Electrical Activity of the Heart

The valves assure that there is unidirectional blood flow through the heart. Between the right

atrium and the right ventricle is the tricuspid valve, with three leaflets, made from connective

tissue and endocardium. It has flaps connected by chordae tendineae and papillary muscles.

The pulmonic or “pulmonary” valve has three leaflets reinforced with connective tissue. There

are no muscles associated with this valve. The mitral valve is a bicuspid valve with two leaflets

and two papillary muscles. The aortic valve has three flaps and no muscles (similar to the

pulmonic valve).

ELECTRICAL ACTIVITY OF THE HEART

Recall that cardiac muscle is different from skeletal muscle in that it has the ability to initiate an

electrical potential at a fixed rate that spreads from cell to cell during cardiac muscle

contraction. This is known as autorhythmicity. This is something that isn’t done in smooth or

skeletal muscle. The heart rate is modulated by the nervous and endocrine system.

There are two types of cardiac muscle cells: a total of 99 percent of them are called “myocardial

contractile cells” that will contract and conduct impulses throughout the heart; 1 percent are

called “myocardial conducting cells” that form the conduction system of the heart. These tend

to be smaller than the contractile cells and do not contract greatly. They act as cells which

propagate the action potential like neuron cells throughout the heart.

Cardiac muscle cells are shorter and smaller than regular muscle cells. There are striations,

which are alternating patterns of darker A bands and lighter I bands, and contractile elements

are identical to skeletal muscle. T tubules or “transverse tubules” penetrate from the

sarcolemma (plasma membrane) to the interior of the cell (like in other muscle cells) and are

found at the junction of the I and A bands, but only at the Z discs. Remember, the Z discs or Z

bands are where the sarcomeres end and another one starts. The calcium comes from both the

sarcoplasmic reticulum in a small part in cardiac cells; most come from outside the cells. This

means the contraction is slower than with skeletal muscle.

There are a lot of branches to cardiac muscle. The junction between two nearby cells is called

an intercalated disc, which supports the synchronized contraction of the cardiac muscle. These