0610 · 14.2Sense organs

Sense organs and receptors

0:00Walk out of bright sunshine into a dark cinema, and for a moment you can hardly see a thing. Then, slowly, the seats and the people start to appear. Look at a book in your hand, then at a sign across the street, and both are sharp, even though they are completely different distances away. And at night, outside, the colours around you seem to fade to grey. All of that is your eyes adjusting, without you ever telling them to.

0:23By the end of this video you will be able to describe what a sense organ is, name the parts of the eye and what each one does, explain the pupil reflex using the muscles of the iris, explain how the eye focuses on near and distant objects, and describe how rods, cones and the fovea let you see in dim light, in colour and in fine detail. In the last video, we saw that a reflex starts at a receptor, a cell that detects a stimulus.

0:47Receptor cells are often grouped together in sense organs. The syllabus describes sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals. Each sense organ responds to its own kind of stimulus. Your eyes respond to light. Your ears respond to sound. Your skin has receptors for touch and for temperature. And your tongue and your nose have receptors for chemicals, which is how you taste and smell.

1:13The receptors turn the stimulus into electrical impulses, which travel along sensory neurones to the brain. In this video, we are going to look at the most studied sense organ of all: the eye.

Eye structures and light pathway

1:23Here is the eye, cut in half from the side, with light coming in from the left. You need to be able to identify seven structures: the cornea, the iris, the pupil, the lens, the retina, the optic nerve and the blind spot. Let's follow the light in, and take them in order. At the very front is the cornea. It is the clear, curved window at the front of the eye.

1:44The cornea refracts light. Refraction means bending light as it passes from one material into another. Because the cornea is curved, it bends the light rays towards each other as they enter the eye. In fact, the cornea does most of the bending. Behind the cornea is the iris. The iris is the coloured part of the eye, the part that makes eyes brown, blue or green. It is a ring of muscle, and in the middle of it is a hole called the pupil.

2:09The pupil looks black because it is a hole leading into the dark inside of the eye. The iris controls how much light enters the pupil, by making the pupil bigger or smaller. Behind the pupil is the lens. The lens is clear and flexible, and it focuses light on to the retina. The cornea does most of the bending, but the lens makes the fine adjustments, so that the image lands exactly on the retina and is sharp.

2:31The retina is the layer lining the back of the eye. It contains the light receptors, and some of them are sensitive to light of different colours. When light hits the receptors, they send impulses along the optic nerve. The optic nerve carries impulses to the brain, and the brain turns them into the picture you see. And the seventh structure is the blind spot. It is the place on the retina where the optic nerve leaves the eye.

2:54All the nerve fibres from the retina gather together there, so there is no room for any light receptors. Any light that falls on the blind spot cannot be detected. You do not normally notice it, because your brain fills in the gap.

Pupil reflex and iris muscles

3:07Now, the iris and the pupil. Go back to the cinema. In bright light, your pupils become small. In dim light, your pupils become large. This is the pupil reflex, and it is a reflex action, just like pulling your hand away from a hot pan. It happens automatically, and you cannot stop it. The syllabus asks you to explain it in terms of light intensity and pupil diameter. Light intensity means how bright the light is.

3:31Pupil diameter means how wide the pupil is. When the light intensity increases, the pupil diameter decreases. That lets less light into the eye, which protects the retina from being damaged by too much light. When the light intensity decreases, the pupil diameter increases. That lets more light into the eye, so that enough light reaches the receptors for you to see. But how does the iris actually change the size of the pupil?

3:55It has two sets of muscles. The circular muscles are arranged in rings around the pupil. The radial muscles run outwards from the pupil, like the spokes of a bicycle wheel. These two sets of muscles are antagonistic. That means they work against each other. When one contracts, the other relaxes, and they pull the pupil in opposite directions. A muscle can only pull when it contracts. It cannot push.

4:18So to move something both ways, you need two muscles pulling in opposite directions. In bright light, the circular muscles contract and the radial muscles relax. When a ring of muscle contracts, the ring gets smaller, like pulling the string of a drawstring bag. So the pupil gets narrower. In dim light, it is the other way round. The radial muscles contract and the circular muscles relax. When the spokes contract, they get shorter, and they pull the edge of the pupil outwards.

4:45So the pupil gets wider. Here is the reflex arc for the pupil reflex, using what we learned in the last video. The stimulus is the change in light intensity. The receptors are the light receptors in the retina. Impulses travel along the optic nerve to the brain, and then back along motor neurones to the effectors, which are the muscles of the iris.

Accommodation for near and distant objects

5:05Next, focusing. Looking at something close, and then at something far away, is called accommodation. The syllabus asks you to explain it in terms of four things: the ciliary muscles, the suspensory ligaments, the shape of the lens and the refraction of light. So first we need two new structures. The lens is held in place by a ring of muscle around it, called the ciliary muscle. The ciliary muscle is joined to the edge of the lens by lots of thin threads called suspensory ligaments.

5:32The lens is naturally elastic. Left to itself, it would spring into a fat, rounded shape. But when the suspensory ligaments are pulled tight, they stretch the lens and pull it thin. Here is the key idea. A fat lens refracts light more, so it bends the rays more strongly. A thin lens refracts light less. Light from a distant object arrives almost parallel, so it only needs a little bending.

