Module 2: Student notes – Electrical conduction of the heart, the ECG and the cardiac cycle
Cellular Level
Cardiac muscle cells (cardiomyocytes) have a unique feature compared to other muscle cells in
the body. They have special intercellular boundaries known as intercalated discs. Intercalated
discs allow for the rapid propagation of the action potential and coordinated contraction of
muscle cells. Within this unique cellular boundary are two intracellular connections: gap
junctions and desmosomes. Gap junctions are communication tunnels that allow the action
potential to transfer to the neighboring cell. The action potential is the movement of Na+, Ca+2
and K+ ions across the cell membrane. The movement of these ions cause the cardiac muscle to
contract. Depolarization and repolarization are terms that describe the movement of the ions
of the action potential.
GIF showing a cell membrane with the different channels for ion movement
https://thumbs.gfycat.com/PointlessLateBeardedcollie-mobile.mp4
The Conduction System
Pathway of the electrical impulse:
1. Sinoatrial node (SA) - Located at the entrance of superior vena cava, this is a group of
specialized cells that start the action potential in the heart by spontaneous
depolarization. It is known as the pacemaker of the heart. The action potential (P wave
on the ECG) travels through both atria to the atrioventricular node and this causes both
atria to contract.
2. Atrioventricular node (AV) - The action potential moves from the atria to the ventricles
through the AV node since the annulus of each Atrioventricular valve acts as an insulator
to the action potential. This is the only route for the action potential to travel to the
ventricles in a normal heart. The AV node is located in the right atrium, near the
interatrial septum by the entrance of the coronary sinus. The AV node is a gatekeeper
causes a slight delay in the action potential, giving the atria time to empty completely
(electrical then mechanical).
3. Bundle of His- Located in the interventricular septum (IVS), the Bundle of His directs the
action potential apically.
4. Right and Left Bundle Branches (RBB & LBB) - In the IVS, the Bundle of His branches into
the right and left bundle branches to direct the action potential to the right and left
ventricles and towards the apex.
5. Purkinje fibers - The bundle branches further divide into Purkinje fibers that carry the
impulse to the endocardium and down toward the apex.
, The result of this AV- Bundle of His - RBB &
LBB- Purkinje fiber organization is that the
contraction starts in the apex and moves up to
the annulus. This allows the squeezing to start
at the apex and move to the base where the
semilunar valves are.
Coordination of Contraction
All cardiac cells have the potential to be self-excitable or are able to contract without being told
to. The SA node (‘the pacemaker of the heart’) coordinates the contraction in the heart. The
specialized intercalated discs of cardiac cells allow for the coordination of contraction. The
desmosomes connect one muscle cell to another so they can contract together and
desmosomes allow for the rapid propagation of the action potential from one cell to the next.
If any other cell decides to take over the role of the SA node or there is a misstep in the
conduction pathway, then this could lead to an arrhythmia. An arrhythmia is simply an irregular
heartbeat. The exact cause of most arrhythmias are not known. If the arrhythmia is a new
finding or it is causing symptoms, then an echocardiogram is ordered to see if the cause can be
identified by ultrasound.
Heart rate
SA node - intrinsic firing rate of 60 to 100 beats per minute (bpm)
AV node - intrinsic firing rate of 40 to 60 bpm
Bundle of His - intrinsic firing rate of 20 bpm
If there was a problem with the SA node (ex. ischemia) and it was no longer firing or controlling
the impulses, the AV node takes over and the heart rate slows to its intrinsic firing rate of 40 –
60 bpm.
