The nervous system

Remember the last time you touched a hot object or stepped on something sharp? Your response would have seemed to be almost instantaneous. Why? How do we interpret pain and the other senses? What keeps us breathing? What actually is involved in thinking and memory? How does a large multicellular organism like a human being manage to act as a single controlled unit? All of these functions involve the body’s nervous system—the most complex communication system on this planet.

The nervous system is made up of highly specialised cells called nerves or neurones. Neurones are able to make electrical signals and then transmit them, like electricity moves along a wire. A nerve message is called an impulse, and can travel in only one direction along a neurone. Impulses can travel at speeds from 1 to 100 metres per second, and so information is carried very quickly.

The nervous system can be divided up into these parts:

• central nervous system (CNS)—this is the brain and spinal cord.
• receptors—specialised cells which detect a stimulus—these make up our senses.
• sensory nerves—these carry messages from the receptors to the CNS.
• motor nerves—these carry messages from the CNS to effectors, which are glands or muscles that carry out the response to the stimulus.

A reflex is a rapid action that helps to protect a part of the body. Consider what happens if you touch a hot object. The messages are carried along nerves to form a reflex arc. This can be represented by a stimulus–response model, with the heat being the stimulus, and the pulling of the hand away being the response. The brain only becomes aware of the pain after you have pulled your hand away.

Nerves are also involved in homeostasis, the maintaining of stable conditions within the body.