5:55Light from a near object is spreading out as it arrives, so it needs to be bent more to meet on the retina. Start with a distant object. The ciliary muscles relax. When the ciliary muscle relaxes, its ring becomes wider. That pulls the suspensory ligaments tight, so there is high tension in the ligaments. The tight ligaments pull the lens thin. A thin lens refracts the light less, which is exactly right for light from a distant object, and it is focused on the retina.

6:20Now a near object. The ciliary muscles contract. When the ciliary muscle contracts, its ring becomes smaller, so it moves closer to the lens. That lets the suspensory ligaments go slack, so there is low tension in the ligaments. Nothing is pulling the lens thin any more, so the elastic lens springs back into a fatter, more rounded shape. A fat lens refracts the light more, which is what light from a near object needs, and it is focused on the retina.

6:44This is the part that trips most students up, because it feels backwards. When the ciliary muscles contract, the ligaments go slack, not tight. Think of it this way: contracting makes the ring smaller, so it closes in on the lens, and the threads have nothing to pull against. So: near object, muscles contract, ligaments slack, lens fat, more refraction. Distant object, muscles relax, ligaments tight, lens thin, less refraction.

Rods, cones and the fovea

7:10Now let's zoom in on the retina itself. The light receptors in the retina are of two kinds, named after their shapes: rods and cones. Rods are much more sensitive to light than cones. They can respond even to very dim light. So rods give you night vision. But rods cannot tell colours apart. That is why, when it is dark outside, you can still see shapes, but the world looks grey.

7:33Cones give you colour vision. There are three different kinds of cones, and each kind absorbs light of a different colour. Roughly, one kind absorbs red light best, one green and one blue. Your brain compares how strongly each kind of cone responds, and that is how you see every colour, not just those three. But cones need bright light to work. In dim light, the cones stop responding, and only the rods are left.

7:56Where are they on the retina? This is the distribution of rods and cones. In the very centre of the retina, directly opposite the lens and in line with the pupil, is a small area called the fovea. The fovea contains only cones, packed very tightly together, and no rods at all. Moving out from the fovea, towards the edges of the retina, there are fewer and fewer cones, and more and more rods.

8:17So cones are concentrated at the fovea, and rods are spread over the rest of the retina, mostly towards the edges. And at the blind spot, there are neither. The function of the fovea is to give the sharpest, most detailed vision, and it sees in colour. When you look straight at something, like the words on this screen, the eye turns so that the image is focused on the fovea.

8:37Because the cones there are packed so tightly, it can pick out fine detail. This also explains a strange trick. At night, a faint star can be easier to see if you look slightly to one side of it. That moves the image off the fovea, which has only cones, and on to the edge of the retina, where the sensitive rods are.

Exam answers and recap

8:54Let's see how this can be tested. A common question says: a student walks from a dark room into bright sunlight. Explain what happens to their pupils. Build the answer in steps. First, the stimulus: the light intensity increases. Receptors in the retina detect it, and impulses go to the brain and back to the iris. Then the muscles: the circular muscles of the iris contract, and the radial muscles relax.

9:19Then the result: the pupil diameter decreases, so less light enters the eye, and the retina is protected from damage. Always name both sets of muscles, and say what each one does. Another question might ask how the eye changes when you look up from your phone to a clock on the far wall. The object is now distant. So the ciliary muscles relax, the suspensory ligaments are pulled tight, the lens becomes thinner, and it refracts the light less, so that the image is focused on the retina.

9:46Now some common mistakes. First, saying that the pupil contracts. The pupil is a hole. It cannot contract. It is the muscles of the iris that contract, and the pupil gets narrower or wider as a result. Second, getting accommodation backwards. When the ciliary muscles contract, the suspensory ligaments go slack, and the lens gets fatter. That is for near objects. Third, mixing up rods and cones. Rods are for dim light and night vision, and they do not see colour.

10:15Cones are for colour vision in bright light. Fourth, confusing the fovea and the blind spot. The fovea is packed with cones and gives the sharpest vision. The blind spot has no receptors at all, because it is where the optic nerve leaves. And fifth, saying the lens does all the focusing. The cornea does most of the refraction. The lens adjusts the focus. Let's recap. Sense organs are groups of receptor cells that respond to specific stimuli: light, sound, touch, temperature and chemicals.

10:46In the eye, the cornea refracts light, the iris controls how much light enters the pupil, the lens focuses light on to the retina, the retina contains the light receptors, and the optic nerve carries impulses to the brain. The blind spot is where the optic nerve leaves, and it has no receptors. In the pupil reflex, bright light makes the circular muscles contract and the radial muscles relax, so the pupil narrows.

11:09Dim light makes the radial muscles contract and the circular muscles relax, so the pupil widens. For a near object, the ciliary muscles contract, the suspensory ligaments slacken, the lens becomes fatter and refracts light more. For a distant object, the ciliary muscles relax, the ligaments are pulled tight, the lens becomes thinner and refracts light less. Rods are very sensitive, so they give night vision. Three kinds of cones absorb different colours, so they give colour vision.

11:37Cones are packed into the fovea, in the centre of the retina, which gives the sharpest vision, and rods are spread towards the edges. So the next time you walk into a dark room and the shapes slowly appear, you will know exactly which muscles, which ligaments and which cells are doing the work.