Sight The structure of the eye Hearing Balance

Our senses are our windows to the outside world through which we receive information about the surrounding environment so that we can respond to ensure our survival.

Sensory receptors contain nerve cells which, when activated, convert stimuli into nerve impulses which are transmitted to the brain. Here they become the experience of sight, sound, taste, touch or smell.

Sight
Animals which live in very dark habitats (e.g. caves or at extreme depths under the sea) have very simple eyes or none at all.

For humans, however, the organs of sight, our eyes, provide us with continuous information about our changing world.

The positioning of our eyes gives us a wide angle of view, and the two eyes working simultaneously (stereoscopic vision) help us determine distance and provides us with 3D images of the world around us.

The structure of the eye
The eye is made up of a lens through which rays of light from a viewed object are directed and focussed, so that an image of the object falls on the retina which is like a screen at the back of the eye.

The light-sensitive rods and cone cells in the retina detect the image, which is converted to nerve impulses and sent immediately to the brain along the optic nerve. In the brain the image is re-formed and we "see" the object. Damage to the optic nerve or the sight centre of the brain result in blindness.

Light in the eye

By the action of small muscles, the lens can change shape to focus on distant or near objects. This is called accommodation.

In people who are long-sighted or short-sighted, the lens does not adjust sufficiently so that the image does not focus on the retina. It therefore appears blurred. Adding other lenses, such as spectacles, overcomes these problems.

To compensate in the dark or bright sunlight, the iris can change the size of the pupil to allow more or less light to enter.

Pupil dilation

Hearing
Sounds are transmitted to our ears by air that has been caused to vibrate. The sound waves are collected and funnelled by the outer ear (the pinna) through the ear canal. This causes the taut eardrum to vibrate. This vibration is conveyed through the middle ear by three tiny bones (hammer, anvil and stirrup) to the oval window. Vibrations here produce pressure changes in the fluid of the cochlea (inner ear) which affect tiny sensitive hairs linking up with nerve fibres through to the auditory nerve and hence to the brain for translation as "sound".

Differences in volume, pitch and rhythm are all conveyed as nerve impulses.

Balance
The inner ear also has fluid-filled semi-circular canals with sensitive hairs which respond to gravity and movement. When "off balance", different cells are stimulated notifying the brain of our disorientation which results in responses being sent to the leg muscles to restore balance. The balance mechanism in our ears works together with special nerve endings in the skeletal muscles (proprioceptors) which also detect minor changes of balance. Both systems together allow immediate, continuous and unconscious readjustment by the body.

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