The document discusses the cardiac impulse and conduction system of the heart. It notes that the heartbeat originates from the sinus node, which acts as the natural pacemaker at a rate of 70-80 beats per minute. The impulse then spreads through the atrioventricular node and Purkinje fibers to contract the atria and ventricles in sequence. The conduction rates vary in different cardiac tissues. The sinus node controls the heartbeat under normal conditions, but abnormal pacemakers can develop elsewhere in rare cases. The conduction system ensures coordinated contraction of the heart chambers to effectively pump blood.

1. RK Goit, Lecturer
Department of Physiology
Origin & Spread of Cardiac Impulse, Pacemaker,
Conducting System of Heart, its Importance

2. • 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

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

10. 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

11. 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

12. 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

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

14. Thank You