neurons

• Features/functions of neurons

  • 100 billion nerve cells (neurons) in the human nervous system (80% of which are located in the brain)
  • By transmitting signals electrically and chemically, these provide the nervous system with its primary means of communication
  • They are how information is transmitted from one part of the nervous system to another
• types of neuron:

  • Sensory neurons:
  • carry messages from the PNS to the CNS
  • Long dendrites and short axons
  • Located in the PNS in clusters called ganglias
  • Relay neurons:
  • Connect sensory neurons to motor or other relay neurons
  • Short dendrites and short axons
  • 97% or all neurons are relay neurons and most are in the brain and visual system
  • motor neurons:
  • Connect the CNS to effectors (such as muscles and glands)
  • Short dendrites and long axons
  • Cell bodies may be in the CNS but long axons form part of the PNS
• structure of a neuron:

  • Cell body (or soma) = includes a nucleus which contains the genetic material of the cell
  • Dendrites = branchlike structures that protrude from the cell body. They carry nerve impulses from neighbouring neurons towards the cell body
  • Axon = carries the electrical impulse away from the cell body and down the length of the neuron
  • Myelin sheaf = a fatty layer that protects the axon
  • Nodes of Ranvier = gaps in the axon that speed up the transmission of the impulse
  • Terminal buttons = at the end of the axon and they communicate with the next neuron in the chain across the synapse
• Direction of impulse:

  • The dendrite (receptor) receives the signal
  • The signal is carried towards a cell body (which contains the nucleus)
  • The signal travels along an axon (which is protected by myelin sheafs) towards the axon terminal
  • Terminal buttons at the end of the axon pass the electrical signal to the next neuron in the chain
• Electrical transmission (the firing of a neuron):

  • When a neuron is in a resting state the inside of the cell 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
• Structure of neurons