Lecture 3

• Impulse Conduction


  The process by which an action potential propagates along an excitable cell membrane
• Topics covered


  • Excitable Cells
  • Cable Properties
  • Myelination
  • Conduction Velocity
  • Axon Terminal
  • Vesicle Release
  • Synapses

  
• Impulse Conduction


  1. Excitable membrane generates an action potential

  2. Influx of Na+ reverses potential difference across membrane

  3. Local reversal in potential serves as source of depolarizing current for adjacent membrane

  4. Na+ channels opened in adjacent membrane

  5. AP propagates from origin across rest of cell
• Excitable Cells


  • Most cells lack voltage-gated Na+ channels and do not generate action potentials
  • Only neurons with long axons and muscle cells generate propagating action potentials
• In biological tissue, a step change in voltage does not propagate like in a copper wire
• Cable Properties


  Mechanisms that shape the form of the signal and prevent signal loss as it travels along the membrane
• Length Constant


  Measure of how quickly a potential difference disappears (decays to zero) as a function of distance
• Length Constant


  • Increased by increasing diameter (less internal resistance)
  • Increased by increasing membrane resistance (less current leakage)
• Myelination


  Wrapping of glial cells (Schwann cells or oligodendrocytes) around axons to increase membrane resistance
• Saltatory Conduction


  The 'jumping' of action potentials from node to node in myelinated axons
• Saltatory Conduction


  • Only the membrane at the nodes is excitable
  • AP generated at one node can depolarize the next 5-10 nodes simultaneously
  • Myelin prevents current leakage between nodes
• Unmyelinated Axons


  • Have lower conduction velocity due to current leakage
  • Na+ and K+ channels are intermixed
• Axon Terminal


  1. AP conducted to end of cell

  2. AP cannot propagate backwards due to refractory period
• Synapses


  Functional association of a neuron with another neuron or effector organ

  
• Types of Synapses


  • Electrical
  • Chemical
• Electrical Synapse


  Adjacent membranes are about 35Å apart, bridged by connexins allowing small ions and depolarization to cross
• Chemical Synapse


  • Transmitter released into extracellular space between presynaptic and postsynaptic membranes
  • Postsynaptic membrane contains specific receptors for the transmitter
• Vesicle Release


  1. Voltage-gated Ca++ channels open when bouton membrane depolarized by AP

  2. Ca++ influx triggers vesicle exocytosis
• Synapses


  • Chemical synapses are processing stations where vesicle release is probabilistic (10-90% chance per AP)