Cardiac muscle, also called the myocardium, is the tissue that forms the walls of the heart. The continuous work of this muscle is fundamental to circulating blood, oxygen, and nutrients throughout the body. This function is a result of its specific cellular structure and properties.
Unique Structure and Properties
The architecture of cardiac muscle allows it to perform its continuous function. The tissue is composed of individual cells known as cardiomyocytes. These cells are shorter than skeletal muscle cells and typically contain just one nucleus. Like skeletal muscle, cardiac muscle appears striped, or striated, because the contractile proteins, actin and myosin, are arranged in organized units called sarcomeres.
Cardiomyocytes are branched and link to one another through structures called intercalated discs. These complex junctions lock the cells together. Within these discs, desmosomes act like anchors to prevent cells from pulling apart during contractions. Also present are gap junctions, which are channels that allow electrical signals to pass rapidly from one cell to the next.
This direct electrical communication, known as electric coupling, ensures all cardiomyocytes contract in a coordinated, wave-like pattern, allowing the heart to function as an effective pump. To power this constant activity, cardiac muscle cells are packed with mitochondria. These organelles produce the large amount of energy, in the form of ATP, that the heart requires.
How Cardiac Muscle Functions
Heart contraction is an involuntary process that occurs without conscious thought. It is regulated by the autonomic nervous system, which adjusts the heart rate based on the body’s needs. For instance, the sympathetic nervous system accelerates the heart rate during exercise or stress, while the parasympathetic nervous system slows it down during rest.
Cardiac muscle has autorhythmicity, the ability to generate its own electrical impulses to trigger contraction. This differs from skeletal muscle, which requires a signal from the nervous system to contract. The heart’s rhythm is established by specialized cells in the sinoatrial (SA) node, the heart’s natural pacemaker, which spontaneously generates these signals at a regular pace.
These electrical impulses spread from the SA node through the atria, causing them to contract, and then travel to the atrioventricular (AV) node. After a brief pause, the signal is transmitted through a network of specialized fibers to the ventricles. This triggers a powerful contraction that begins at the bottom of the ventricles and moves upward, pumping blood out of the heart and into the major arteries.
Comparison to Other Muscle Types
The human body contains three distinct types of muscle tissue: cardiac, skeletal, and smooth muscle, each differentiated by its structure and function. Skeletal muscles are the tissues that attach to bones and are responsible for voluntary movements, from walking to lifting. Smooth muscle is located in the walls of hollow internal organs like the intestines and blood vessels, where it performs involuntary contractions.
In terms of appearance, both cardiac and skeletal muscle are striated, while smooth muscle lacks these striations, giving it a uniform appearance. Control is another major difference. Skeletal muscle is under voluntary control, while both cardiac and smooth muscle are involuntary. Cardiac muscle is unique for its autorhythmicity, while smooth muscle contractions are controlled by the autonomic nervous system.
Cellular structure also varies. Skeletal muscle consists of long, cylindrical fibers with multiple nuclei. Smooth muscle cells are spindle-shaped with a single nucleus. Cardiac muscle cells are branched, have one or two nuclei, and are connected by intercalated discs. This disc structure is a feature not found in the other muscle types.
Health and Conditions of Cardiac Muscle
The cardiac muscle’s high energy and oxygen demand makes it vulnerable to disruptions in blood supply. A myocardial infarction, or heart attack, occurs when blood flow to a part of the heart is blocked. This deprives the muscle cells of oxygen, causing them to die and weakening the heart’s ability to pump blood.
Cardiomyopathy is a group of diseases affecting the heart muscle, making it enlarged, thickened, or stiff. These changes impair the heart’s pumping efficiency and can lead to heart failure. Ischemic cardiomyopathy, for example, is caused by reduced blood flow from coronary artery disease, which weakens the heart muscle over time.
Maintaining cardiac muscle health involves supportive lifestyle choices. Regular cardiovascular exercise, such as brisk walking or cycling, helps strengthen the heart muscle and makes it more efficient. A heart-healthy diet low in saturated fats and sodium can prevent plaque buildup in arteries, ensuring the muscle receives the oxygen-rich blood it needs.