CNS) is the brain and the spinal cord. It is the body's processing centre: it takes in information from receptors all over the body, coordinates a response, and sends instructions out to effectors.
The peripheral nervous system (PNS) is all the nerves that carry information to and from the CNS. A nerve is a bundle of many neurones wrapped together. The PNS links the CNS to every receptor and every effector.
flowchart TD
A["Nervous system"] --> B["Central nervous system (CNS)<br/>brain + spinal cord"]
A --> C["Peripheral nervous system (PNS)<br/>nerves to and from the CNS"]
B --> D["Processes information<br/>and coordinates responses"]
C --> E["Carries impulses between<br/>receptors / effectors and the CNS"]
The flow of information runs: receptor → (neurone carrying the impulse in) → CNS → (neurone carrying the impulse out) → effector. Neurones that carry impulses towards the CNS are sensory neurones; those that carry impulses away from the CNS to effectors are motor neurones.
A neurone is a cell specialised to carry electrical impulses quickly over long distances. There are three types, each with a slightly different job and shape.
| Neurone | Carries impulses... | Key feature |
|---|---|---|
| Sensory neurone | From receptors to the CNS | Long fibre bringing the impulse in; cell body on a side-branch |
| Relay neurone | Within the CNS (links sensory to motor) | Short; many connections; found in the brain and spinal cord |
| Motor neurone | From the CNS to effectors (muscles/glands) | Long axon carrying the impulse out to the effector |
All neurones share the same basic parts:
The diagram below shows a motor neurone, the type you are asked to label most often.
Key:
| # | Part | Function |
|---|---|---|
| 1 | Dendrites | Receive impulses from other neurones |
| 2 | Cell body (with nucleus) | Controls the neurone |
| 3 | Axon | Carries the impulse away from the cell body |
| 4 | Myelin sheath | Insulates the axon and speeds up the impulse |
| 5 | Nerve endings | Pass the impulse on to the effector (here, a muscle) |
A sensory neurone has a similar set of parts, but its cell body lies on a small side-branch part-way along, and it carries the impulse the other way — from a receptor into the CNS. A relay neurone is short, lies entirely within the CNS, and has many connections so that it can link incoming sensory neurones to outgoing motor neurones.
Exam Tip: Neurones are adapted cells. Two adaptations score marks again and again: they are very long (carrying impulses over large distances) and they have an insulating myelin sheath (speeding the impulse up). Mention both in a "how is a neurone adapted?" question.
When a receptor detects a stimulus it sets off an electrical impulse that travels along the neurone. You do not need the detailed biophysics for combined science, but you should describe the journey clearly:
Electrical transmission along a neurone is extremely fast, which is exactly why the nervous system is the body's system for rapid responses. This speed is one of the key differences between nervous control and the slower, longer-lasting hormonal control you will meet later in B3.
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 weigh up the situation could mean a serious burn. The body's answer is the reflex action — a rapid, automatic response to a stimulus that does not involve conscious thought. Reflexes happen the same way every time, and often you become aware of the stimulus only after the response has already happened.
Most reflexes are protective. Familiar examples include:
Because these responses are automatic and do not wait for a conscious decision, they are extremely fast — and that speed is 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 a reflex impulse follows is called a reflex arc. It is a special version of the coordination pathway, 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 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 (in the spinal cord for many reflexes) and links the incoming sensory neurone straight to the outgoing motor neurone. Between each pair of neurones there is a synapse.
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 and goes straight back out to the effector, without waiting for the conscious brain.
This short-cut has one huge advantage — speed. By skipping the decision-making stage, the response happens in a fraction of a second, fast enough to protect you from a burn or a falling object. 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 common misconception is that a reflex "does not go to the brain at all". The safe phrasing is that it does not require the conscious decision-making part of the brain — the impulse passes through the spinal cord (or unconscious 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. 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 makes the impulse travel in one direction only, because the neurotransmitter is released on one side and the receptors are on the other. Second, it is a point where 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 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 essential verb is diffuses — make sure it appears.
It helps to compare an automatic reflex response with a conscious voluntary response.
| Feature | Reflex response | Voluntary response |
|---|---|---|
| Conscious thought? | No — automatic | Yes — you decide to do it |
| Speed | Very fast | Slower |
| Part of CNS involved | Spinal cord / unconscious brain | Conscious part of the brain |
| Main purpose | Often protective | Carrying out chosen actions |
| Example | Pulling away from a pin; blinking | Picking up a pen; writing |
A useful way to remember the difference: a voluntary action is one you choose to make and can stop or change part-way through; a reflex action happens whether you want it to or not and cannot be stopped once it has started.
