Physiological PropertiesThe heart has the follow properties1.AutomaticityAutomaticity is defined as the ability of a cell to alterits resting membrane potential toward the excitationthreshold without the influence of an externalstimulus.The changes of automaticity are associated with theresting membrane potential, the threshold potential.and the steepest slope of Phase 4
Physiological Properties The heart has the follow properties: 1. Automaticity Automaticity is defined as the ability of a cell to alter its resting membrane potential toward the excitation threshold without the influence of an external stimulus. The changes of automaticity are associated with the resting membrane potential, the threshold potential, and the steepest slope of Phase 4
Myocytes within the sinoatrial node possess the mostrapid intrinsic rate of automaticity; therefore, thesinoatrial node serves as the normal pacemaker of thecellsthewithinheart.Specializedatria,atrioventricular (A-V) node, and His-Purkinje systemare capable ofspontaneous depolarization, albeit at aslower rate. The more rapid rate of depolarization ofthe sinoatrial nodal cells normally suppresses all ofthe other cells with the potential for automaticity. Theother cells will become pacemakers when their own
Myocytes within the sinoatrial node possess the most rapid intrinsic rate of automaticity; therefore, the sinoatrial node serves as the normal pacemaker of the heart. Specialized cells within the atria, atrioventricular (A-V) node, and His-Purkinje system are capable ofspontaneous depolarization, albeit at a slower rate. The more rapid rate of depolarization of the sinoatrial nodal cells normally suppresses all of the other cells with the potential for automaticity. The other cells will become pacemakers when their own
intrinsic rate of depolarization becomes greater thanthat of the sinoatrial node or when the pacemakercells within the sinoatrial node are depressed. Whenimpulses fail to conduct across the A-V node toexcite the ventricular myocardium (heart block)spontaneous depolarization within the His-Purkinjesystem may become the dominant pacemakermaintaining cardiac rhythm and cardiac output
intrinsic rate of depolarization becomes greater than that of the sinoatrial node or when the pacemaker cells within the sinoatrial node are depressed. When impulses fail to conduct across the A-V node to excite the ventricular myocardium (heart block), spontaneous depolarization within the His-Purkinje system may become the dominant pacemaker maintaining cardiac rhythm and cardiac output
The rate of pacemaker discharge within thesespecialized myocytes is influenced by the activity ofboth divisions of the autonomic nervous system.Increased sympathetic nerve activity to the heart, therelease of catecholamines from the adrenal medullaor the exogenous administration of adrenomimeticamines willeintherateofcauseanincreaseofstimulationpacemakerthroughactivityadrenoceptors on the pacemaker cells
The rate of pacemaker discharge within these specialized myocytes is influenced by the activity of both divisions of the autonomic nervous system. Increased sympathetic nerve activity to the heart, the release of catecholamines from the adrenal medulla, or the exogenous administration of adrenomimetic amines will cause an increase in the rate of pacemaker activity through stimulation of - adrenoceptors on the pacemaker cells
2.Conductivitythe cardiac impulse begins in the sinoatrial node inthe high lateral right atrium near the junction of superiorvena cava and the right atrium. excitation leaves thesinoatrial node and spreads throughout the atrium. Thenit enters the atrioventricular node, after passing throughthe A-V node, the electrical signal is carried via the rightand left bundle branches to the body of the right and leftventriclesIt is associated with the maximum velocity (Vmax) ofdepolarization of Phase O, the threshold potential, andthe resting membrane potential. The Vmax is alsoassociated with the membrane responsibility
2. Conductivity the cardiac impulse begins in the sinoatrial node in the high lateral right atrium near the junction of superior vena cava and the right atrium. excitation leaves the sinoatrial node and spreads throughout the atrium. Then it enters the atrioventricular node, after passing through the A-V node, the electrical signal is carried via the right and left bundle branches to the body of the right and left ventricles. It is associated with the maximum velocity (Vmax) of depolarization of Phase 0, the threshold potential, and the resting membrane potential. The Vmax is also associated with the membrane responsibility