M04.01.014 Cardiac muscle

Cardiac Muscle: Cardiac muscle, also known as myocardium, is the muscle tissue that makes up the walls of the heart. It is a specialized type of muscle that is responsible for the contraction and pumping action of the heart to circulate blood throughout the body.

Structure of Cardiac Muscle: Cardiac muscle is striated like skeletal muscle but differs in its structural organization. It is composed of branching muscle fibers called cardiomyocytes. Cardiomyocytes are interconnected through intercalated discs, which contain specialized junctions called desmosomes and gap junctions. The desmosomes provide mechanical strength, while the gap junctions allow for electrical and chemical communication between cells.

Cardiac Muscle Fiber Types: Cardiac muscle fibers are predominantly of a single type, known as cardiomyocytes or cardiac myocytes. These cells are small, cylindrical, and contain a single centrally located nucleus. Unlike skeletal muscle, there are no distinct fiber types based on contractile properties.

Motor Unit: In cardiac muscle, the concept of motor units, as seen in skeletal muscle, does not apply. Instead, cardiac muscle fibers contract as a syncytium, meaning that electrical signals propagate across the entire myocardium, leading to coordinated contractions of the entire heart.

Excitation-Contraction Coupling: The excitation-contraction coupling in cardiac muscle involves a complex series of events that ensure coordinated and efficient contractions of the heart. Here is a simplified overview of the process:

1. Electrical Excitation: The cardiac cycle begins with the generation of electrical impulses by the sinoatrial (SA) node, the natural pacemaker of the heart. These electrical signals travel through specialized conducting pathways, including the atrioventricular (AV) node and the bundle of His, eventually reaching the Purkinje fibers, which distribute the signals throughout the ventricles.
2. Calcium Release: When the electrical impulse reaches the cardiomyocytes, it triggers the release of calcium ions from the sarcoplasmic reticulum (SR) within the cells. Calcium acts as the key regulator of muscle contraction in cardiac muscle.
3. Actin-Myosin Interaction: The released calcium ions bind to the protein complex troponin, leading to a conformational change that exposes the binding sites on actin filaments. Myosin heads interact with actin, forming cross-bridges.
4. Cross-Bridge Cycling: ATP is hydrolyzed to provide energy for the myosin heads to undergo a series of cyclic interactions with actin, resulting in the sliding of actin filaments over myosin filaments. This leads to muscle contraction.
5. Relaxation and Repolarization: After contraction, calcium is actively transported back into the SR by calcium pumps, causing muscle relaxation. The electrical signals also cease, allowing the muscle to repolarize and prepare for the next cycle of contraction.

Clinical Relevance: Understanding the structure, function, and regulation of cardiac muscle is crucial in diagnosing and managing various cardiovascular conditions:

1. Cardiac Arrhythmias: Abnormalities in the electrical conduction system of the heart can lead to arrhythmias, which can affect the normal rhythm and contraction of the heart.
2. Heart Failure: Impaired contractility of cardiac muscle, often due to conditions like ischemic heart disease or cardiomyopathies, can result in heart failure. Dysfunction of the excitation-contraction coupling process contributes to the reduced pumping ability of the heart.
3. Cardiomyopathies: Cardiac muscle disorders, such as hypertrophic cardiomyopathy or dilated cardiomyopathy, involve structural and functional abnormalities in the myocardium, leading to compromised heart function.
4. Myocardial Infarction: Interruption of blood flow to the cardiac muscle, as in a myocardial infarction (heart attack), can cause damage to the muscle tissue, impairing its contractile function.

Understanding the physiology of cardiac muscle and its clinical relevance is crucial for diagnosing, treating, and managing various cardiovascular diseases and conditions, and for designing interventions to optimize cardiac function.