VPB 121: Veterinary Physiology - II (2+1)

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).