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The evolution of hormones and the nervous system

The nervous system

Multicellular organisms need a way for cells [cell: Basic unit of life. Unicellular organisms only have one cell. Multicellular organisms have many cells. ] to communicate with one another. This has led to the development of nervous communication systems, as well as hormonalhormones: Chemical messengers produced in glands and carried by the blood to specific organs in the body systems.

The nervous system uses electrical impulses to bring about fast, but short-lived, responses. It consists of the:

  • brain and spinal cord, which make up the central nervous system (CNS)
  • neurones

Nerve cells are also called neurones. The diagram below shows a typical neurone. It has tiny branches at each end, and a long fibre to carry electrical signals.

a neurone has a 'head' at one end where the nucleus, cytoplasm, cell membrane and dendrite are. The axon is tail-like, with nerve endings at the end which look like branches.

A motor neurone

Receptors are groups of specialised cells. They can detect changes in the environment, called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs such as the ear, eye and skin.

Different receptors

Sense organsReceptors sensitive to
A baby holding its mother's finger


touch, pressure, pain and temperature
A womens tongue


chemicals in food


chemicals in the air




sound and position of the head

Effectors are parts of the body that produce a response. Here are three examples:

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

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