The nervous system and the eye — Edexcel International GCSE Biology
The central nervous system, nerve impulses, synapses, the reflex arc, and the structure and function of the eye.
Revision notes
5 short notes, in the order of the specification. Each one 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.
A synapse is a gap between two neurones that an electrical impulse cannot cross. When an impulse arrives at the end of the first neurone, it releases a neurotransmitter into the synapse. The neurotransmitter diffuses across the gap and binds to receptors on the membrane of the next neurone, triggering a new electrical impulse there.
A reflex arc is the pathway of a reflex, a fast, automatic response that does not involve conscious thought. Impulses pass from a receptor along a sensory neurone to a relay neurone in the spinal cord, then along a motor neurone to an effector, such as a muscle. Reflexes are fast and protect the body from harm.
The eye is a sense organ that detects light and sends impulses to the brain along the optic nerve. The cornea and lens refract light to focus it on the retina, which contains rod cells, sensitive to dim light, and cone cells, which detect colour. The iris controls the size of the pupil and so how much light enters.
The eye focuses by changing the shape of the lens. For a near object the ciliary muscles contract, the suspensory ligaments loosen and the lens becomes thicker. For a distant object the reverse happens. In bright light the circular muscles of the iris contract and the pupil gets smaller, protecting the retina; in dim light the radial muscles contract.
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Quick check questions
What two organs make up the central nervous system?
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The brain and the spinal cord.
What carries the message across a synapse?
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A neurotransmitter, which diffuses across the gap.
Name the three types of neurone in a reflex arc, in order.
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Sensory neurone, relay neurone, motor neurone.
Which part of the eye contains the light receptors?
Show answer
The retina.
In bright light, do the circular muscles of the iris contract or relax?
Show answer
They contract, making the pupil smaller.
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.
How does the eye focus on near objects?
To focus on a near object, the ciliary muscles contract and the suspensory ligaments loosen, so the lens becomes thicker and more curved. The thicker lens refracts light more, focusing it on the retina. For a distant object the ciliary muscles relax, the ligaments are pulled tight and the lens is pulled thin.
What happens to the pupil in bright light?
In bright light the pupil gets smaller because the circular muscles of the iris contract and the radial muscles relax. Less light enters the eye, which protects the retina from damage. In dim light the radial muscles contract and the circular muscles relax, so the pupil gets larger and more light enters. This is a reflex.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026