The nervous system and neurones

Coordination and response · Nervous control · note 1 of 3

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The nervous system and neuronesSpec 14.1.1, 14.1.2, 14.1.3, 14.1.4

In short

The nervous system coordinates and regulates body functions using electrical impulses that travel along neurones. The central nervous system is the brain and spinal cord, and the peripheral nervous system is the nerves outside them. Sensory neurones carry impulses from receptors to the CNS, relay neurones work within the CNS, and motor neurones carry impulses to effectors.

Electrical impulses travel along cells called neurones. Neurones carry information quickly between the parts of the body.

The mammalian nervous system has two parts.

The central nervous system (CNS) consists of the brain and the spinal cord.
The peripheral nervous system (PNS) consists of the nerves outside of the brain and spinal cord.

The role of the nervous system is the coordination and regulation of body functions. Receptors detect a change (a stimulus), impulses pass along neurones to the CNS, and the CNS sends impulses to effectors (muscles and glands), which produce a response.

Stimulus (a change in the surroundings)
Receptor detects the stimulus
Impulses travel along a sensory neurone to the CNS
The CNS coordinates the response
Impulses travel along a motor neurone to an effector
Effector (muscle or gland) produces the response

Flow chart of the nervous pathway: stimulus, receptor, sensory neurone, CNS (relay neurone), motor neurone, effector, response. — The pathway from **stimulus** to **response**.

Three types of neurone

Sensory, relay and motor neurones
Neurone	Where it carries impulses	How to identify it in a diagram
Sensory neurone	From a receptor to the CNS	Receptor at one end; the cell body is off to the side of the long fibre; the fibre enters the CNS
Relay neurone	Within the CNS, from the sensory neurone to the motor neurone	Short; found entirely inside the CNS; connects the other two types
Motor neurone	From the CNS to an effector (a muscle or a gland)	Cell body inside the CNS at one end; long fibre leaving the CNS to the effector

Exam tip:

To identify a neurone, ask where it is and which way the impulse goes. Only the relay neurone sits wholly inside the CNS. Remember the PNS is the nerves, not the brain or spinal cord.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway that impulses follow in a reflex action. It has five parts in order: a receptor detects the stimulus, a sensory neurone carries impulses to the CNS, a relay neurone passes them on within the CNS, and a motor neurone carries them to an effector, a muscle or gland, which responds.

Why are reflexes faster than normal responses?

Reflexes are fast because they are automatic, so you do not have to think about the response first. Impulses pass straight from the receptor through the sensory, relay and motor neurones to the effector. This rapid, automatic response protects the body from harm, such as pulling your hand away from something hot.

How does a nerve impulse cross the synapse?

Supplement A nerve impulse crosses a synapse by chemical means, because the electrical impulse cannot cross the gap. The impulse makes vesicles release neurotransmitter molecules into the synaptic gap. These diffuse across the gap and bind with receptor proteins on the next neurone, which stimulates a new impulse in that neurone.

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