Dr. Aisha Riaz Department of Physiology

parts of the heart normally beat in orderly sequence contraction of the atria (atrial systole) is followed by contraction of the ventricles (ventricular systole) during diastole all four chambers are relaxed heartbeat originates in a specialized cardiac conduction system & spreads via this system to all parts of the myocardium

The Sinus Node as the Pacemaker of the Heart SA node discharge at an intrinsic rhythmical rate of /min (AV nodal fibers 40-60/min, & Purkinje fibers /min Located in superior posterolateral wall of the right atrium each time the SA node discharges, its impulse is conducted into both the A-V node & the Purkinje fibers SA node discharges again before either the A-V node or the Purkinje fibers can reach their own thresholds for self- excitation the sinus node controls the beat of the heart

Mechanism of sinus nodal rhythmicity : THREE types of channels : fast sodium channels, slow sodium-calcium channels and potassium channels RMP : -55 to 60 mV At this RMP, fast sodium channels have already become “ inactivated” Action potential is caused only by slow sodium- calcium channels So nodal potential is slower to develop than Ventricular muscle

Inherent leakiness of the sinus nodal fibres to sodium and calcium ions causes their self-excitation. WHY DOES THIS LEAKINESS TO SODIUM AND CALCIUM IONS NOT CAUSE THE SINUS NODAL FIBRES TO REMAIN DEPOLARIZED ALL THE TIME? Two events prevent this : First, Sodium-calcium channels become inactivated within about msec after opening Second, increased number of potassium channels open Both of them causes negativity inside the cells

Transfers the action potential from sinus node to the atrial muscle fibres Three bands : Anterior, Middle and Posterior All internodal bands terminate in the AV-node

Delay of impulse conduction from the atria to the ventricles to empty blood into ventricles Total delay in the A-V nodal and A-V bundle system is about 0.13 seconds CAUSE OF SLOW CONDUCTION : Diminished numbers of gap junctions

Inability of action potential to travel backwards from ventricles to the atria Fibrous tissues b/w atria and ventricles act as an insulator and prevent passage of cardiac impulse In abnormal cases, cardiac impulse can re-enter the atria from the ventricles causing serious cardiac arrythmias

Purkinje fibers are very large fibres m/sec Rapid tranmission is due to high level of permeability of gap junctions at intercalated discs Have few myofibrils, therefore contract little or not at all

TissueConduction rate (m/s) SA node0.05 Atrial muscle0.3 Atrial pathways1 AV node0.05 Bundle of His1 Purkinje system4 Ventricular muscle

Once the impulse reaches the purkinje fibres it is transmitted through the ventricular mass by the ventricular muscle fibres Velocity now is m/sec It takes another 0.03 sec for transmission of impulse from endocardial surface to the epicardial surface of ventricles

Abnormal Pacemakers—“Ectopic” Pacemaker occasionally some other part of the heart develops a rhythmical discharge rate that is more rapid than that of the sinus node a pacemaker elsewhere than the sinus node is called an “ectopic” pacemaker an ectopic pacemaker causes an abnormal sequence of contraction of the different parts of the heart another cause of shift of the pacemaker is blockage of transmission of the cardiac impulse from the sinus node to the other parts of the heart

Important functions: pacemaker setting the rhythm of electrical excitation that causes contraction of the heart conduction system ensures that cardiac chambers become stimulated to contract in a coordinated manner, which makes the heart an effective pump

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