Blood supply to Conduction System
SA node - supplied by a branch off the right coronary artery in 60% of the population (40% of
the population supplied by the circumflex)
AV node - supplied by the AV nodal artery, a branch off the right coronary artery for 85% of the
population (10% Circumflex artery)
Bundle of His - supplied by the right coronary artery (RCA)(90%), 10% of the population it is
supplied by the left anterior descending
Bundle branches – same as the Bundle of HIS
Cellular Level
Cardiac muscle cells (cardiomyocytes) have a unique feature compared to other muscle cells in
the body. They have special intercellular boundaries known as intercalated discs. Intercalated
discs allow for the rapid propagation of the action potential and coordinated contraction of
muscle cells. Within this unique cellular boundary are two intracellular connections: gap
junctions and desmosomes. Gap junctions are communication tunnels that allow the action
potential to transfer to the neighboring cell. The action potential is the movement of Na+, Ca+2
and K+ ions across the cell membrane. The movement of these ions cause the cardiac muscle to
contract. Depolarization and repolarization are terms that describe the movement of the ions
of the action potential.
GIF showing a cell membrane with the different channels for ion movement
https://thumbs.gfycat.com/PointlessLateBeardedcollie-mobile.mp4
The Conduction System
Pathway of the electrical impulse:
1. Sinoatrial node (SA) - Located at the entrance of superior vena cava, this is a group of
specialized cells that start the action potential in the heart by spontaneous
depolarization. It is known as the pacemaker of the heart. The action potential (P wave
on the ECG) travels through both atria to the atrioventricular node and this causes both
atria to contract.
2. Atrioventricular node (AV) - The action potential moves from the atria to the ventricles
through the AV node since the annulus of each Atrioventricular valve acts as an insulator
to the action potential. This is the only route for the action potential to travel to the
ventricles in a normal heart. The AV node is located in the right atrium, near the
interatrial septum by the entrance of the coronary sinus. The AV node is a gatekeeper
causes a slight delay in the action potential, giving the atria time to empty completely
(electrical then mechanical).
3. Bundle of His- Located in the interventricular septum (IVS), the Bundle of His directs the
action potential apically.
4. Right and Left Bundle Branches (RBB & LBB) - In the IVS, the Bundle of His branches into
the right and left bundle branches to direct the action potential to the right and left
ventricles and towards the apex.
5. Purkinje fibers - The bundle branches further divide into Purkinje fibers that carry the
impulse to the endocardium and down toward the apex.
, The result of this AV- Bundle of His - RBB &
LBB- Purkinje fiber organization is that the
contraction starts in the apex and moves up to
the annulus. This allows the squeezing to start
at the apex and move to the base where the
semilunar valves are.
Coordination of Contraction
All cardiac cells have the potential to be self-excitable or are able to contract without being told
to. The SA node (‘the pacemaker of the heart’) coordinates the contraction in the heart. The
specialized intercalated discs of cardiac cells allow for the coordination of contraction. The
desmosomes connect one muscle cell to another so they can contract together and
desmosomes allow for the rapid propagation of the action potential from one cell to the next.
If any other cell decides to take over the role of the SA node or there is a misstep in the
conduction pathway, then this could lead to an arrhythmia. An arrhythmia is simply an irregular
heartbeat. The exact cause of most arrhythmias are not known. If the arrhythmia is a new
finding or it is causing symptoms, then an echocardiogram is ordered to see if the cause can be
identified by ultrasound.
Heart rate
SA node - intrinsic firing rate of 60 to 100 beats per minute (bpm)
AV node - intrinsic firing rate of 40 to 60 bpm
Bundle of His - intrinsic firing rate of 20 bpm
If there was a problem with the SA node (ex. ischemia) and it was no longer firing or controlling
the impulses, the AV node takes over and the heart rate slows to its intrinsic firing rate of 40 –
60 bpm.
Blood supply to Conduction System
SA node - supplied by a branch off the right coronary artery in 60% of the population (40% of
the population supplied by the circumflex)
AV node - supplied by the AV nodal artery, a branch off the right coronary artery for 85% of the
population (10% Circumflex artery)
Bundle of His - supplied by the right coronary artery (RCA)(90%), 10% of the population it is
supplied by the left anterior descending
Bundle branches – same as the Bundle of HIS
