SA NODE, AV NODE and Purkinje System are specialized cells of the heart having unstable phase IV. SA Node has no role of Voltage gated sodium channels(although they are present in SA Node) and so the depolarization in it occurs through voltage gated slow calcium channels The membrane of SA Node is Inherently leaky to Sodium and Calcium Ions. It is the Pre Potential Slope or spontaneous slow depolarization which accounts for the Pace maker activity of SA node i.e. Automaticity It is caused by the inherent leakiness of SA Nodal membrane to Sodium and Calcium leading to influx of Na+ , causing a slow rise in the RMP in the positive direction. Thus, the “resting” potential gradually rises between each two heartbeats. When the potential reaches a threshold voltage of about -40 millivolts, the Sodium-Calcium channels become “activated,” thus causing the action potential. It is the upstroke of action potential When the membrane potential reaches the thresh hold level i.e. -40 mV, voltage gated slow calcium channels open up leading to influx of calcium causing depolarization Voltage gated sodium channels has no role in SA nodal depolarization because at the level of -55 mV, the fast sodium channels mainly have already become “inactivated,” which means that they have become blocked. The cause of this is that any time the membrane potential remains less negative than about -55 mV for more than a few milliseconds, the inactivation gates on the inside of the cell membrane that close the fast sodium channels become closed and remain so. Therefore, only the slow sodium-calcium channels can open (i.e., can become “activated”) and thereby cause the action potential.

1. SA NODAL ACTION POTENTIAL, CONDUCTING
SYSTEM OF HEART AND SPREAD OF CARDIAC
IMPULSE
Maryam Fida (o-1827)
Source: The Guyton and Hall Physiology

2. SA NODAL ACTION POTENTIAL
 SA NODE, AV NODE and Purkinje System are specialized cells of the heart
having unstable phase IV.

3. SA NODAL AP
 SA Node has no role of Voltage gated sodium channels(although they are
present in SA Node) and
 so the depolarization in it occurs through voltage gated slow calcium
channels
 The membrane of SA Node is Inherently leaky to Sodium and Calcium
Ions.

4. SA NODAL AP– PHASE IV
 It is the Pre Potential Slope or spontaneous slow depolarization which
accounts for the Pace maker activity of SA node i.e. Automaticity

5.  It is caused by the inherent leakiness of SA Nodal membrane to Sodium and
Calcium leading to influx of Na+ , causing a slow rise in the RMP in the
positive direction.
 Thus, the “resting” potential gradually rises between each two heartbeats.
 When the potential reaches a threshold voltage of about -40 millivolts, the
Sodium-Calcium channels become “activated,” thus causing the action
potential.

6. SA NODAL ACTION POTENTIAL
Opening of
slow Ca++
channels
Opening of voltage
gated K+ Channels
Ca++
Influx
K+ Eflux
Na+ and Ca++
influx because of
Inherent
leakiness

7. SA NODAL ACTION POTENTIAL – PHASE 0
 It is the upstroke of action potential
 When the membrane potential reaches the thresh hold level i.e. -40 mV, voltage
gated slow calcium channels open up leading to influx of calcium causing
depolarization

8.  Voltage gated sodium channels has no role in SA nodal depolarization because at
the level of -55 mV, the fast sodium channels mainly have already become
“inactivated,” which means that they have become blocked.

9.  The cause of this is that any time the membrane potential remains less
negative than about -55 mV for more than a few milliseconds, the
inactivation gates on the inside of the cell membrane that close the fast
sodium channels become closed and remain so. Therefore, only the slow
sodium-calcium channels can open (i.e., can become “activated”) and
thereby cause the action potential.

10. SA NODAL ACTION POTENTIAL
 Phase I and Phase II are absent in SA Nodal Action Potential.

11. SA NODAL ACTION POTENTIAL – PHASE III
 It is the repolarization phase
 When the potential reaches around 0 mv, Voltage gated Potassium Channels
open up leading to the Efflux of Potassium which brings the membrane
potential back to around -55 to -60 mV

12. EXCITATORY AND CONDUCTIVE SYSTEM
OF HEART
Two major functions
 Spontaneous generation of cardiac AP or cardiac impulses without external
stimulus.
 Rapid conduction of cardiac impulses into heart resulting into rhythmic heart
rate

13. CONDUCTING SYSTEM OF HEART
 SA Node
 Internodal Pathways
 AV Node
 AV Bundle
 Bundle Branches
 Peripheral Purkinje Network

14. CONDUCTING VELOCITIES IN VARIOUS PARTS
OF HEART
 Atrial Muscle: 0.3 m/sec
 Inter nodal pathway: 1 m/sec
 AV Node: 0.1 m/sec (slowest)
 Purkinje System: 1.5 – 4 m/sec
 Ventricular muscle: 0.3 m/sec

17. SA NODE
 1.5 cm long, 3 mm wide, 1 mm thick.
 Crescent shaped.
 Present in right atrium near the opening of superior vena cava.
 It is composed of small modified cardiac cells which are called P Cells.
 In between P Cells there are collagen fibers
 Supplied by Right Vagus Nerve and Sympathetic nerve fibers.

