General Characteristics

• Results from a reversal of the resting potential.
• K+ and Cl- are in equilibrium: Na+ has both chemical & electrical forces acting to push into cell.
• It is kept outside by the cell membrane.
• The action potential results because the Na+ permeability of the membrane is changed allowing Na+ to rush into the cell.
• The resting potential may be altered in 1 of 4 ways:
  1. Hyperpolarization - increase in resting potential (inhibitatory).
  2. Hypopolarization - reduce the resting potential.
  3. Depolarization - abolish resting potential.
  4. Action potential - complete polarity reversal
• Axon hillock
  • A spike is initiated in the axon.
  • Permeability for Na+ is immediately lowered.
  • Na+ ions rush in & locally cancel resting potential.
  • Threshold - point where membrane no longer offers resistance (area of approx. 1 cm).
  • Voltage inside the cell reverses to +30mv.
  • K+ and Cl- ions are no longer in equilibrium.
  • Change is brief & involves few particles.
  • Na+ pump's operation then works to exchange Na+ and K+ ions.

Characteristics of Action Potentials

• Passive - heat liberated during recovery.
• Ion movement is in & out, not down the axon.
• Cl- movement is negligible
• Ion movement studied by use of voltage clamp: Feedback device which can prevent depolarization of axon.
• Works by delivering current which would exactly equal flow of ions across membrane.
• Allows this required voltage to be recorded.

Graded vs Action Potentials

Excitability of the Neuron

Excitability: Reciprocal of the stimulation threshold.

Absolute refractory period: Period when Na+ is rushing in (1 msec).

Relative refractory period: (negative after potential)

• Higher threshold
• K+ moving out
• Repeated stimuli can prolong period

Subnormal period

• Reduced excitability
• Tetanus increases time of occurrence
• Stimuli can be timed to inhibit activity during subnormal period.

Terms to Know