Your cells only work well when the conditions around them stay within narrow limits: body temperature, blood glucose concentration and water level all have to be held steady. Keeping internal conditions within set limits is homeostasis, and it happens automatically.

Every control system has three parts:

• Receptors detect a stimulus (a change inside or outside the body).
• A coordination centre (the brain, the spinal cord or an organ such as the pancreas) processes the information and decides on a response.
• Effectors (muscles or glands) carry out the response.

The body sends the messages in two ways: as electrical impulses along neurones (the nervous system) or as hormones in the blood (the endocrine system).

Receptors detect and effectors respond. A muscle or a gland is always an effector, never a receptor.

The central nervous system (CNS) is the brain and the spinal cord. Neurones carry electrical impulses to and from it:

• a sensory neurone carries impulses from a receptor to the CNS
• a relay neurone links neurones inside the CNS
• a motor neurone carries impulses from the CNS to an effector

Impulses are fast. To find how long one takes, use time = distance ÷ speed with the distance in metres:

1. A motor neurone is 0.9 m long; impulses travel at 75 m/s
2. time = 0.9 ÷ 75 = 0.012 s
3. 0.012 s × 1000 = 12 ms

Where two neurones meet there is a tiny gap, a synapse. The impulse arriving at the end of the first neurone causes a chemical to be released. The chemical diffuses across the gap and binds to receptors on the next neurone, which starts a new impulse there.

The electrical impulse does not jump the gap: a chemical carries the signal across, and only in one direction.

A reflex is an automatic, rapid response that does not involve the conscious part of the brain. It protects you, for example by pulling your hand off something hot before you have even noticed the pain.

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

There are synapses between the sensory and relay neurones and between the relay and motor neurones. Because the route goes through the spinal cord and skips conscious thought, the response is very fast.

The relay neurone sits in the spinal cord. The brain may learn about the stimulus a moment later, but it does not make the decision.