Action potential of Nerve and Muscle fibers
The action potentials are the rapid changes in the resting membrane potential. Nerve signals are transmitted by action potentials.
Each action potential begins with a sudden change from the normal resting negative potential to a positive potential, and ends with an equally rapid change back again to normal negative potential.
The action potential initiated moves along the fiber until it comes to the end of the fiber and thus spreads the impulse or signal along the fiber.
The stages of action potential
Depolarization stage
The resting membrane suddenly becomes very permeable to sodium ions (stimulus applied causes this permeability) allowing tremendous number of Na+ to flow to the interior of the fiber. The normal resting state of -75mV is lost with the potential rising or changing rapidly in the positive direction. This is called depolarization.
In large fibers the potential over shoots beyond the zero level and becomes slightly positive (reversal potential). In some smaller fibers as well as in many CNS neurons the potential approaches the zero but does not overshoot or reverse.
Repolarization stage
Within a fraction of a second (few 10,000 ths of a second) after the membrane becomes highly permeable for Na+ the channels for the Na+ close almost as rapidly as they had opened. Then rapid diffusion of K+ to the exterior re-establishes the normal negative resting potential. This is called repolarization of the membrane.
Generation of the action potentials
When a fiber is stimulated at a point the action potential is initiated at the point. A local current or EMF (Electromotive force) is produced at the point.
A “local circuit” of current is established between the depolarized point and the adjacent resting point.
Positive electric charges flow through the circuit into the fiber through the depolarized membrane and continue to flow along the core of the fiber. This kind of flow increases the voltage (to above threshold value) along the core through a distance of 1-3 mm. This strong impulse promptly activates the Na+ channels present on the course and the explosive action potential spreads.
Spike potential
The initial very large action potential is called the spike potential. In large myelinated nerve fibers the spike potential lasts for about 0.4 seconds. It is also called as nerve impulse.
Negative after potential
At the termination of the action potential the membrane potential sometimes fail to return to its resting level for an additional few milliseconds. Often instances will be there after a series of rapidly repeated action potentials. The situation is due to the build up of K+ immediately outside the membrane and the side becomes more positive than normal and this increase the time for the potential to return to the normal resting level. This less negative membrane potential through the additional few milliseconds is called the negative after potential.
Positive after potential
Once the resting value of the membrane potential is reached, it further becomes little more negative than the normal resting value. This excess negativity is from a fraction of an mV to a few volts than the normal resting level and this potential is known as “Positive after potential”. The after potential state can last from 50 milliseconds to as long as many second. It is principally the recharging process (the electrogenic pumping of excess Na+ outward).