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.
Sa nodal action potential, conducting system of heart and spread of cardiac impulse
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
15.
16.
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.
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
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,
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.
28.
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
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.
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.