Neurones (Cambridge (CIE) A Level Biology): Revision Note

Neurones

• A neurones has a long fibre known as an axon

• The axon is insulated by a fatty sheath with small, uninsulated sections along its length (called nodes of Ranvier)

  • The sheath is made of myelin, a substance that is made by specialised cells known as Schwann cells

  • Myelin is made when Schwann cells wrap themselves around the axon along its length

• This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next

• This means that less time is wasted transferring the impulse from one cell to another

• Their cell bodies contain many extensions called dendrites

  • This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication

• There are three main types of neurone: sensory, relay and motor

  • Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)

  • Intermediate (aka relay) 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 extend from the cell body, providing a large surface area for the axon terminals of other neurones

• Sensory neurones have the same basic structure as motor neurones, but have:

  • A cell body that branches off in the middle of the cell - it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion