Neurons

• Structure and Function of Neurons:

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

  • Sensory.
  • Relay.
  • Motor.

  
• Sensory Neurons:

  • Carry messages from the PNS to the CNS.
  • They have long dendrites and short axons.
• Relay Neurons:

  • Connect the sensory neurons to the motor neurons.
  • They have short dendrites and short axons.
• Motor Neurons:

  • Connect the CNS to the effectors (muscles and glands).
  • They have short dendrites and long axons.
• Neurons vary in size.

  Less than a millimetre - a meter long.
• Cell Body (Soma):

  • Includes a nucleus which contains genetic material of the cell - chromosomal DNA.
  • Control centre.
• Dendrites:

  • Protrude from the cell body.
  • Carry nerve impulses from neighbouring neurons towards the cell body.
  • Receives the impulse.
• Axon:

  • Carries impulse away from the cell body down the length of the neuron.
  • Where electrical signals pass along.
• Myelin Sheath:

  • Protects/insulates the axon.
  • Keeps the impulse going down the axon.
• Nodes of Ranvier:

  • Speed up the transmission of the impulse by forcing it to ‘jump’ across the gaps along the axon.
• Terminal Buttons:

  • Communicate with the next neurons in the chain across a gap known as the synapse.
• Synapse:

  • Between two neurons.
• Location of Neurons:

  • The cell bodies of motor neurons may be in the CNS, but they have long axons which form part of the PNS.
  • Sensory neurons are located outside of the CNS, in the PNS in clusters known as ganglia.
  • Relay neurons make up 97% of all neurons and most are found within the brain and the visual system.
• Electrical Transmission: Firing of a Neuron

  • When a neuron is in a resting state inside of the cell is negatively charged compared to the outside.
  • When a neuron is activated by a stimulus, 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.