1.
Slow response action potential
• Mainly of 3 components:
1. Phase 0: depolarization
2. Phase 3: repolarization
3. Phase 4: slow depolarization
• Phase 1 and phase 2 are usually absent in
this type of AP
• In SA node , phase 4 is characterized by
pacemaker potential

2.
Slow response action potential

3.
• It is the membrane potential of the
rhythmically discharging cells, that after each
impulse, declines to firing level and triggers
next impulse.
Pacemaker potential

4.
• Phase 0: rapid
Depolarization
– due to
1. opening of L (long)
type calcium channel
causing Ca influx
2. Increase in Na+ influx

5.
• Phase 3: Repolarization
- due to :
• 1. Closure of calcium channels
• 2. opening of K+ channel
causing K+ efflux

6.
• Slow depolarization
• Ist part due to – slow decrease
in K efflux .
• Later part due to– opening of
T (transient) calcium channels
and potential become less
negative.
 Thus, phase 4 shows
spontaneous depolarization
from RMP to firing potential
which triggers the next AP
Phase 4 or Pacemaker potential

7.
• RMP of pacemaker tissue is -55mV.
• Pacemaker tissues has an unstable resting membrane
potential because of the continuous change in
membrane permeability[The ion channels in the cell
membrane of pacemaker tissues appear to open and
close spontaneously] .
• Therefore, membrane potential decreases steadily
after each AP until firing level is reached and another
AP is fired.
• So, When potential reaches -40mV[firing level in
pacemaker tissue] an action potential is generated.

8.
• Vagal /parasympathetic stimulation---flattens
the slope of prepotential /pacemaker
potential----decreased heart rate
• sympathetic stimulation- makes the slope of
prepotential /pacemaker potential steep----
increased heart rate

9.
Electrical activity in the Sino-atrial
node or Pacemaker Potential
• Pacemaker tissues: SA node, AV node
• SA node is the pacemaker of heart since it
produce maximum number of impulse in unit
time.

10.
Cardiac conducting system
• The heartbeat originates in a specialized conduction system of
the heart called cardiac conduction/conducting system
• Peculiarities are :
- it can generate rhythmical electrical impulses
- It can conduct these impulses rapidly throughout the heart
to cause rhythmical contraction of the myocardium
- the purkinje system allows all portions of the ventricle to
contract simultaneously so that sufficient pressure is created
in the ventricles to pump blood into arterial trunks

11.
EXCITATORY AND CONDUCTING
SYSTEM OF HEART
Components of Cardiac Conducting System
1. Sinoatrial node ( SA node )
2. Internodal pathways
3. Atrioventricular node (AV node)
4. Bundle of His
5. Purkinje fibers

12.
SA Node
• Situated immediately below the opening of
superior vena cava in the right atrium.
• Contains P cells called Pacemaker cells.
• P cells have the property of self excitation & can
spontaneously generate action potentials.
• SA node discharge rapidly & depolarization of
other part of the conducting system occurs
before they discharge spontaneously.
• So SA node is considered as pacemaker of heart.

13.
Internodal Atrial Pathways
• Conduct impulse at a faster rate to AV node
and are called internodal pathways.
1. Anterior internodal tract of Bachmann
2. Middle internodal tract of Wenckebach.
3. Posterior internodal tract of Thorel.

14.
AV Node & Bundle of His
• AV node is located in the posterior wall of right
atrium immediately behind the tricuspid valve.
• AV node & Bundle of His is the only conducting
pathway between atria & ventricle.
• Impulse reaching the AV node are not
immediately transmitted to bundle of His. There
is a delay of 0.08-0.1 second, called AV nodal
delay.
• Because of AV nodal delay atria gets enough time
to empty blood into the ventricle.

15.
• Causes of AV nodal delay
AV node made of small diameter fibers, which
have low conduction velocity.
Multiple branching system in AV node.
Number of gap junction is less between
successive cells. So, velocity of conduction is
reduced.

16.
• AV node is continuous
with bundle of his
• Bundle of his gives off
left bundle branch at
the top of
interventricular
septum & continues as
the right bundle
branch.
• Left bundle branch
divides into an
anterior fascicle &
posterior fascicle.

17.
Purkinje fibers
• Are fast conducting fibers.
• Has maximum number of tight junctions.

18.
Spread of Cardiac Impulse
• Action potential generated in the SA node
spreads through the atria and converge on the AV
node.
• Atrial depolarization is completed in 0.1 seconds.
• In AV node there is delay of 0.1s before the
impulse reaches the ventricle.
• In the interventricular septum the depolarization
wave spread rapidly through the Purkinje fibers
to all parts of ventricles.

19.
• Depolarization is always from endocardial
surface to epicardial surface.
• Depolarization of papillary muscles occurs
when the impulse reaches the middle of
interventricular septum.

20.
Spread of impulse from SA
node to AV node causing
atrial activation
Interventricular

21.
late activation of posterobasal
portion of left ventricle, pulmonary
conus and upper part of
interventricular septum
to epicardial surfaces