IONIC BASIS OF THE ACTION POTENTIAL
A. Mechanism of passive movement of Na+ and K+
1. via ion specific channels, with separate channels for Na+
and K+ and Cl-
2. permeability ("P") of Na+ and K+ channels controlled
by one or more gates in the respective channel
a. Na+ channel has two gates, an Activation gate
(A) and an Inactivation gate (I)
b. Na+ Activation gate is mainly closed at the membrane
resting potential but opens rapidly upon membrane depolarization
c. Na+ Inactivation gate is normally open at the membrane
resting potential but closes with delay upon membrane depolarization;
reopens slowly following repolarization
d. K+ channel involved in the action potential
opens with delay upon depolarization; returns slowly to its resting
level following repolarization
Note: The Na+ and K+ channel
are termed electrically gated channels |
 |
3. Summary
| |
P-Na |
P-K |
DEPOLARIZATTION:
Immediate |
Opening of Na channel A gate, leading to large
(600x) increase of P-Na |
No Change |
DEPOLARIZATION:
Next |
Closing of Na channel I gate, leading to decrease
of P-Na below its normal resting level |
Opening of K channel gate, leading to P-K increase
(10x) |
REPOLARIZATION
Gradual |
Return of P-Na gates to their resting state
(capable of another activation) |
Return of P-K to its resting level (decrease) |
B. Permeability Changes During Action Potential
1. Changes on initial depolarization (upstroke of AP)
2. Hodgkin cycle
| If depolarization proceeds until the P-Na increase
causes Na influx > K efflux + Cl influx
(threshold exceeded)
then depolarization becomes regenerative: Hodgkin
Cycle (example of regenerative behavior or positive feedback)
Membrane potential moves toward (but does not reach)
E-Na (sodium equilibrium potential) |
 |
3. Subsequent event (down stroke of AP)
P-K increase and P-Na decrease causes
Vm to return to its resting
level approaching E-K
4. Final event (refractory period)
Following repolarization, several msec are required for P-Na
to increase and P-K to decrease to their resting levels; until this occurs,
the membrane is absolutely or relatively refractory
SUMMARY: In each action potential, a
small amount of Na enters the neuron and a small amount of K leaves the neuron
Question: What would be the expected
effect of hypokalemia (low blood potassium) on neuron excitability?
C. Role of Active Transport
1. Establishes original Na+ and K+ concentration gradients
(energy source)
2. Not directly involved in the action potential
3. Eventually restores intracellular concentrations of Na+
and K+ after the action potential is over (minutes)
Note: In the peripheral nervous system, the normal concentrations
of Na+ and K+ are sufficient to sustain a number of action potentials without
additional active transport, but without active transport nerve axons eventually
will lose their ability to generate action potentials