Question

In some cases, the excited membrane does not immediately repolarize after depolarization. Instead, the potential remains on a plateau near the peak of the spike potential for some milliseconds, then depolarization begins. This happens in the cardiac muscle fibers (0.2 to 0.3 second delay) due to what type of channels?
Explain using the principle of all-or-nothing, absolute refractory period and the directionality of propagation of an action potential.

Answer 1

The excited membrane does not immediately repolarize after depolarization. Instead, the potential remains on a plateau near the peak of the spike potential for some milliseconds, then depolarization begins. This happens in the cardiac muscle fibers (0.2 to 0.3 second delay) due to combination of several factors'

First, In heart muscle there are two types of channels responsible for depolarization. They are:

1) The usual voltage activated sodium channels also called as FAST CHANNELS

2) The slow opening voltage activated calcium sodium channels(L Type) also called as SLOW CHANNELS.

Opening of the fast channels causes the spike of the action potential whereas the prolonged opening of slow channels mainly allows the calcium entry into the fibres which is responsible for the plateau phase of action potential.

Second factor that is partially responsible for plateau phase is voltage gated potassium channels which takes longer to open than normal often does not open much until the end of plateau. This delays the return of membrane potential to its normal negative value of -80 to - 90 millivolts.

All-or-none law, a physiological principle that relates response to stimulus in excitable tissues.An induction shock or stimulus produces a contraction or fails to do so according to its strength; if it does so, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.

An organized rhythmic contraction of the heart requires adequate propagation of electrical impulses along the conduction pathway. The impulses in the His-Purkinje system travel in such a way that papillary muscle contraction precedes that of the ventricles, thereby preventing regurgitation of blood flow through the AV valves.

The period from the initiation of the action potential to immediately after the peak is referred to as the absolute refractory period (ARP) . This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. Thus, because Na⁺ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials. The absolute refractory period takes about 1-2 ms.

This recovery period is the relative refractory period during which a stronger than normal stimulus is needed to initiate a new action potential.