Neurones (OCR A Level Biology)

• Neurones are specialised cells of the nervous system which carry electrical impulses around the body
• A bundle of neurones is known as a nerve
• There are different types of neurones
• The following features are found in all types of neurone
  • Neurones have a long fibre known as an axon
  • They have a cell body that contains the nucleus and other cellular structures
  • The end of the axon, known as the axon terminal, contains many nerve endings
  • The nerve endings at the axon terminal allow neurones to connect to many other neurones which receive impulses from the axon terminals; this forms a network for easy communication

• Some neurones are myelinated, their axon is insulated by a myelin sheath with small uninsulated sections along its length (called nodes of Ranvier)
  • The myelin sheath is formed by specialised cells known as Schwann cells which wrap themselves around the axon.
  • This means that electrical impulses do not travel down the whole axon, but jump from one node to the next so that less time is wasted transferring the impulse from one cell to another

• In non-myelinated neurones the axon is uninsulated
  • The impulse travels more slowly as it moves through the entire length of the axon

The diagram shows the structure of a myelinated neurone

• There are three main types of neurone: sensory, relay and motor
  • Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)
  • Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)
• Each type of neurone has a slightly different structure
• Motor neurones have:
  • A large cell body at one end that lies within the spinal cord or brain
  • A nucleus that is always in its cell body
  • Many highly-branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones
  Relay neurones have:
  • Short, but highly branched, axons and dendrites
• Sensory neurones have:
  • A cell body that branches off in the middle of the cell
  • A single long dendron that carries impulses to the cell body and a single long axon that carries impulses away from the cell body

The three types of neurone – the red line shows the direction of impulses. Note that the axon always carried impulses away from the cell body.