15.1.3 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

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 – 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 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:
  • 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, 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

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 pathway of a reflex arc