Receptors and the Eye

In the last lesson you followed the nervous pathway from stimulus to response. That pathway always begins with a receptor — a cell, or group of cells, that detects a change and turns it into an electrical impulse. This lesson looks more closely at receptors, and then at the body's most important sense organ, the eye. You will learn how receptors are grouped into sense organs, label the main parts of the eye and give their jobs, and study the pupil reflex, an automatic response that protects the light receptors of the eye from bright light. This is part of Topic B3 of OCR Gateway Combined Science A.

By the end of this lesson you should be able to explain what a receptor does and where the main sense organs are, label the parts of the eye and state their functions, describe how light is focused onto the retina, and explain how the pupil reflex controls the amount of light entering the eye.

This lesson mainly develops AO1 (recalling the parts of the eye and their functions), with AO2 when you apply the reflex-arc model to explain how the pupil reflex adjusts to bright and dim light.

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Receptors and Sense Organs

A receptor detects a stimulus and converts its energy into an electrical impulse that travels along a neurone to the CNS. Receptors are specific: each kind responds to only one kind of stimulus. Light receptors respond to light, sound receptors to sound, and so on.

Receptors are often grouped together in sense organs — structures whose job is to detect a particular kind of stimulus.

Sense organ	Stimulus it detects	Receptors
Eye	Light	Light receptors in the retina
Ear	Sound (and balance)	Sound receptors
Skin	Touch, pressure, temperature, pain	Touch and temperature receptors
Tongue	Chemicals in food (taste)	Taste receptors
Nose	Chemicals in the air (smell)	Smell receptors

Whichever sense organ is involved, the same rule holds: the receptor detects the stimulus and converts it into an electrical impulse. The impulse then travels along a sensory neurone to the CNS, exactly as you saw in the coordination pathway.

Exam Tip: The verb examiners want for receptors is convert (or "transduce"): a receptor converts the energy of the stimulus into an electrical impulse. Saying it "sends a message" is too vague to score the top mark.

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The Eye as a Sense Organ

The eye is the sense organ that detects light. It contains light receptors, focuses light to form an image, and converts that image into electrical impulses that travel to the brain along the optic nerve. Everything the eye is built from serves one of these jobs: letting light in, focusing it, or detecting it.

Structure of the eye

#	Part	Function
1	Cornea	Transparent front of the eye; refracts (bends) light, doing most of the focusing
2	Iris	Coloured ring of muscle; controls the size of the pupil
3	Pupil	The hole in the middle of the iris that lets light in
4	Lens	Focuses light onto the retina
5	Ciliary muscles	Ring of muscle that changes the shape of the lens
6	Suspensory ligaments	Connect the ciliary muscles to the lens
7	Retina	Light-sensitive layer at the back containing light receptors
8	Optic nerve	Carries electrical impulses from the retina to the brain

When light enters the eye it is refracted first by the cornea and then by the lens, which together focus it to form a sharp image on the retina. The light receptors in the retina convert the image into electrical impulses, which travel along the optic nerve to the brain, where the image is interpreted.

Exam Tip: Be precise about which structures do what. The cornea does most of the focusing (it refracts the light most strongly); the lens sits behind the pupil and focuses light onto the retina. The retina is the layer that contains the light receptors — it is the actual detector.

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How the Eye Detects Light

It is worth being clear about the order of events when you look at something, because it links straight back to the coordination pathway from the last lesson.

1. Light (the stimulus) enters the eye through the transparent cornea, which refracts it.
2. Light passes through the pupil, the hole in the middle of the iris.
3. The lens focuses the light so that a sharp image forms on the retina.
4. Light receptors in the retina detect the light and convert it into electrical impulses.
5. The impulses travel along the optic nerve to the brain, which interprets them as an image.

So the eye is a receptor system: the stimulus is light, the receptors are in the retina, and the coordinator that makes sense of the impulses is the brain.

Two features of the eye deserve a second look, because they show how well the structure is suited to the job. First, the cornea and lens are both transparent, so light can pass through them, and both refract (bend) light so that it is brought to a sharp focus rather than being spread out. If the light were not focused, the image on the retina would be blurred. Second, the retina is packed with light receptors across the whole back of the eye, so light arriving from any part of your field of view can be detected. Together these features let the eye do all three of a receptor's jobs — let light in, focus it, and detect it — in one compact organ. This is a useful thing to remember, because "explain how the eye is adapted to detect light" is a question that rewards linking each structure to its job.

Exam Tip: A frequent misconception is that "the eye sees" or "the retina makes the picture". The retina only detects light and converts it into impulses; it is the brain that interprets those impulses as an image. Keep the two jobs separate.

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Focusing on Near and Distant Objects

The eye does not only control how much light gets in — it also has to bring light from objects at different distances to a sharp focus on the retina. Light from a distant object arrives at the eye in nearly parallel rays and needs only a little bending; light from a near object spreads out more and needs bending more strongly. The eye adjusts the amount of bending by changing the shape of the lens, and this adjustment is called accommodation.

The shape of the lens is controlled by the ciliary muscles and the suspensory ligaments. Because the two work against each other, it is easy to muddle them, so it is worth setting the two cases out side by side.

Looking at a distant object

• The ciliary muscles relax.
• This lets the suspensory ligaments pull tight.
• The taut ligaments pull the lens thin (less curved).
• A thinner lens bends the light less, which is all that is needed for the nearly parallel rays from a distant object, so the image is sharp on the retina.

Looking at a near object

• The ciliary muscles contract, forming a smaller ring.
• This slackens the suspensory ligaments, so they no longer pull on the lens.
• Freed from that pull, the lens becomes thicker (more curved) because of its own elasticity.
• A thicker lens bends the light more, which is what the diverging rays from a near object need, so the image is again sharp on the retina.

Object viewed	Ciliary muscles	Suspensory ligaments	Lens shape	Light bent
Distant	Relaxed	Tight (pull hard)	Thin, less curved	Less
Near	Contracted	Slack (loose)	Thick, more curved	More

The confusing part is that when the ciliary muscles contract (do work), the suspensory ligaments go loose, and the lens gets fatter. Contracting the muscle does not stretch the lens — it releases the ligaments so the lens can spring into its rounder shape. Learning the table in full avoids the classic mix-up.