Transmission within neutrons

• TRANSMISSION

  • ability to relay signals within or between neurons
  • electrical --> within neurons
  • chemical --> between neurons
  • dendrites --> comes from the greek word for tree

  1. each 'branch' can receive input signals from other neurons and its the sum of these inputs that determines if the signal will be passed on via the neurons axon
  2. if a current is added to the dendrite --> possible to generate an AP
  3. current further in the cell body becomes attenuated --> smaller current

  *current leaks out the cell membrane in dendrites

  *not a big issue -> dendrites are short + many inputs
• SALTATORY CONDUCTION

  • axon is capable of 1m passive transmission --> needs to have a diameter of 1cm
  • axon transmit without attenuation
  • properties of axon

  1. has a higher density of sodium channels compared to dendrites
  2. key to non-attenuating transmission
  3. sent as an AP wave
  4. axon hillock and a bit of the axon have a high density of sodium channels --> allows a generation of action potential
  5. high density down the axon transmits AP as a wave and allows it to be transmitted down
• INCREASING SALTATORY CONDUCTION- myelination

  • faster and more efficiency
  • nodes of ranvier --> high density of sodium channels (decrease rise time of AP)
  • internodes --> spaced at regular internodes (channel density is low)
  • AP is generated in the first node --> current is spread passively through the internode really quickly --> next node triggers the next AP
  • a wave of action potential interspersed by rapid cable conduction
• MYELINATION SPEED GAIN DUE TO:

  • density of sodium channels (node 10x higher than unmyelinated neuron)
  • decreased capacitance of membranes so cable transmission through internodes is much faster than an action potential wave
• MYELIN FORMATION

  • formed by oligodendrocyte or Schwann cells
  • schwann cells wrapped around the axon
  • myelin --> fatty like substance --> good insulator
  • myelination is specific to axons

  
• INCRREASING CONDUCTION- cable diameter

  • increase cable diameter

  1. used in their escape reflex
  2. 1/2 mm - 1mm in diameter --> been used in experiments
  3. relatively primitive nervous system
• MULTIPLE SCLEROSIS

  • auto-immune disease in Northern Europe
  • common in women under 50
  • demyelination of CNS neurons
  • caused by an immune attack on the oligodendrocytes
  • genetic susceptibility, infection with viruses and vitamin d --> suggested contributing factors
• GUILLIAN-BARRE SYNDROME

  • autoimmune disease
  • Schwann cells are attacked and peripheral nervous system neurons become demyelinated
  • leads to progressive weakness, numbness and pain --> sensory + motor neurons lose their ability to transmit signals efficiently