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The central nervous system and nerve impulses

Structure and functions in living organisms · The nervous system and the eye · note 1 of 5

Spec 2.87, 2.88Download all The nervous system and the eye notes (PDF)Download all notes (PDF)7 pages

The central nervous system and nerve impulsesSpec 2.87, 2.88

In short

The central nervous system is made up of the brain and spinal cord. When a receptor in a sense organ is stimulated, it sends electrical impulses along neurones in nerves into the central nervous system, which coordinates a response. Impulses then travel out along nerves to effectors, which are muscles that contract or glands that secrete. The response is rapid.

The nervous system lets you react to your surroundings and coordinate your behaviour. The central nervous system (CNS) is made up of the brain and spinal cord. It is linked to the sense organs (such as the eye and the skin) by nerves.

When a receptor in a sense organ is stimulated, it sends electrical impulses along neurones in the nerves into the CNS. The CNS sends electrical impulses out along nerves to effectors. Effectors are muscles (which contract) or glands (which secrete). Impulses travel quickly, so the response is rapid.

  1. Stimulus (a change in the surroundings)
  2. Receptor in a sense organ is stimulated
  3. Electrical impulses travel along nerves into the CNS (brain and spinal cord)
  4. Electrical impulses travel out along nerves to an effector
  5. Effector (muscle or gland) produces the response
Parts of the nervous pathway
PartJob
ReceptorsDetect a stimulus and start electrical impulses
NervesBundles of neurones that carry impulses to and from the CNS
CNS (brain and spinal cord)Receives impulses and coordinates the response
EffectorsMuscles or glands that carry out the response
Exam tip:

The CNS is the brain and spinal cord only. Nerves are not part of the CNS; they link it to the sense organs and effectors.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway of a reflex, a fast, automatic response that does not involve conscious thought. A receptor detects the stimulus, a sensory neurone carries impulses to the spinal cord, a relay neurone passes them to a motor neurone, and the motor neurone carries them to an effector, such as a muscle that contracts.

Why are reflexes faster than normal responses?

Reflexes are faster because the impulses do not have to reach the conscious part of the brain before the response happens. The pathway is short: sensory neurone, relay neurone in the spinal cord, then motor neurone to the effector. Because no conscious thought is involved, the response is automatic, which helps protect the body from harm.

How does a nerve impulse cross the synapse?

A nerve impulse crosses a synapse using a chemical, because the electrical impulse cannot cross the gap. The first neurone releases a neurotransmitter into the synapse. The neurotransmitter diffuses across the gap and binds to receptors on the membrane of the next neurone, which triggers a new electrical impulse in that neurone.

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