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•Energy Requirements •Cardiac muscle –Has many mitochondria •Great dependence on aerobic respiration •Little anaerobic respiration ability –Readily switches fuel source for respiration •Even uses lactic acid from skeletal muscles •Heart Physiology: Electrical Events •Heart depolarizes and contracts without nervous system stimulation –Rhythm can be altered by autonomic nervous system •Heart Physiology: Setting the Basic Rhythm •Coordinated heartbeat is a function of –Presence of gap junctions –Intrinsic cardiac conduction system •Network of noncontractile (autorhythmic) cells •Initiate and distribute impulses àcoordinated depolarization and contraction of heart •Autorhythmic Cells •Have unstable resting membrane potentials (pacemaker potentials or prepotentials) due to opening of slow Na+channels
–Continuously depolarize •At threshold, Ca2+channels open •Explosive Ca2+influx produces the rising phase of the action potential •Repolarization results from inactivation of Ca2+channels and opening of voltage-gated K+channels •Action Potential Initiation by Pacemaker Cells •Three parts of action potential: –Pacemaker potential•Repolarization closes K+channels and opens slow Na+channels àion imbalance à–Depolarization•Ca2+channels open àhuge influx àrising phase of action potential –Repolarization•K+channels open àefflux of K+•Figure 18.14 Pacemaker and action potentials of pacemaker cells in the heart. •Sequence of Excitation •Cardiac pacemaker cells pass impulses, in order, across heart in ~220 ms –Sinoatrial node à–Atrioventricular node à–Atrioventricular bundle à–Right and left bundle branches à–Subendocardial conducting network(Purkinje fibers) •Heart Physiology: Sequence of Excitation •Sinoatrial (SA) node –Pacemakerof heart in right atrial wall •Depolarizes faster than rest of myocardium –Generates impulses about 75X/minute (sinus rhythm) •Inherent rate of 100X/minute tempered by extrinsic factors •Impulse spreads across atria, and to AV node •Heart Physiology: Sequence of Excitation •Atrioventricular (AV) node –In inferior interatrial septum –Delays impulses approximately 0.1 second
•Because fibers are smaller diameter, have fewer gap junctions •Allows atrial contraction prior to ventricular contraction –Inherent rate of 50X/minute in absence of SA node input •Heart Physiology: Sequence of Excitation •Atrioventricular (AV) bundle (bundle of His) –In superior interventricular septum –Only electrical connection between atria and ventricles •Atria and ventricles not connected via gap junctions •Heart Physiology: Sequence of Excitation •Right and left bundle branches –Two pathways in interventricular septum –Carry impulses toward apex of heart •Heart Physiology: Sequence of Excitation •Subendocardial conducting network –