BiologyCoachOnlineBlog

Cardiac muscle is one of three types of muscles tissues found throughout the body. [The other two are skeletal muscle and smooth muscle.] Cardiac muscle (a.k.a. "heart muscle") composes the thick, middle layer - or myocardium - (between the endocardium and the epicardium) of tissue that forms the walls of the heart. The cells that make up cardiac muscle are called myocardiocytes.

Like all muscle tissue, cardiac muscle contracts. It is the serial contractions of cardiac muscle that forces deoxygenated blood from the right ventricle to the lungs (via the pulmonary circulation) and oxygenated blood from the left ventricle to the other tissues of the body (via the systemic circulation). [By the way, the contraction of heart (i.e. cardiac muscle) registers as the systolic blood pressure while relaxation of the heart registers as the diastolic blood pressure.]

Morphologically, myocardiocytes are very often branched (unlike the cells of smooth and skeletal muscle which are both linear). [Branching is easily seen microscopically and is one of the distinguishing characteristics of cardiac tissue.] Like skeletal muscle, cardiac muscle is striated. Striations are the pattern of light and dark banding representing the alternating segments of thick (myosin) and thin (actin) protein filaments that make up myocardiocytes. Most generally, cardiomyocytes have a single nucleus (while the myocytes of skeletal muscle are multinucleated).

Finally, unique to cardiac muscle are the numerous "intercallated discs" which, again, are clearly visible on the microscopic specimen and are useful when identifying cardiac muscle. Intercallated discs (at arrows below) are structures which connect the ends of cardiomyocytes to one another and which form the cardiac "syncytium." The cardiac syncytium enables the rapid transmission of electrical impulses through the cardiac network yielding a coordinated contraction of the heart.