18. INTER NODAL PATHWAYS
 Pass from SA node to AV Node.
 Anteior ( Tract of Bachman),
 Middle (Wenckebach)
 Posterior ( Tract of Thoral).
 Velocity is faster in these pathways but slow in atrial musculature.
 4th Pathway passes from SA node to left Atrium
 Between the internodal pathway and AV node there are transitional fibers.

19. AV NODE
 Structure is similar to SA Node.
 Present in right atrium in interatrial septum near the opening of coronary
sinus.
 Supplied by Left Vagus Nerve and Sympathetic fibers.

20. AV BUNDLE
 Arises from AV node and divide into two branches.

21. BUNDLE BRANCHES
 Right bundle branch and left bundle branch.
 These branches are subendocardial and present along the side of inter
ventricular septum.

22. PERIPHERAL PURKINJE NETWORK
 Penetrates the myocardium
 AV Bundle, Bundle Branches and Peripheral Purkinje Network are collectively
Known as Purkinje Network

23. SPREAD OF CARDIAC IMPULSE IN HEART

24. SPREAD OF CARDIAC IMPULSE IN HEART
 Cadiac impulse originates from SA Node and then it spreads into heart.
 From SA Node cardiac impulse spreads radially into atrial muscle at a
velocity of 0.3 m/sec and also through the internodal pathways at the
velocity of 1 m/sec.
 Cardiac impulse take 0.03 seconds to reach from SA Node to AV node,

25. SPREAD OF CARDIAC IMPULSE IN HEART

26. SPREAD OF CARDIAC IMPULSE IN HEART
 At .16 seconds, it enters AV bundle.
 So cardiac impulse takes .13 seconds to pass through AV node.
 This delay is known as AV Nodal delay.
 Out of 0.13 sec 0.09 sec is taken in AV node and 0.04 seconds is taken in the
penetrating portion of AV bundle.

27. CAUSE OF AV NODAL DELAY
 Slow velocity of conduction in AV nodal fibers i.e. 0.1 m/sec.
 The velocity is slower in AV node because of
1. Few gap junctions between AV nodal fibers. (Junctions are other than gap
junctions having high electrical resistance.)
2. In the AV nodal fiber membrane, channels are slow calcium sodium channels,
which are slow to open so velocity becomes slowed down.

29. SIGNIFICANCE OF AV NODAL DELAY
 It allows the atria to complete their contraction, before the ventricles begin
to contract so that with atrial systole filling of the ventricles is completed.
 Sympathetic stimulation shortens while vagus prolongs the AV nodal delay

30. SPREAD OF CARDIAC IMPULSE IN HEART

31. SPREAD OF CARDIAC IMPULSE IN HEART
 The velocity of conduction is fastest in AV bundle and its branches.
 The branches of AV bundle are present under the endocardium.

32.  So at 0.16 seconds, first the endocardium at the sides of septum, then apex,
then wall and then endocardium at the base of heart is depolarized.
 At 0.19 seconds, the endocardium of both ventricles is depolarized.
 So 0.03 sec is taken to depolarize whole of the endocardium

33. SPREAD OF CARDIAC IMPULSE IN HEART
 From the endocardium cardiac impulse enters the epicardium at a velocity of 0.3 to
0.5 m/sec.
 So cardiac impulse takes another 0.03 sec to pass from the endocardium to
epicardium.
 In right ventricle cardiac impulse takes 0.02 seconds to pass from the endocardium
to epicardium because of less thickness of right ventricle wall.

34. SPREAD OF CARDIAC IMPULSE IN HEART

35. SPREAD OF CARDIAC IMPULSE IN HEART
 Once Cardiac impulse enters the AV bundle, it takes 0.06 sec to depolarize both the
ventricles.
 So the time taken by the cardiac impulse to depolarize whole of the heart after its
origin from SA node is 0.22 seconds.

36. FUNCTIONS OF PURKINJE SYSTEM
 Purkinje system rapidly transmits cardiac impulse to both ventricles so that all parts
of both ventricles contract simultaneously.
 The purkinje system prevents the development of ventricular arrhythmias.The
refractory period is 25 % longer than that of ventricles. So purkinje system allows
the cardiac impulse to pass onto the ventricles when the ventricles are out of their
refractory period, so this prevents the development of arrhythmia.