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Neurones (CIE A Level Biology)

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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

A Myelinated Neurone Diagram

The structure of a myelinated neuroneAn 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)
    • 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)

The Three Types of Neurone Diagram

three-types-of-neurone-intermediate

The three types of neurone – the red line shows the direction of impulses. Note that axons always carry nerve impulses away from the cell body

  • 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

Reflex arc

  • Sensory neurones, intermediate (relay) 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 the eye on an object
    • Controlling how much light enters the eye

The Reflex Action Diagram

reflex action neurones

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 an intermediate neurone in the spinal cord
  • The intermediate 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 (the quadriceps in this case) to contract and pull the leg up and away from the sharp object (the response)

The Pathway of a Reflex Arc Diagram

reflex arc flowchart

The pathway of a reflex arc

Examiner Tip

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

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Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.