The nervous system detects changes and sends electrical impulses to bring about a response. Its control centre is the central nervous system (CNS), which is made up of the brain and the spinal cord. Everything else is nerves that carry impulses to and from the CNS.
Sensory receptors are cells that detect a stimulus, such as light, sound, heat or pressure. A sensory neurone carries impulses from a receptor to the CNS. Inside the CNS, a relay neurone passes the impulse on to a motor neurone. The motor neurone carries impulses out to an effector. An effector is a muscle or a gland: muscles respond by contracting and glands respond by releasing a substance.
Neurones do not touch each other. Where two neurones meet there is a tiny gap called a synapse. The impulse cannot cross the gap as electricity, so the first neurone releases a chemical. This chemical diffuses across the gap and starts a new impulse in the next neurone.
Sensory and motor neurones are both long, so one cell can carry an impulse a long way. Each has a fatty myelin sheath around the axon, which insulates it and speeds up the impulse. A motor neurone has its cell body at one end, with short branches called dendrites, and its axon ends at the effector. A sensory neurone has dendrites at the receptor end and its cell body sits off to one side of the fibre.
