Cardiac conduction system

So, electrical conduction in the heart refers to the electrical signals that go from cell to cell in the heart. This happens in the form of action potentials, which get sent out by the pacemaker cells in the heart.

The pacemaker cells, also called conducting cells, are a relatively tiny group -- only about 1% of the heart cells -- but they’re a pretty influential minority.

They’re special ability is that they are autorhythmic, which means that they are able to continually generate new action potentials that go out to the rest of the heart -- the other 99%.

This is different from how it works in skeletal muscle cells, which get their action potential signals directly from neurons.

The cells that receive the cardiac action potential from the pacemaker cells are called myocytes - they make up the myocardium, which is the muscular middle layer of the heart.

Myocytes are also called contractile cells because they contract and that’s how the heart pumps blood.

Action potentials are initiated by depolarization, which is the opposite of polarization. In this case polarization is when there are more positive ions outside the cell than inside.

This difference in charge is called the membrane potential and is negative since there are more positive ions outside the cell.

So, depolarization is when the membrane potential gets smaller making a cell slightly more positive than it normally would be - imagine a negative, gloomy cell enjoying a moment of joy.

If one cell after another depolarizes, then there’s a depolarization wave which is just like a crowd of people doing the wave at a football stadium.

So, there’s a group of pacemaker cells in the sinoatrial node or SA node, which is a small sinus or cavity tucked up into the right atrium. During each heartbeat, one pacemaker cell out of the group will automatically depolarize first.

In fact, each heart beat might be led by a different cell in the group, but eventually at least one of them will fire because they’re all autorhythmic, meaning that every pacemaker cell has the ability to self-generate a new action potential, given enough time.

So as a group, the pacemaker cells of the SA node act like a drill sergeant that gives orders to the rest of the heart. They decide when the heart contracts and when it relaxes, so they set the heart rate.

The depolarization wave that comes out of the SA node moves really fast through pacemaker cells throughout the heart, and moves more slowly through atrial and ventricular myocytes.

Some pacemakers lie along atrial internodal tracts, also called Bachmann's bundle, which connect the SA node to spots in the right and left atria, so that the depolarization wave can quickly reach atrial myocytes in both atria.

When the atrial myocytes get depolarized, they contract, pushing blood from the atria into the ventricles.

While this is happening, the depolarization wave also travels from the SA node through pacemaker cells to the atrioventricular or AV node. Conduction velocity slows way down in the AV node for two reasons.

First, the AV nodal cells have very small diameters which increases resistance to electrical flow, and second, the AV nodal cells use the relatively slower opening calcium ion channels rather than the faster opening sodium ion channels.