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Sinus node is pacemaker because of its faster discharge rate.Normal rate of discharge in sinus node is 70-80/min.Parasympathetic: Increase the permeability of the cardiac cells to K+ and decrease its permeability to Na+ and Ca++.Sympathetic: increase the permeability of the cardiac cells to Na+ and Ca++ i.e Positive Chronotropic and positive Inotropic action.Parasympathetic from Vagus nerves supply mainly the atria, SA and AV nodes, very little supply to ventricles.Sympathetic from the cardiac plexus supplies all parts of the heart (atria, ventricle and all parts of the conduction system).Sympathetic – increases heart rate by  Ca+2 & If channel (net Na+) flow Parasympathetic – decreases rate by  K+ efflux &  Ca+2 influx Autonomic neurotransmitters affect ion flow to change rate What part of the graph is not changed by autonomic influences?Įffect of Sympathetic & Parasympathetic Stimulation Effect of Sympathetic & Parasympathetic Stimulation Sympathetic Sympathetic Parasympathetic Parasympathetic 0 0 3 3 4 4 Ectopic Pacemaker- Abnormal site of pacemaker.Purkinje fibers: Fastest speed of conduction.Ventricular and Atrial muscle: Moderate speed.Purkinje 15-40 action potential /min Conduction Speed.SA node 60-80 action potential /min (Pacemaker).Intrinsic rate and speed of conduction of the components of the system Slow Response Action Potential (Pacemaker Potential) Pacemaker and Action Potentials of the Heart After 100-150 msec Ca++ channels close and K+channels open more thus returning membrane potential to -55mV.įast Response Action Potential of Contractile Cardiac Muscle Cell.When membrane potential reaches -40 mV, slow Ca++ channels open causing action potential.Acts as pacemaker because membrane leaks Na+ and membrane potential is -55 to -60mV.Specialized cardiac muscle connected to atrial muscle.SA node AV node Bundle of His Bundle Branches

Purkinje fibers Intrinsic Conduction System Function: initiate & distribute impulses so heart depolarizes & contracts in orderly manner from atria to ventricles.

Intrinsic Cardiac Conduction System Approximately 1% of cardiac muscle cells are autorhythmic rather than contractile 75/min 40-60/min 30/min Objectives List the parts that comprise the conduction system Explain the mechanism of slow response action potential (pacemaker potential) Point out the regulation of the conduction system potential by Autonomic Nerves 5 Mechanism of Cardiac Muscle Excitation, Contraction & Relaxation The importance of calcium influx through the slow voltage gated calcium channels 2