Focusing and the pupil reflex

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

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Focusing and the pupil reflexSpec 2.92

The eye can focus on both near and distant objects. The lens changes shape to do this. The ciliary muscles (a ring of muscle around the lens) and the suspensory ligaments (which hold the lens in place) control the lens shape.

Focusing on near and distant objects
	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Pulled tight
Lens shape	Thicker (more curved)	Pulled thin (flatter)
Refraction of light	Light rays refracted more	Light rays refracted less

Two lens diagrams: for a near object the ciliary muscles contract, the suspensory ligaments loosen and the lens is thicker; for a distant object the ciliary muscles relax, the suspensory ligaments are pulled tight and the lens is pulled thin. In both, light rays focus on the retina. — Near object: **muscles contract, ligaments loosen, lens thicker**. Distant object: muscles relax, ligaments tight, lens thin.

Common mistake:

For a near object the ciliary muscles contract and the ligaments go slack. Many students reverse this. Remember: contracted muscle, loose ligaments, fat lens.

Changes in light intensity: the pupil reflex

The size of the pupil changes automatically in response to light intensity. This is a reflex, so it is fast and protects the retina. The iris has two sets of muscle: circular muscles (in rings around the pupil) and radial muscles (running out from the pupil like spokes).

The pupil reflex
	Bright light	Dim light
Circular muscles	Contract	Relax
Radial muscles	Relax	Contract
Pupil	Gets smaller (constricts)	Gets larger (dilates)
Effect	Less light enters, so the retina is protected from damage	More light enters, so you can see better

Front view of the iris twice: in bright light the circular muscles are contracted and pull inwards, the radial muscles are relaxed and the pupil is small; in dim light the radial muscles are contracted and pull outwards, the circular muscles are relaxed and the pupil is large. — The pupil reflex. Bright light: **circular muscles contract**, pupil smaller. Dim light: **radial muscles contract**, pupil larger.

Exam tip:

Name the muscles and say what each does. ‘Circular muscles contract’ makes the pupil smaller; ‘radial muscles contract’ makes it larger.

Quick check

What two organs make up the central nervous system?
Show answer
The brain and the spinal cord.
What carries the message across a synapse?
Show answer
A neurotransmitter, which diffuses across the gap.
Name the three types of neurone in a reflex arc, in order.
Show answer
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.

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