neurons

• Structure and function

  There are 100 billion nerve cells in the human nervous system , 80% located in the brain. By transmitting signals electrically and chemically, these provide the nervous system with its primary means of communication
• Three types of neurons

  1. sensory
  2. relay
  3. motor
• Sensory neurons

  Sensory neurons carry messages from the peripheral nervous system to the central nervous system. They have long dendrites and short axons. They are located in the peripheral nervous system in clusters called ganglias
• Relay neurons

  Relay neurons connect sensory neurons to motor or other relay neurons. They have short dendrites and short axons. Of all neurons, 97% are relay neurons and most are in the brain and visual system.
• Motor neurons

  Motor neurons connect the central nervous system to effectors such as muscles and glands. They have short dendrites and long axons. Cell bodies may be in the central nervous system but long axons form part of the nervous peripheral nervous system.
• Structure of a neuron

  . Cell body (soma) - includes a nucleus which contains the genetic material of the cell.

  . Dendrites - branchlike structures that protude from the cell body. These carry nerve impulses from neighbouring neurons towards the cell body.

  . Axon - carries the electrical impulse away from the cell body down the length of the neuron. It is covered in a fatty layer of myelin sheath that protects the axon. Gaps in the axon called nodes of ranvier speed up the transmission of the impulse.
• Electrical transmission - the firing of a neuron

  When a neuron is in a resting state the inside of the call is negatively charged compared to the outside. When a neuron is activated, the inside of the cell becomes positively charged for a split second causing an action potential to occur. This creates an electrical impulse that travels down the axon towards the end of the neuron.