• 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