The neurone
A neurone (nerve cell) is specialised to carry an electrical impulse. It has a cell body containing the nucleus, many short dendrites that receive impulses, and one long axon that carries the impulse away. Many axons are wrapped in a fatty myelin sheath that speeds up the impulse and insulates it. This shape — one long fibre with many receiving branches — lets a neurone carry signals over long distances within the body.
Three types of neurone
- Sensory neurone — carries impulses from a receptor to the CNS.
- Relay neurone (intermediate/interneurone) — found within the CNS; passes the impulse from a sensory neurone to a motor neurone.
- Motor neurone — carries impulses from the CNS to an effector (muscle or gland).
Key idea
Receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector.
The synapse
Neurones do not touch. Where one ends and the next begins there is a tiny gap called a synapse. An impulse cannot jump the gap as electricity, so it is carried across by a chemical. When the impulse reaches the end of the first neurone, it triggers the release of a neurotransmitter. This chemical diffuses across the gap and binds to the next neurone, starting a new impulse there.
Because the neurotransmitter is stored on only one side, the synapse acts like a one-way valve: impulses travel in a single direction only. Synapses also let one neurone communicate with many others, giving the nervous system its huge range of connections.
Example
In the spinal cord, a sensory neurone from your fingertip releases neurotransmitter onto a relay neurone. The relay neurone then signals a motor neurone, which drives the arm muscle — all through synapses passing the message chemically from cell to cell.
Remember
Along a neurone the signal is electrical; across a synapse it is chemical (a neurotransmitter). Synapses make transmission one-way.