Ans. is 'a' i.e.. There is spike in action potentialo Na+ influx causes spike potential (i.e. phase of rapid depolarization in AP).o Spike potential (phase of rapid depolarization of AP) is due to Na+ influx.o Action potentials in nonautomatic fibers (Atria, ventricles) and in automatic fibers (S A node, AV node) have different ionic mechanisms for their generation.Action potential in nonautomatic fibers, i.e.. Cardiac muscles (atria, ventricles)o Normal RMPin myocardial fibers is about -90 mV. The AP in myocardial fibers has 5 phases : 0, 1,2, 3,4.Phase 0 : - It is the phase of rapid depolarization and is due to opening offast sodium channels.Phase 1 : - It is the initial phase of rapid repolarization. It is due to closure offast Na+ channels.Phase 2 : - This is plateu phaseQ. It is due to opening of'voltage gated slow Ca+ channelsQ. also called calcium- sodium channelsQ which causes calcium influxQ.Phase 3 : - This phase of final repolarization is due to opening of K+ channels. The membrane potential comes back to resting membrane potential.Phase 4 : - It is the resting phase, i.e., the phase of resting membrane potential.Action potential in automatic fibers fSA node and AY node)o The resting membrane potential of nodal fibers is less, i.e., about -65 mV (as compared to myocardium : -90 mV). Further, Resting membrane potential, even during resting phase (phase 4), moves steadily towards the depolarization, without any stimulus from nerves, hormones etc. When depolarization reaches a threshold level, an action potential is fired. Thus, between two action potentials (between the end of one action potentials and begining of other) there is slow and gradual depolarization which is called pacemaker potential or prepotential. The functional significance of the unsteady RMP is that nodal tissues generates rhythmic impulse "spontaneously"o Beside unsteady RMP (unsteady phase 4) other differences of nodal action potential from myocardial (atrial orventricular) AP are:- (i) Rapid repolarization and the platea (phase 1 and 2) are absent; and (ii) the late rapiddepolarization is slow.o Ionic basis of AP in S A and AV node are as follows : - The prepotential or pacemaker potentialQ (i.e., slow depolarization) starts due opening of funny (F) channelsQ which produce funny current. These channels are called funny channels because (i) Unlike most other channels, these are activated by hyperpolarization, and (ii) They can pass both Na+ and K+ (However, the dominant effect is that of Na" influx). The later part of prepotential is due to opening of "T" type of calcium channelsQ (T stands for transient ) with calcium influx.When prepontial depolarizes the membrane upto threshold level, action potential develops. The action potential spike is largely due to opening of L type of calcium channels (L stands for long lasting) which causes Ca+2 influx.The repolarization is produced by opening of K+ channels which causes K+ efflux.