5.3 & 5.4

• Action Potentials:

  -move by continuous regeneration (re-triggering)

  -move in 1 direction (due to refractory periods)
• Once initiated, an action potential moves along the membrane of a (1.) until it reaches the (2.)

  1. Neuron's axon

  2. Synaptic end bulbs
• Three Phases of an Action Potential

  • Depolarizing phase
  • Repolarizing phase
  • After-hyperpolarizing phase
• Propagation of an Action Potential

  1. Entry of Na+ produces a local current → spreads laterally to depolarize adjacent areas of membrane

  2. Adjacent membrane area is depolarized to threshold

  3. Na+ channels open in 'new' region of membrane

  4. Entry of Na+ in 'new' region of membrane produces another local current → spreads to adjacent areas of membrane

  5. The 'new' adjacent membrane area is depolarized to 'threshold' → Depolarization Phase of action potential in this 'new new' region of membrane
• 1st Factor for Velocity of Propagation of an Action Potential

  1. Size (axon diameter): bigger = faster as diameter increases → resistance to current flow In axoplasm decreases
• 2nd Factor for Velocity of Propagation of an Action Potential

  2. Myelination (insulation): insulation increases speed of action potential propagation

  -glial cells form an insulating sheath around axons

  *oligodendrocytes (CNS)

  *Schwann cells (PNS)

  -sheath is not continuous (has gaps) → saltatory conduction
• Saltatory vs. continuous conduction

  Saltatory: in myelinated axons (faster)

  Continuous: in unmyelinated axons
• Importance of refractory periods

  • Establishes maximum rate (frequency) of action potentials
  • Ensures forward propagation (axon hillock → axon terminal)
• Saltatory Conduction

  -Action potentials only occur at nodes of Ranvier

  -Ions can only be exchanged at the nodes of Ranvier
• Demyelination:

  absence of myelin 'insulation' → action potential fails
• Demyelination

  Disruption of myelin sheath

  1. Multiple Sclerosis (central)

  2. Guillan-Barre (peripheral)
• Fugu (pufferfish) contains tetrodotoxin
• How is an electrical signal (action potential) transmitted from one excitable cell to another?

  Synaptic transmission
• Spatial summation

  Signals from MANY pre-synaptic neurons add up at the trigger zone (axon hillock)
• Temporal summation

  Increased FREQUENCY of action potentials from pre-synaptic neurons add up at the trigger zone
• Graded potentials (post-synaptic electrical events) are either...

  1. Depolarizing (if Na+ enters): Excitatory Post-Synaptic Potential - EPSP 2. Hyperpolarizing (if K+ leaves or Cl- enters): Inhibitory Post-Synaptic Potential - IPSP