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The conditions inside our body must be carefully controlled if the body is to function effectively. The conditions are controlled in two ways with chemical and nervous responses.

All control systems include:

• cells called receptors, which detect stimuli
• the coordination centre, such as the brain, spinal cord or pancreas, which receives and processes information from receptors around the body
• effectors bring about responses, which restore optimum levels, such as core body temperature and blood glucose levels
• effectors include muscles and glands, and so responses can include muscle contractions or hormone release

Nerve cells

Nerve cells are called neurones. They are adapted to carry electrical impulses from one place to another. A bundle of neurones is called a nerve.

There are three main types of neurone:

• sensory
• motor
• relay

They have some features in common:

• A long fibre (axon) which is insulated by a fatty (myelin) sheath. They are long so they can carry messages up and down the body.
• Tiny branches (dendrons) which branch further as dendrites at each end. These receive incoming impulses from other neurones.

Where two neurones meet there is a small gap called a synapse. Here the electrical signal must be converted into a chemical one, which is converted back to an electrical one on the other side of the synapse where the next neurone starts.

1. an electrical impulse travels along the first axon
2. this triggers the nerve-ending of a neurone to release chemical messengers called neurotransmitters
3. these chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the second neurone
4. the receptor molecules on the second neurone bind only to the specific neurotransmitters released from the first neurone
5. this stimulates the second neurone to transmit the electrical impulse.

Receptors to effectors

Receptor cells detect a change in the environment (a stimulus) and start electrical signals along neurons. These move towards the central nervous system (CNS). The CNS is the brain and spinal cord. It coordinates the responses. Messages are then sent back along different neurones to muscles which contract or relax, and glands which secrete hormones. Muscles and glands are called effectors.

The diagram summarises how information flows from receptors to effectors in the nervous system.

Receptors are groups of specialised cells. They detect a change in the environment and stimulate electrical impulses in response. Sense organs contain groups of receptors that respond to specific stimuli.

Effectors include muscles and glands. They produce a specific response to a detected stimulus. For example:

• a muscle contracting to move an arm
• muscle squeezing saliva from the salivary gland
• a gland releasing a hormone into the blood.