PROPAGATION OF THE ACTION POTENTIAL.
The depolarization process travels along the entire length of the fiber = NERVE/MUSCLE IMPULSE.
Direction of propagation.
An excitable membrane has no single direction of propagation → the action potential travels in all directions away from the stimulus (even along all branches of a nerve fiber) until the entire membrane has become depolarized.
All-or-nothing principle.
Once an action potential has been elicited at any point, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right → ALL-OR-NOTHING PRINCIPLE (applies to all normal excitable tissues).
RE-ESTABLISHING SODIUM AND POTASSIUM IONIC GRADIENTS AFTER ACITON POTENTIALS.
The transmission of each action potential along a nerve fiber reduces slightly the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization and potassium ions diffuse to the outside during repolarization.
For a single action potential, this effect is so minute that it cannot be measured. 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases.
Even so, with time, it becomes necessary to re-establish the sodium and potassium membrane concentration differences → achieved by action of the Na+-K+ pump:
- - Sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na+-K+ pump.
- - Because it requires energy, it is an active metabolic process → using energy from ATP.
Na -K adenosine triphosphatase pump.
- - Its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane.
- - The pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration → the “recharging” process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to “run down.”