Alila Academy

The heart is essentially a muscle that contracts and pumps blood. It consists of specialized muscle cells called cardiac myocytes. The contraction of these cells is initiated by electrical impulses, known as action potentials. Unlike skeletal muscles, which have to be stimulated by the nervous system, the heart generates its own electrical stimulation. In fact, a heart can keep on beating even when taken out of the body. The nervous system can make the heartbeats go faster or slower, but cannot generate them.

The Cardiac Conduction System:

The impulses start from a small group of myocytes called the pacemaker cells, which constitute the cardiac conduction system. These are modified myocytes that lose the ability to contract and become specialized for initiating and conducting action potentials. The SA node is the primary pacemaker of the heart. It initiates all heartbeats and controls heart rate. If the SA node is damaged, other parts of the conduction system may take over this role. The cells of the SA node fire spontaneously, generating action potentials that spread though the contractile myocytes of the atria.
The cardiac conduction pathway: Sinoatrial node (SA node) => Atrioventricular node (AV node) => Atrioventricular (AV) bundle (or bundle of His) => right and left bundle branches (with several fascicles) => Purkinje fibers (Purkinje system)

Will talk more about the cardiac conduction system in the next lesson. 

So there are 2 types of cells:

- Pacemaker cells = modified myocytes (cannot contract): they initiate electrical impulses

- Contractile myocytes = "real" muscle cells that contract, they make up the bulk of myocardium

The term "cardiac myocyte" is often used interchangeably with "contractile myocyte" 

The myocytes are connected by gap junctions, which form channels that allow ions to flow from one cell to another. This enables electrical coupling of neighboring cells. An action potential in one cell triggers another action potential in its neighbor and the signals propagate rapidly. The impulses reach the AV node, slow down a little to allow the atria to contract, then follow the conduction pathway  and spread though the ventricular myocytes. 

Subscribe to one of the courses below to continue!