Neurones: Structure & Function (Edexcel A (SNAB) 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, but the following features are found in all types
  • A long fibre known as an axon
  • A cell body that contains the nucleus and other cellular structures
  • The end of the axon, known as the axon terminal, has many nerve endings
    • The nerve endings at the axon terminals allow neurones to connect to and receive impulses from other neurones, forming a network for easy communication
• Some neurones are myelinated, meaning that their axon is insulated by a fatty layer known as the myelin sheath 
  • The myelin sheath is made up of specialised cells known as Schwann cells which wrap themselves around the axon
  • There are uninsulated gaps between the Schwann cells known as the nodes of Ranvier
  • Electrical impulses in myelinated cells do not travel down the whole axon, but jump from one node to the next, speeding up impulse transmission
• In non-myelinated neurones the axon is not insulated by Schwann cells
  • The impulse travels more slowly as it moves through the entire length of the axon

Neurones have a long axon, a cell body, and an axon terminal, and some neurones are myelinated

• There are three main types of neurones
  • Sensory neurones carry impulses from receptors to the brain and spinal cord in the CNS
  • Relay neurones are found entirely within the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effector muscles or glands
• Each type of neurone has a slightly different structure
  • Motor neurones 
    • A large cell body at one end that lies within the spinal cord or brain
    • Many highly-branched dendrites extending from the cell body, providing many connections with the axon terminals of other neurones
  • Relay neurones 
    • Short neurones with axons and highly branched dendrites
  • Sensory neurones
    • A cell body that branches off in the middle of the axon
    • The dendrites are attached to a receptor cell 
    • The section of neurone that links the axon terminal (at the receptors) with the cell body is known as a dendron; it delivers the electrical impulse to the cell body
    • The section of neurone that connects the cell body with the CNS is the axon

Different types of neurone differ in both structure and function