Rate of impulse generation is maximum in -

Ans. is 'a' i.e., SA node Automaticityo Cells of the conductive system also possess automaticity, i.e., the capability of spontaneous excitation,o However, the rate at which different parts of conductive system discharge impulses is not the same,o The SA node discharges impulses at fastest rateQ, hence the rate at which SA node fires determines the heart rate, therefore, SA node is normal pacemaker of the heartQ, i.e., it determines the pace of the heart,o The AV node has the next highest automaticity after SA node; therefore, if the SA node stops discharging, the AV node takes over the role of cardiac pacemaker.o The automaticity' of the SA node, being the highest, paces the excitatory' drive to all other cardiac fibers, suppressing their intrinsic automaticity. This phenomenon of suspension of pacemaker activity in cells other than those of the SA node is called overide suppression. Overide suppression occurs because an impulse originating in the SA node spreads throughout the heart by means of the conduction system and results in sequential contraction of different cardiac chambers. Before an impulse from AV node or other parts of conduction system can initiate impuse, the next impulse from the SA node is discharged because its rate of discharge is highest, i.e., before other parts of conducting system can generate their own impulse, they are overwhelmed by the wave of impulse generated and coming from the SA node.TissueRate o f impulse generationSA nodeAV nodeBundle of HisPurkinje system70-80/minute (Highest)40-60/minute40/minute24/minite