Neuron Action Potential

All or None Principle - if the graded potential causes a threshold level depolarization, action potentials will be generated in the neuron.

Action potential

1. Rapid depolarization
   1. Graded potential that reaches threshold causes many voltage-gated Na⁺ channels to open (in addition to Na⁺ leakage channels) and voltage-gated K⁺ channels to begin to open
   2. Membrane becomes more permeable to Na⁺, which rapidly diffuses into the axon
   3. Na⁺ influx carries positive charges into the axon and decreases potential difference until polarity is reversed. 
2. Repolarization
   1. Membrane returns to normal Na⁺ permeability as voltage-gated Na⁺ channels close (+30mV) and are inactivated (cannot reopen)
   2. Voltage-gated K⁺ channels open more slowly and now allow K⁺ to diffuse out of the cell along its electrochemical gradient
   3. K+ efflux carries positive charges out of the axon and increases the potential difference
   4. During repolarization, voltage-gated Na⁺ channels are closed but no longer inactivated
   5. Voltage-gated K⁺ channels begin to close at resting membrane level resulting in slight hyperpolarization or afterpotential
3. Na-K exchange pump returns concentrations of Na⁺ and K⁺ to resting state levels
4. Absolute refractory period
   1. Time during which a second stimulus cannot produce an action potential
   2. Voltage-gated Na⁺ channels already open or are inactivated
5. Relative refractory period
   1. Time during which only a second very strong stimulus produces an action potential
   2. Voltage-gated Na⁺ channels are closed but no longer inactivated
6. Figure in class (see text Figure 11.20)

T₀ = resting membrane pot
T₁ = depolarized
T₂ = reverse polarity
T₃ = repolarized
T₄ = hyperpolarized
Absolute refractory period
Relative refractory period

Propagation of action potential - self-propagating change in polarity along an axon

1. Na⁺ flows into the axon and depolarization begins at the axon hillock
2. Na⁺ flows into adjacent areas causing a graded depolarization or local current
3. In adjacent axon area the graded depolarization causes voltage-gated Na⁺ channels to open and Na+ diffuses in.  (Axon hillock does not respond to a local current.)
4. Process continues as a chain reaction or wave of depolarization along the axon
5. Depolarization wave is followed by a wave of repolarization and then a wave of refraction, thus the action potential only travels in one direction

Continuous propagation - action potential moves in series of small steps along the unmyelinated axon

Saltatory propagation - action potential jumps from node to node along the myelinated axon, 5-7X faster, uses less ATP energy

Propagation speed:

• Myelination - myelinated axons conduct faster than unmyelinated axons
• Axon diameter - larger diameter axons conduct faster than smaller diameter axons
  • Type A fibers: 4-20 µm axon diameter, myelinated, 15-120 m/sec
  • Type B fibers: 2-4 µm axon diameter, myelinated, 3-15 m/sec
  • Type C fibers:  less than 2 µm axon diameter, unmyelinated,  0.5-2 m/sec