Conduction System of the Heart

discs are where the cells connect to each other and are essentially desmosomes, tight

junctions, and gap junctions that allow the passage of ions between the cells, helping to bind

the cells together. There is also intercellular connective tissue, which binds the cells together

during contraction.

Cardiac muscle undergoes aerobic respiration just like other muscle cells. Lipids and

carbohydrates are metabolized in the mitochondria to make energy. Cardiac muscle cells have

long refractory periods with brief relaxation periods. The relaxation period is necessary to

allow the heart to fill with blood for the next cardiac cycle. The refractory period is long in order

to prevent tetany of the heart muscle, which isn’t compatible with life.

Damaged cardiac muscle cells cannot easily repair or replace themselves if the cell is damage.

There are a few cardiac muscle stem cells that can potentially replace dead cells but those that

replace dead cells aren’t as functional as the original cells. Dead cells are often replaced by

inactive scar tissue.

CONDUCTION SYSTEM OF THE HEART

If embryonic heart cells are grown in vitro (in a Petri dish), they can generate their own

electrical impulse and contract. When they are connected, they contract together from the

faster cell through to the slower cell. The heart can generate its own electrical impulse and the

fastest cells lead the way for slower cells. The major components of the cardiac conduction

system are the SA node (or sinoatrial node), AV node or atrioventricular node, the bundle

branches, and the Purkinje cells.

The SA node is where the cardiac conduction cycle begins. It is located in the upper back wall of

the right atrium near where the superior vena cava enters the heart. The SA node has the

fastest rate of depolarization and is considered the pacemaker of the heart. Impulse spreads

from the SA node via internodal pathways through the atria to the AV node. There are three

bands of internodal pathways (anterior, middle, and posterior) that lead onto the next node in

the electrical pathway. It takes 50 milliseconds to travel between the nodes.

There is a specialized pathway called the Bachmann’s bundle or the interatrial band connects

the left and right atrium. The connective tissue of the cardiac skeleton prevents the impulse

from getting to the ventricles by any other pathway but through the AV node. Figure 82

describes the conduction system of the heart:

The AV or atrioventricular node receives impulses from the AV node. It is located near the part

of the right atrium near the atrioventricular septum. The AV node depolarizes and sends

impulses down to the apex and back up the sides of the ventricles in what’s called the bundle of

His. It takes about 100 milliseconds for the impulse to pass through the node. This is a crucial

pause that is critical to heart function. It allows the atrial cardiac cells to complete their

contraction so blood pumps into the ventricles.

The bundle of His divides at the apex into the left and right bundle branches. The left bundle

branch supplies the left ventricle and the right bundle branch supplies the right ventricle. The

left bundle branch is bigger than the right bundle branch because of the difference in size of

these ventricles. Each papillary muscle receives the signal at the same time so they contract

simultaneously before the ventricles. The passage of the electrical impulse through the bundle

of His takes 25 milliseconds.

The Purkinje fibers are conductive fibers that spread the impulse throughout the ventricles.

They extend throughout the myocardium from the apex toward the base of the heart. They

reach the entire ventricle in about 75 milliseconds. The pathway allows the contraction of the

heart to go from the apex to the base of the heart. This allows the blood to be squeezed out of

the heart in a total of about 225 milliseconds.

The action potentials in the cardiac conductive cells is different than the cardiac contractive

cells. Sodium, potassium, and calcium are all important in both cell’s action potential activity.

There is not a stable resting potential in cardiac muscle cells. Sodium ions leak into the cell

continuously, allowing for spontaneous depolarization of the cell. The sodium ions cause the

membrane potential to increase from -60 mV to -40 mV. Then the calcium enters the cell,

causing further depolarization to +15 mV. Finally, the potassium channels open, allowing the

membrane potential to go back to -60 mV before the cycle begins again.

The electrical pattern of contractile cells in the heart is different from the conductive cells.

There is a more rapid depolarization, followed by a plateau phase and repolarization. This

allows the heart muscle to pump blood out of the heart before they can fire a second type.

They usually wait for an impulse to come to them although they can pump and generate an

action potential on their own if necessary.

Contractile cells have a much more stable resting phase when compared to the conductive

cells. The resting potential is -80 mV for the atria and -90 mV for the ventricles. The rest of the

action potentials are the same for the atria and ventricles. When stimulated, the influx of ions

causes the potential to go up to +30 mV, with a rapid depolarization again. This is followed by a

plateau phase, where the membrane potential drops slowly. The absolute refractory period is

200 milliseconds, while the relative refractory period is about 50 milliseconds. This long period

is necessary for the heart to pump blood out of the ventricles. It prevents premature

contractions, which would be fatal.