The eye is a great example of a receptor and it also illustrates some reflex arcs too. But first we need to revise its structure.
Have a look at this cross-section of the human eye:
You will need to know about light receptor cells (photoreceptors). There are two groups: the rods and the cones.
The rods respond to shades of light - really they 'see' in black and white.
The cones respond to colours of light. There are 3 exciting varieties - blue, green and red. So the red cones respond to red light, and so on.
To see full colour, our brains invent all the other colours based on how many impulses it receives from each variety of cones. Clever stuff, eh?
Too much light would damage the receptor cells in the retina. Too little light wouldn't allow them to work properly.
The iris contains a pigment that gives our eyes their nice colour, this protects the retina from getting too much light. A darker iris colour gives more protection. But that isn't flexible enough to cope with constant changes in light intensity.
Instead, there is yet another reflex!
The iris contains two sets of muscles
One muscle is radial, arranged round the pupil like spokes in a wheel.
The other is circular, arranged concentric rings around the pupil.
When there is too much light, the pupil is closed down by contracting the circular muscle.
When there isn't enough light, the radial muscles in the iris contract, pulling the pupil wider. This lets more light through to the retina.
Each iris has both circular and radial muscles in it at the same time. Have a look in a mirror, or at your friend's eye - much more fun!
How is this movement of the iris controlled?
The rods and cones in the retina send impulses to the brain about the light they receive. The brain then sends back impulses by motor neurones to tell muscles in the iris to contract and change the shape of the pupil. This controls how much light gets in - a very cunning feedback system!
The eye also has to adapt to be able to focus a clear image of an object no matter how far away it is from the eye. Again, this is under the control of the nervous system.
When the eye looks at an object that is far away the ciliary muscle pulls on the suspensory ligaments. These pull on the lens and make it flatter (less convex). This brings the rays of light from the object into crisp focus on the retina.
But with an object closer to the eye, the lens needs to be more convex (fatter). To do this, the ciliary muscles relax to allow the rubbery lens to return to its naturally rounder shape.
The brain's careful control of the ciliary muscles allow it to adjust the convexity of the lens to give a perfectly focussed image on the retina.
This retinal image is not the same as the object that is being looked at. The image is inverted.
This is like looking through a magnifying glass or convex lens. Try it!
Sometimes our eyes need a bit of extra help to get the focussing right. This may be due to problems with the ciliary muscles, the lens, or the overall shape of the eye. Then we need to wear contact lenses or glasses for help.