RK Goit, LecturerDepartment of Physiology

Origin & Spread of Cardiac Impulse, Pacemaker, Conducting System of Heart, its Importance

• 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

Tissue Conduction rate (m/s)SA node 0.05Atrial muscle 0.3Atrial pathways 1AV node 0.05Bundle of His 1Purkinje system 4Ventricular muscle 0.3-0.5

The Sinus Node as the Pacemaker of the Heart• SA node discharge at an intrinsic rhythmical rate of 70-

80/min (AV nodal fibers 40-60/min, & Purkinje fibers 15- 40/min

• 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

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

References

• Ganong Review of Medical Physiology, 23/E• Textbook of Medical Physiology, 12/E Guyton & Hall • Understanding Medical Physiology, 4/E Bijlani &

Manjunatha

Thank You