Pacemaker potential ? (instead of resting memb potential) b/c memb potential of myocardial autorhythmic cells never ?rest? at a constant value; generate A.P. spontaneously at the absence of input; unstable memb pot starts at -60 mV If ? permeable to both K+ and Na+; open when myocardial autorhythmic cells is at -60 mV (Na+ influx > K+ efflux) Depolarize If gradually close, some Ca2+ channels open influx of Ca2+ continues the depolarize memb pot moves toward threshold Speed at which pacemaker cells depolarize determines rate at which heart contracts (HR) Can be modified by altering the permeability of the autorhythmic cells to diff ions Increased perm to Na+ and Ca2+ speeds up depolarization & HR Decreased Ca2+ or increased K+ perm slows depolarization & HR Sympathetic stimulation of pacemaker cell speeds up HR NE and E increase ion flow through both If and Ca2+ channels; bind to ?1-adrenergic receptors on autorhythmic cells, which then use cAMP messenger system If belongs to family called HCN channels (hyperpolarization activated cyclic nucleotide-gated channels); cAMP binds to If open longer Parasympathetic stimulation of pacemaker cell slows HR ACh binds to muscarinic cholinergic receptors increase K+ perm hyperpolarizes cell pacemaker potential begins at > neg value decrease Ca2+ perm slows HR Pg.476, table 14-3 Fibrillation ? condition in which myocardial cells contract in a disorganized manner; ventricular fibrillation very dangerous b/c w/o coordinated contraction, ventricles cannot pump enough blood to supply oxygen to brain Can be corrected by administering electrical shock to heart Sinoatrial node (SA node)? area in R atrium containing autorhythmic cells that begin depolarization in the heart; sets the pace of the heart?fastest and primary pacemaker Internodal pathway ? branched pathway that connects SA node to Atrioventricular node (AV node) - group of autorhythmic cells near floor of R atrium that (1) provide the ONLY pathway through which A.P. can reach contractile fibers of ventricles, to ensure that blood gets out (2) slightly delay transmission of A.P.?s, allowing atria to complete their contraction before ventricular contraction begins = AV node delay Purkinje fibers ? specialized conducting cells that transmit electrical signals very rapidly; in the atrioventricular bundle (AV bundle) in septum btwn ventricles Bundle branches ? divided from AV bundle fibers Electrical conduction in heart: SA node internodal pathways AV node A-V bundle bundle branches Purkinje fibers SA node depolarizes Electrical activity goes rapidly to AV node via internodal pathways Depolarization spreads more slowly across atria; conduction slows through AV node Depolarization moves rapidly through ventricular conducting system to apex of heart Depolarization wave spreads upward from apex Spiral arrangement of muscles in walls help pull apex & base of heart closer together as they contract
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