Neurones (AQA A Level Biology)

Myelinated Motor Neurones

• 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