Cardiac muscle

Blausen 0470 HeartWall

2006 Heart Musculature

Cardiac Muscle

Cardiac sarcomere structure

Myocardiocyte

Cardiac muscle is a specialized form of muscle tissue that is found only in the heart. Its primary function is to contract and relax rhythmically in order to pump blood throughout the body, ensuring the delivery of oxygen and nutrients to tissues and the removal of carbon dioxide and other waste products. Unlike skeletal muscle, which can be controlled voluntarily, cardiac muscle operates involuntarily and is self-excitable, meaning it has the inherent ability to generate electrical impulses that trigger heart contractions without the need for external stimuli.

Structure

Cardiac muscle shares some similarities with both skeletal and smooth muscle but has unique structural features that enable its continuous, rhythmic activity. Cardiac muscle cells, also known as cardiomyocytes, are shorter than skeletal muscle fibers and are connected end-to-end at junctions called intercalated discs. These discs contain gap junctions and desmosomes that facilitate rapid electrical conduction and strong physical coupling between cells, allowing the heart muscle to contract as a unified whole.

Each cardiomyocyte contains a single nucleus, although occasionally they can be binucleated. These cells are rich in mitochondria, which provide the energy needed for continuous contraction. The presence of a large number of mitochondria ensures that cardiac muscle is highly resistant to fatigue.

Cardiac muscle fibers are striated, similar to skeletal muscle, due to the organized arrangement of actin and myosin filaments within the sarcomeres, the basic contractile units of muscle. This striation is responsible for the muscle's ability to contract forcefully and efficiently.

Function

The primary function of cardiac muscle is to pump blood throughout the body. This is achieved through a sequence of events known as the cardiac cycle, which includes both the contraction (systole) and relaxation (diastole) phases of the heart. The inherent rhythmicity of cardiac muscle contractions is regulated by the heart's own conduction system, a network of specialized muscle cells that generate and propagate electrical impulses.

The sinoatrial node (SA node), located in the right atrium, acts as the natural pacemaker of the heart, initiating each heartbeat and setting the pace for the heart rate. These electrical impulses spread through the atria, causing them to contract and push blood into the ventricles. The impulses then reach the atrioventricular node (AV node), which acts as a gatekeeper, slowing the electrical signal before it passes to the ventricles. This delay ensures that the atria have emptied their blood into the ventricles before the ventricles contract. Finally, the impulse travels along the His-Purkinje system, causing the ventricles to contract and eject blood into the arteries.

Regulation

The activity of cardiac muscle is influenced by the autonomic nervous system (ANS), which modulates heart rate and force of contraction in response to the body's needs. The sympathetic nervous system increases heart rate and contractility during times of stress or physical activity, while the parasympathetic nervous system slows the heart rate during restful periods.

Cardiac muscle's ability to respond to different physiological demands is also aided by the presence of various receptors that can sense changes in chemical and mechanical stimuli, allowing for adjustments in heart function.

Pathology

Diseases of the cardiac muscle, known as cardiomyopathies, can significantly impair heart function. These conditions can be caused by a variety of factors, including genetic mutations, long-standing high blood pressure, heart valve problems, and chronic exposure to toxins, including alcohol and certain drugs. Cardiomyopathies can lead to heart failure, arrhythmias, and other serious cardiovascular complications.