1. Cardiovascular system- L2
Faisal I. Mohammed, MD, PhD
University of Jordan

2. Autorhythmic Fibers Specialized cardiac muscle fibers Self-excitable
Repeatedly generate action potentials that trigger heart contractions
2 important functions
Act as pacemaker
Form conduction system
University of Jordan

3. Conduction system Begins in sinoatrial (SA) node in right atrial wall
Propagates through atria via gap junctions
Atria contact
Reaches atrioventricular (AV) node in interatrial septum
Enters atrioventricular (AV) bundle (Bundle of His)
Only site where action potentials can conduct from atria to ventricles due to fibrous skeleton
Enters right and left bundle branches which extends through interventricular septum toward apex
Finally, large diameter Purkinje fibers conduct action potential to remainder of ventricular myocardium
Ventricles contract
University of Jordan

4. Anterior view of frontal section SINOATRIAL (SA) NODE
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
RIGHT AND LEFT
BUNDLE BRANCHES 1 2 3 4
Right atrium
Right ventricle
Frontal plane
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
Left atrium
Left ventricle
Anterior view of frontal section
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
RIGHT AND LEFT
BUNDLE BRANCHES
PURKINJE FIBERS 1 2 3 4 5
Right atrium
Right ventricle
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS) 1 2 3
Right atrium
Right ventricle
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE 1 2
Right atrium
Right ventricle
Frontal plane
Right atrium
Right ventricle
Left atrium
Left ventricle
Anterior view of frontal section
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE 1
Right atrium
Right ventricle
University of Jordan

5. Intrinsic Cardiac Conduction System
Approximately 1% of cardiac muscle cells are autorhythmic rather than contractile
75/min
40-60/min
30/min

6. Autorhythmicity
During embryonic development, about 1% of all of the muscle cells of the heart form a network or pathway called the cardiac conduction system. This specialized group of myocytes
is unusual in that
they have the ability
to spontaneously
depolarize.
For the conduction system, the heart acts like a “top” and a “bottom” 6

7. Autorhythmicity
The rhythmical electrical activity they produce is called autorhythmicity. Because heart muscle is autorhythmic, it does not rely on the central nervous
system to sustain a lifelong heartbeat.
When transplanted hearts are re-
warmed following cardiopulmonary
bypass, they once again begin to beat
without the need to connect outside
nerves or use life-long pacemaker devices. 7

8. Autorhythmicity
Autorhythmic cells spontaneously depolarize at a given rate, some groups faster, some groups slower. Once a group of autorhythmic cells reaches threshold and starts an action potential (AP), all of the cells in that area of the heart also depolarize.
Membrane of two cells clearly seen. The spread of ions through gap junctions of the Intercalated discs (I) allows the AP to pass from cell to cell 8

9. Cardiac Conduction
The self-excitable myocytes that "act like nerves" have the 2 important roles of forming the conduction system of the heart and acting as pacemakers within that system.
Because it has the fastest rate of depolarization, the normal pacemaker of the heart is the
sinoatrial (SA) node, located in the
right atrial wall just below
where the superior vena
cava enters the chamber. 9

10. Cardiac Conduction
Spontaneous Depolarization of autorhythmic fibers in the SA node firing about once every 0.8 seconds, or 75 action potentials per minute 10

11. Fast Response Action Potential of Contractile Cardiac Muscle Cell

12. Pacemaker and Action Potentials of the Heart

13. Slow Response Action Potential (Pacemaker Potential)

14. Cardiac Conduction
The action potential generated from the SA node reaches the next pacemaker by propagating throughout the wall of the atria to the AV node in the interatrial septum.
At the AV node, the signal is
slowed, allowing the atrium
a chance to mechanically
move blood into
the ventricles. 14

15. Cardiac Conduction
From the AV node, the signal passes through the AV bundle to the left and right bundle branches in the interventricular septum towards the
apex of the heart.
Finally, the Purkinje
fibers rapidly
conduct the action
potential throughout
the ventricles (0.2
seconds after atrial contraction). 15

16. Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
RIGHT AND LEFT
BUNDLE BRANCHES 1 2 3 4
Right atrium
Right ventricle
Frontal plane
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
Left atrium
Left ventricle
Anterior view of frontal section
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
RIGHT AND LEFT
BUNDLE BRANCHES
PURKINJE FIBERS 1 2 3 4 5
Right atrium
Right ventricle
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE
ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS) 1 2 3
Right atrium
Right ventricle
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE
ATRIOVENTRICULAR
(AV) NODE 1 2
Right atrium
Right ventricle
Frontal plane
Left atrium
Left ventricle
Anterior view of frontal section
SINOATRIAL (SA) NODE 1
Right atrium
Right ventricle
Frontal plane
Right atrium
Right ventricle
Left atrium
Left ventricle
Anterior view of frontal section

17. Conduction System SA node acts as natural pacemaker
Faster than other autorhythmic fibers
Initiates 100 times per second
Nerve impulses from autonomic nervous system (ANS) and hormones modify timing and strength of each heartbeat
Do not establish fundamental rhythm
University of Jordan

18. Action Potentials and Contraction
Action potential initiated by SA node spreads out to excite “working” fibers called contractile fibers
Depolarization
Plateau
Repolarization
University of Jordan

19. Autonomic regulation
Originates in cardiovascular center of medulla oblongata
Increases or decreases frequency of nerve impulses in both sympathetic and parasympathetic branches of ANS
Noreprinephrine has 2 separate effects
In SA and AV node speeds rate of spontaneous depolarization
In contractile fibers enhances Ca2+ entry increasing contractility
Parasympathetic nerves release acetylcholine which decreases heart rate by slowing rate of spontaneous depolarization
University of Jordan

20. Thank You 20