Key Stage 4 science · Biology – Coordination and control

GCSE nervous system and reflex arc: a guide for parents

The nervous system lets the body detect changes and respond to them within a fraction of a second. At GCSE, students learn how information passes from receptors through the central nervous system to effectors, how the structure of a neurone suits its job, and how a reflex arc produces a rapid, automatic response without conscious thought. It is the first of two coordination systems; the second, hormones, is covered separately.

Nervous system and reflexes, explained

Coordination means making sure the parts of the body work together and respond to changes. The nervous system does this with electrical impulses. A change in the surroundings is a stimulus. Receptors, such as the cells in the skin, eye or ear, detect it. The information goes to the central nervous system (CNS), which is the brain and spinal cord. The CNS coordinates a response, and effectors, which are muscles or glands, carry it out. For example, a muscle contracts or a gland releases a hormone.

Information travels along neurones. A sensory neurone carries impulses from a receptor towards the CNS, a relay neurone passes them on within the CNS, and a motor neurone carries them from the CNS to an effector. Neurones are long, thin cells, which suits them to carrying signals over distance. Many are wrapped in a fatty myelin sheath that insulates them and makes impulses travel faster, and they have branched endings that connect to other cells. Bundles of neurones form nerves.

Neurones do not touch. Between two neurones is a tiny gap called a synapse. When an electrical impulse reaches the end of one neurone, it releases chemicals called neurotransmitters, which diffuse across the gap and bind to receptors on the next neurone. This sets off a new impulse. Because the chemicals are released on one side only, impulses can pass across a synapse in one direction, from receptor towards effector. Some courses, often the separate biology GCSE, also cover the roles of regions of the brain, for example that the cerebral cortex deals with conscious thought and memory, the cerebellum with balance and coordination and the medulla with automatic functions such as heart rate and breathing.

A reflex is an automatic, rapid response to a stimulus that does not need conscious thought, which makes it protective. Pulling a hand away from something hot, blinking when something approaches the eye and the pupil narrowing in bright light are reflexes. The route is the reflex arc: stimulus, receptor, sensory neurone, relay neurone in the spinal cord (or in some cases the brain), motor neurone, effector, response. Because the relay neurone links straight to a motor neurone, the signal does not have to be processed by the conscious part of the brain, which saves vital time. Impulses also travel up the spinal cord to the brain, but by the time you feel the pain the hand is already moving.

Words your child will hear

Stimulus: A change in the surroundings that is detected, such as heat, light or pressure.
Receptor: A cell or organ that detects a stimulus and starts an impulse.
Effector: A muscle or gland that carries out a response.
Neurone: A nerve cell. Sensory, relay and motor neurones carry impulses between receptors, the CNS and effectors.
Synapse: The tiny gap between two neurones, crossed by chemicals called neurotransmitters.
Central nervous system: The brain and spinal cord, which coordinate responses.
Reflex arc: The pathway of neurones that produces a fast automatic response.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about principles of nervous coordination and control in humans
Students should be taught about the relationship between the structure and function of the human nervous system
Students should be taught about the relationship between structure and function in a reflex arc

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model exam answer

A student touches a hot pan and pulls their hand away before they feel any pain. Describe the pathway that produces this reflex and explain why it is useful.

The stimulus is the heat. Temperature receptors in the skin of the fingertip detect it and start an impulse.
The impulse travels along a sensory neurone towards the spinal cord, which is part of the central nervous system.
In the spinal cord the sensory neurone passes the impulse across a synapse to a relay neurone. Neurotransmitter chemicals diffuse across the gap and trigger a new impulse.
The relay neurone passes the impulse across another synapse to a motor neurone, which carries it out of the spinal cord to the muscle in the arm.
The muscle is the effector. It contracts and pulls the hand away. Because the impulse did not need to be processed by the conscious part of the brain, the response is very fast and protects the skin from damage.

Receptor, sensory neurone, relay neurone, motor neurone, effector (the muscle). The reflex is fast and automatic, so it reduces injury.

Common mistakes and how to help

The message goes up to the brain first, and the brain decides to move the hand

It feels as if we decide to pull away, and most responses we think about do go through the brain.

What helps: In this reflex arc a relay neurone in the spinal cord links the sensory neurone straight to the motor neurone, so the conscious brain is not needed to start the movement. Impulses do reach the brain too, which is why the pain is felt a split second after the hand has moved.

Neurones are joined together like wires

Diagrams often show neurones end to end, and the electrical part of the story is easy to remember.

What helps: There is a small gap, the synapse. The signal crosses it chemically, with neurotransmitters, and that is the reason impulses can only pass in one direction.

Catching a falling ruler is a reflex

It is fast and feels automatic, and it is commonly used in reaction-time investigations.

What helps: It involves the brain and a conscious decision, so it is a voluntary response. A reflex is involuntary and uses a shorter pathway, so it is faster still.

Mixing up sensory and motor neurones

Both carry impulses and look similar on diagrams, and textbooks lay the arc out in different ways.

What helps: Describe direction from the CNS. Sensory neurones bring information in from the senses, and motor neurones take instructions out to muscles that move.

How to help at home

Try a safe reflex. With your teenager sitting on a chair with their legs hanging loose, tap lightly just below the kneecap with the side of your hand. The leg kicks without a decision. Ask them to trace the arc: receptor, sensory neurone, spinal cord, motor neurone, muscle. This reflex is unusual because the sensory neurone connects directly to the motor neurone, without the relay neurone found in most reflex arcs.
Compare a reflex and a reaction. Drop a ruler for someone to catch between finger and thumb, and see how far it falls. This is a voluntary reaction. Ask why it is slower than a reflex and which extra parts of the nervous system are involved.
Watch the pupils. In a dim room, watch a family member’s pupils as the room light is switched on. They narrow in a moment. Never shine a torch into the eyes. Ask what the stimulus, receptor and effector are.
Draw it from memory. Ask for a labelled reflex arc with arrows showing the direction of the impulse, including both synapses. Check that the sensory neurone goes into the spinal cord and the motor neurone goes out.