Senses and responses

1. Detecting changes and responding

Crossing the road can be dangerous. In order to cross safely, we rely on various senses:

• sight to see the traffic,

• hearing to hear the traffic, and

• touch to move off the kerb.

Without our senses we would not survive for very long. Our senses detect changes in the environment to which we respond. They warn us about danger and help us to find food.

If someone tapped you on the shoulder, you would probably look round. The tap on your shoulder causes a change in your sense of touch. Changes like this which can be detected by our senses are called stimuli (singular: stimulus). Our reactions to stimuli are called responses.

When one of our sense organs detects a stimulus, impulses (messages) pass along nerves to the central nervous system (CNS). The central nervous system consists of the brain and the spinal cord. When the central nervous system receives a message, it must decide whether to respond. If it responds, then impulses pass along nerves and cause our muscles to move. The flow diagram below shows what happens when you touch a hot plate.

Stimulus 	Detection 	Transmission 	Transmission 	Response
Heat from hot plate	by skin (sense organ)	of messages to CNS via nerves	of messages to muscles from CNS via nerves	from muscles to move hand away from heat

Our bodies have another system for responding to stimuli as well as nerves. This second system involves the action of chemicals called hormones. Most animals have these two methods of responding to stimuli. Plants, however, do not have a nervous system. They rely on hormones alone.

2. Detecting light – the eye

Our eyes enable us to detect light. They are very sensitive to changes in the intensity of light (light and dark). Although our eyes are very complex organs, they have three essential parts:

1) the iris, which controls the amount of light entering the eye. Your iris might be blue, brown or hazel. In bright light, the hole in the centre of your iris (the pupil) becomes smaller. In a dark room, the pupil becomes larger to let in more light.

2) the lens and cornea, which help us to form an image of the outside world on the retina. This is called focusing.

3) the retina at the back of the eye, which is sensitive to light. The retina contains more than 100 million light-sensitive cells. These cells send messages (nerve impulses) to the brain along the optic nerve. Our brains can use these messages to form a picture of the outside world.

How do our eyes focus?

The eye focuses on objects at different distances by changing the shape of its lens. It does this using the ciliary muscle which forms a ring around the lens.

Suppose you look at something close up, e.g. a book. The ciliary muscle tightens and pushes on the lens. This makes the lens fatter and more powerful.

Now suppose you look at something far away. In this case, the ciliary muscle relaxes and the lens becomes thinner and less powerful. This change in shape of the lens to focus on near or distant objects is called accommodation.

If you have good eyesight, you will be able to focus on objects as near as 25 cm (the near point) or as far away as the stars (the far point).