Absolute refractory period

After an electrical impulse is initiated and conducted, there is a period of time during which cells and fibers cannot be depolarized again. This period of time is referred to as the absolute refractory period (Fig. 6-2),2 and corresponds to phases 1,2, and approximately half of phase 3 repolarization on the action potential. The absolute refractory period also corresponds to the period from the Q wave to approximately the first half of the T wave on the ECG (Fig. 6-2). During this period, if there is a premature stimulus for an electrical impulse, this impulse cannot be conducted, because the tissue is absolutely refractory. 

The process of reentry is depicted in Fig. 6-3.4 Under normal circumstances, when a premature impulse is initiated, it cannot be conducted in either direction down either pathway because the tissue is in its absolute refractory period from the previous beat. A premature impulse may be conducted down both pathways if it is only slightly premature and arrives after the tissue is no longer refractory. However, when refractoriness is prolonged down one of the pathways, a precisely timed premature beat may be conducted down one pathway, but cannot be conducted in either direction in the pathway with prolonged refractoriness because the tissue is still in its absolute refractory period (Fig. 6-3, panel la).4 When the third condition for reentry is present, that is, when the velocity of impulse conduction in the other pathway is slowed, the impulse traveling forward down the other pathway still cannot be conducted. However, because the impulse in the other pathway is traveling so slowly, by the time it circles around and travels upward down the other pathway, that pathway is no longer in its absolute refractory period, and now the impulse may travel upward in that pathway. In other words,... 

Distinguish between the absolute refractory period and the relative refractory period... 

Phase 2 A plateau occurs owing to the opening of L-type Ca2+ channels, which offset the action of K+ channels and maintain depolarization. During this phase, no further depolarization is possible. This is an important point to demonstrate and explains why tetany is not possible in cardiac muscle. This time period is the absolute refractory period (ARP). The plateau should not be drawn completely horizontal as repolarization is slowed by Ca2+ channels but not halted altogether. 

This is the time from the absolute refractory period until the cell s membrane potential is less than the threshold potential. It corresponds to the period of increased K+ conductance. 

Because of the absolute refractory period of the voltagegated Na channels and the brief hyperpolarization resulting from efflux, the action potential Is propagated in one direction only, toward the axon terminus. 

Besides the existence of a threshold for stimulation, excitable systems are characterized by the existence of a refractory period during which the response to further stimulation is reduced or even totally absent (Fitzhugh, 1961). As shown in fig. 5.33, the cAMP signalling system of D. discoideum shares this property. Represented in this figure is the ratio Aic/Aiii of the maxima of two successive peaks of intracellular cAMP, as a function of the time interval separating the second stimulus from the maximum of the first response, which defines time zero. For 4 min, no cAMP synthesis can be elicited by the second stimulus. This phase defines an absolute refractory period. Thereafter, the response to the second stimulus increases gradually, until the second maximum reaches the value of the first, after some 15 min. This second phase defines a relative refractory period. The two types of refractory period are known in other excitable systems, for example in nerve cells (Fitzhugh, 1961). 

The depolarization and repolarization phases of action events can occur quickly over intervals of tens of microseconds, although the actual durations depend very much on die cell type. During the time when the cell is depolarized, it cannot be restimulated to another action event. This interval is known as the cell s absolute refractory period. The cell s relative refractory period is the interval... 

Fig. 3.16 Example of AAI pacing from Figure 3.1 with refiactory periods added. The ARP has an initial absolute refractory period (abs). No sensed events are noted in the marker channel during the absolute refractory period. During the remaining portion of the ARP, the relative refractory period (rel), sensed events may be noted, but the LRL will not be reset.

All signals are ignored during the absolute refractory period, and noise sampling occurs within the relative refractory period. The purpose of the ARP is to prevent oversensing of the output pulse stimulus afterpotential, the atrial electrogram produced by the atrial stimulus, or ventricular depolarization. 

The antianhythmic activity was evaluated in vitro according to the extent to which the compounds affected the prolongation of the myocardial absolute refractory period in an isolated rat atrium of the heart stimulated by electric impulses [3, 138]. This method does not allow for the identification of all of the antiarrhythnuc effects of new substances, but the technique is valid at the stage of primaiy screening. MEC, the minimum effective concentration of a substance (mol/L) preventing the atrium from adopting the rhythm forced on it, served as a measure of the antiarrhythmic activity. Moricizine (CAS 31883-05-3) was studied as the drag for comparison its MEC was found to be 5.10-10 M. 

Each sensed ventricular heartbeat undergoes a classification determined by its relationship in time to the previous sensed/paced ventricular beat, and the number of zones of detection that have been programmed. "Time zero" below marks the point of initial pacing or sensing. An absolute refractory period ( ) serves to prevent this beat from being sensed should it be a ventricular paced complex, or from being sensed a second time should the complex be a particularly wide native beat. 

Drugs that prolong absolute refractory period of SA node... 

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