Reaction time is the time taken to respond to a stimulus. It is usually very short — a fraction of a second — because the nervous pathway is so fast. Reaction time can be increased (made slower) by tiredness, age, distractions and the drug alcohol, and it can sometimes be shortened a little by practice or by caffeine.
Two students do a ruler-drop test, in which a partner drops a ruler without warning and they catch it as fast as they can. Student A catches the ruler after it falls a mean of 12 cm; student B after a mean of 18 cm. Which student has the faster reaction time?
Step 1 — interpret the distances. A shorter distance means the ruler was caught sooner, so the reaction time was shorter.
Step 2 — compare: student A's ruler fell 12 cm and student B's fell 18 cm, so student A has the faster (shorter) reaction time.
Step 3 — suggest a reason. Student B might be more tired, more distracted or less practised; any factor that increases reaction time would lengthen the distance fallen.
Answer: Student A has the faster reaction time, because their ruler fell the shorter distance.
Exam Tip: In the ruler-drop test the variable you actually measure is the distance fallen, which is then used to work out reaction time. Keep the conditions the same each trial, drop the ruler without warning, and take repeats and a mean, ignoring anomalies.
Question (6 marks): Explain how a reflex action allows a person to pull their hand away from a hot object very quickly, and describe how the impulse passes from one neurone to the next.
Mid-band response: "When you touch something hot, receptors in the skin send an impulse to the spinal cord and then to a muscle, which pulls your hand away. It is quick because it is a reflex. The impulse passes between neurones at a synapse using chemicals."
Examiner-style commentary: This identifies the receptor, spinal cord, muscle and synapse, and links the speed to it being a reflex. It is held back by missing the named neurones and by not describing the synapse steps. To improve, name the sensory, relay and motor neurones and describe the neurotransmitter diffusing and binding.
Stronger response: "Receptors in the skin detect the heat and send an electrical impulse along a sensory neurone to a relay neurone in the spinal cord, then along a motor neurone to a muscle, which contracts to pull the hand away. It is fast because it does not involve conscious thought. At a synapse, a chemical neurotransmitter is released and diffuses across the gap to the next neurone."
Examiner-style commentary: A strong answer with the correct neurone order and a sensible account of the synapse. To reach the top band, state that the receptor converts the stimulus into the impulse, that the reflex bypasses the conscious part of the brain, and complete the synapse description with the neurotransmitter binding to receptors and triggering a new impulse.
Top-band response: "Receptors in the skin detect the heat and convert it into an electrical impulse. The impulse travels along a sensory neurone to a relay neurone in the spinal cord, then out along a motor neurone to an effector — a muscle, which contracts to pull the hand away. The response is very rapid and protective because it follows a reflex arc through the spinal cord and does not involve the conscious decision-making part of the brain. Where one neurone meets the next there is a synapse: the impulse causes a neurotransmitter to be released, which diffuses across the gap, binds to receptors on the next neurone and triggers a new electrical impulse that carries the signal on."
Examiner-style commentary: Full marks. The answer gives the full reflex arc with correctly named neurones, states that the receptor converts the stimulus, explains why bypassing the conscious brain makes the reflex fast and protective, and gives all four synapse steps including diffusion, binding and the new impulse. A complete, precise response.
| Misconception | The correct idea |
|---|---|
| "The stimulus is the object (e.g. the hot pan)" | The stimulus is the change detected (e.g. the rise in temperature) |
| "An effector is always a muscle" | An effector is a muscle or a gland |
| "Nerves and neurones are the same thing" | A neurone is a single nerve cell; a nerve is a bundle of many neurones |
| "Impulses travel along neurones as chemicals" | Impulses travel along a neurone as electrical signals; chemicals are used only at the synapse |
| "The order is sensory → motor → relay" | The order is sensory → relay → motor |
| "The electrical impulse jumps across the synapse" | The impulse is carried across by a neurotransmitter that diffuses across the gap |
This content is aligned with OCR Gateway Combined Science A (J250), Topic B3 Organism-level systems. Refer to the official OCR specification for exact wording.