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What is Resting pacemaker potential in cardiac tissue (in mv)? and explain in detail?
Ans. b (-55) (Ref Guyton 11th/p. 55)# The RMP of Pacemaker in cardiac tissues -55.# Cardiac muscle potential = -90.# The RMP of spinal motor neurons = -90mV (close to the equilibrium potential of K+). Structure/TissueResting membrane potential (RMP)1RBCs-7 mV2Smooth muscles-50 mV3Nerve-70 mV 4Skeletal and cardiac muscle-90 mVPHYSIOLOGICAL PROPERTIES OF CARDIAC MUSCLE# Myocardial fibers have a resting membrane potential of approximately -90 mV.# The individual fibers are separated by membranes, but depolarization spreads radially through them as if they were a syncytium, because of the presence of gap junctions.# The transmembrane action potential of single cardiac muscle cells is characterized by rapid depolarization, a plateau, and a slow repolarization process.# The initial depolarization is due to Na+ influx through rapidly opening Na+ channels (the Na+ current, INa). Ca2+ influx through more slowly opening Ca2+ channels (the Ca2+ current, ICa) produces the plateau phase, and repolarization is due to net K+ efflux through multiple types of K+ channels.# Recorded extracellularly, the summed electrical activity of all the cardiac muscle fibers is the ECG.In phase 4 of action potential of cardiac muscle there exists a constant resting membrane potential of -85 to -90 mV. In pacemaker (SA node) fibres, however, the resting membrane potential is only of - 55 to -60. - Textbook Of Medical Physiology by Indu Khurana - Pg. 258.Salient features of pacemaker potential# The action potentials in the SA and AV nodes are largely due to Ca2+, with no contribution by Na+ influx.# Consequently there is no sharp, rapid depolarizing spike before the plateau, as there is in other parts of the conduction system and the atrial and ventricular fibers.# These are of two types in the heart, the T (for transient) channels and the L (for long-lasting) channels.# The calcium current (ICa) due to opening of T channels completes the prepotential, and ICa due to opening of L channels produces the impulse.Educational Table:MCQsAnsResting membrane potential depends onECF K+Resting membrane potentialEquilibrium potential of Cl-In resting and excitable tissues the most diffusible ion isCl-In resting and excitable tissues the least diffusible ion isNa+Magnitude of action potential is affected byECF Na+ ionsFreely permeable isWaterMost diffusible substance isUrea