CIE A Level Biology (9700) 2019-2021

Revision Notes

15.1.2 Neurones


  • Neurones have 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 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

A neurone, IGCSE & GCSE Biology revision notes

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)

The three types of neurone, IGCSE & GCSE Biology revision notes

The three types of neurone – the red line shows the direction of impulses

  • 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 that 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:
    • One long axon with a cell body that branches off in the middle of the axon – it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion

Reflex arc

  • Sensory neurones, relay (intermediate) neurones and motor neurones work together to bring about a response to a stimulus
  • A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
  • As it does not involve the brain, a reflex response is quicker than any other type of nervous response
  • Examples of simple reflex actions that are coordinated by these pathways are:
    • Removing the hand rapidly from a sharp or hot object
    • Blinking
    • Focusing of the eye on an object
    • Controlling how much light enters the eye

A reflex, IGCSE & GCSE Biology revision notes

How sensory neurones, intermediate (relay) neurones and motor neurones work together to carry out a reflex action

  • In the example above:
    • A pin (the stimulus) is detected by a pain receptor in the skin
    • The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
    • Electrical impulses are passed on to relay neurone in the spinal cord
    • The relay neurone connects to the motor neurone and passes the impulses on
    • The motor neurone carries the impulses to the muscle in the leg (the effector)
    • The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)

The reflex pathway, IGCSE & GCSE Biology revision notes

The pathway of a reflex arc

Exam Tip

You may be asked to identify the different types of neurones in a diagram. It can be helpful to memorise the key differences between them – such as the location and size of the cell body.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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