Sense Organs and Homeostatic Mechanisms
Senior 3 · Biology
Sense Organs and Homeostatic Mechanisms
Lesson 1 of 5
25 min
Walk from the bright sunshine of a school yard into a dim storeroom and, for a few seconds, you can hardly see the shelves. Then, slowly, the room appears. Walk out again and the yard is dazzling until you have blinked a few times. Two things are happening: your pupils are changing size, and the light-sensitive cells of your retina are adjusting. Your eye is a sense organ, a group of receptors with tissues around them that help them work, and it is always adjusting itself.
Definition
Sense organ: an organ containing receptors that detect one kind of stimulus. The eye detects light; the ear, sound and movement of the head; the nose and tongue, chemicals; the skin, touch, pressure, pain and temperature.
Retina: the light-sensitive layer at the back of the eye, containing two kinds of receptor cell: rods, which work in dim light and do not detect colour, and cones, which work in bright light and detect colour.
Accommodation: the change in the shape of the lens that keeps the image focused on the retina when you look at objects at different distances.
Pupil reflex (iris reflex): the automatic change in the size of the pupil in response to the brightness of light.
Figure 1: simplified; not to scale. The iris, ciliary muscle and suspensory ligaments are rings, so a section cuts each of them twice.
| Part | Function |
|---|---|
| Conjunctiva | Thin, transparent layer over the front of the eye; protects the cornea and is kept moist by tears |
| Cornea | Transparent front of the sclera; refracts (bends) light, doing most of the focusing |
| Aqueous humour | Watery fluid that keeps the front of the eye in shape |
| Iris | Coloured ring of muscle; controls the size of the pupil and so the amount of light entering |
| Pupil | The hole in the centre of the iris through which light enters |
| Lens | Transparent and elastic; changes shape to fine-tune the focus |
| Ciliary muscle | Ring of muscle that changes the shape of the lens |
| Suspensory ligaments | Fibres that hold the lens and pass the pull of the ciliary muscle to it |
| Vitreous humour | Transparent jelly that keeps the eyeball in shape |
| Retina | Contains the rods and cones that turn light into nerve impulses |
| Fovea | Part of the retina packed with cones; gives the sharpest, colour vision |
| Blind spot | Where the optic nerve leaves; it has no rods or cones, so nothing is seen there |
| Optic nerve | Carries impulses from the retina to the brain |
| Choroid | Dark layer with blood vessels; nourishes the retina and absorbs stray light |
| Sclera | Tough white outer coat that protects the eye |
Light from an object is refracted by the cornea and then by the lens, and it comes to a focus on the retina. The image is real, smaller than the object, and inverted (upside down). Rods and cones turn the light into nerve impulses, which travel along the optic nerve to the brain, and the brain interprets them so that you see the object the right way up.
| Looking at | Ciliary muscle | Suspensory ligaments | Lens | Refraction |
|---|---|---|---|---|
| A near object (a book) | Contracts | Slacken | Becomes fatter (more convex) | More: diverging light from a near object is bent strongly |
| A distant object (the board) | Relaxes | Pulled tight | Pulled thinner | Less: almost parallel light needs less bending |
It may seem odd that the ligaments slacken when the muscle contracts. The ciliary muscle is a ring: when it contracts, the ring becomes smaller, so it moves closer to the lens and the ligaments are no longer pulled. The elastic lens then springs into a fatter shape.
The iris has two sets of muscle: circular muscles, in rings round the pupil, and radial muscles, like the spokes of a wheel.
| Light | Circular muscles | Radial muscles | Pupil | Why |
|---|---|---|---|---|
| Bright | Contract | Relax | Constricts (smaller) | Less light enters, protecting the retina from damage |
| Dim | Relax | Contract | Dilates (wider) | More light enters, so you can see |
This is a reflex: the receptors are the rods and cones, and the coordinator is the brain, so it is a cranial reflex (Unit 9).
Worked example 1: how much more light?
In bright light a pupil is typically about 2 mm across; in dim light about 8 mm (typical values). How many times more light can enter the dilated pupil?
Worked example 2: from your notebook to the board
You are reading your notebook, 25 cm away, and then look up at the board, 6 m away. Describe the changes in your eye.
Warning
Common mistakes. (1) "The ciliary muscle contracts to pull the lens thin": it is the other way round; contraction makes the ligaments slack and the lens fat. (2) Saying the lens does all the focusing: the cornea does most of it; the lens fine-tunes. (3) Mixing up rods and cones: cones for colour. (4) Safety: never look straight at the Sun, even briefly or through dark glass; it can permanently damage the retina.
How the Eye Works (Part 1): Structure of the Eye and Iris Reflex, Cognito. Watch for the parts of the eye and the two sets of iris muscles; compare them with Figure 1 and the table.
Tip
Memory hook: "near is fat and slack; far is thin and tight". For a near object the lens is fat and the ligaments slack; for a far object the lens is thin and the ligaments tight.
Practice: the eye
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the tables, the worked examples (typical values) and the practice questions. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
