Neurones (AQA A Level Biology)

• Information is sent through the nervous system as nerve impulses – electrical signals that pass along nerve cells known as neurones
• A bundle of neurones is known as a nerve
• Neurones coordinate the activities of sensory receptors (eg. those in the eye), decision-making centres in the central nervous system, and effectors such as muscles and glands
• Neurones have a long fibre known as an axon
• The axon of some neurones 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 made by specialised cells known as Schwann cells
  • Myelin is made when Schwann cells wrap themselves around the axon along its length

• The presence of Schwann cells means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next
  • The 'jumping' of the electrical impulse between nodes of Ranvier is called saltatory conduction
  • This speeds up the conduction of the impulse and its transfer from one cell to another

• Neurone 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

An example of a 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