Download The Sinus Node as the Pacemaker of the Heart

Chapter 10 : Rhythmical Excitation of the Heart
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 70-
80/min (AV nodal fibers 40-60/min, & Purkinje fibers 1540/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 selfexcitation
 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 sodiumcalcium 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 100-150 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
 1.5-4.0 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
Tissue
SA node
Atrial muscle
Atrial pathways
AV node
Bundle of His
Purkinje system
Ventricular muscle
Conduction rate
(m/s)
0.05
0.3
1
0.05
1
4
0.3-0.5
 Once the impulse reaches the purkinje fibres it is
transmitted through the ventricular mass by the
ventricular muscle fibres
 Velocity now is 0.3-0.5 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
Thank You