Cardiac Muscle

Structure and Function of Cardiac Muscle

Cardiac muscle, also known as myocardium, is a specialized form of muscle tissue found only in the heart. It is responsible for the rhythmic contractions that pump blood throughout the body. Unlike skeletal muscle, which is under voluntary control, cardiac muscle contractions are involuntary and are regulated by the autonomic nervous system and intrinsic conduction systems.

Cellular Composition

1. 1. Cardiomyocytes

Cardiac muscle is composed primarily of cardiomyocytes, which are the muscle cells of the heart. These cells are characterized by their striated appearance, similar to skeletal muscle, but they are shorter, branched, and interconnected by specialized junctions known as intercalated discs. These discs contain gap junctions and desmosomes, which facilitate the rapid transmission of electrical impulses and mechanical force between cells, respectively.

1. 1. Intercalated Discs

Intercalated discs are unique to cardiac muscle and play a crucial role in synchronizing heart contractions. They contain three types of cell junctions: fascia adherens, desmosomes, and gap junctions. Fascia adherens anchor actin filaments and transmit contractile forces, while desmosomes provide mechanical strength. Gap junctions allow ions to pass freely between cells, enabling the rapid spread of action potentials.

Electrophysiology

1. 1. Action Potentials

Cardiac muscle cells generate action potentials that are essential for initiating contractions. The action potential in a cardiomyocyte consists of five phases (0-4). Phase 0 is the rapid depolarization due to the influx of sodium ions through voltage-gated sodium channels. Phase 1 is a brief repolarization caused by the efflux of potassium ions. Phase 2, the plateau phase, is characterized by the influx of calcium ions through L-type calcium channels, which prolongs the action potential and is crucial for sustained contraction. Phase 3 is the repolarization phase, where potassium efflux restores the resting membrane potential. Phase 4 is the resting phase, maintained by the sodium-potassium pump.

1. 1. Conduction System

The heart's conduction system ensures the coordinated contraction of the atria and ventricles. It includes the sinoatrial node (SA node), atrioventricular node (AV node), bundle of His, and Purkinje fibers. The SA node, located in the right atrium, acts as the natural pacemaker, initiating action potentials that spread through the atria to the AV node. From the AV node, the impulse travels down the bundle of His and through the Purkinje fibers, resulting in a coordinated ventricular contraction.

Metabolism and Energy Supply

Cardiac muscle has a high demand for energy and relies predominantly on aerobic respiration for ATP production. The mitochondria in cardiomyocytes are abundant and densely packed, accounting for about 30% of the cell volume. Fatty acids are the primary energy substrate, although glucose, lactate, and ketone bodies can also be utilized. The heart's continuous activity necessitates a constant supply of oxygen and nutrients, delivered via the coronary circulation.

Pathophysiology

1. 1. Ischemic Heart Disease

Ischemic heart disease, also known as coronary artery disease, occurs when the blood supply to the cardiac muscle is reduced due to the narrowing or blockage of the coronary arteries. This can lead to myocardial infarction (heart attack), where the affected cardiac muscle tissue becomes necrotic due to prolonged ischemia.

1. 1. Cardiomyopathies

Cardiomyopathies are a group of diseases that affect the structure and function of the heart muscle. They can be classified into several types, including dilated cardiomyopathy, hypertrophic cardiomyopathy, and restrictive cardiomyopathy. These conditions can lead to heart failure, arrhythmias, and sudden cardiac death.

1. 1. Heart Failure

Heart failure occurs when the heart is unable to pump blood effectively, leading to inadequate perfusion of tissues. It can result from various conditions, including ischemic heart disease, hypertension, and cardiomyopathies. The symptoms of heart failure include dyspnea, edema, and fatigue.

Regeneration and Repair

Unlike skeletal muscle, cardiac muscle has limited regenerative capacity. Cardiomyocytes have a low proliferative potential, and the heart relies on the formation of scar tissue (fibrosis) to repair damage. Recent research has focused on regenerative therapies, such as stem cell therapy and gene therapy, to enhance cardiac repair and improve outcomes for patients with heart disease.

Clinical Implications

1. 1. Diagnostic Techniques

Several diagnostic techniques are used to assess cardiac muscle function and detect abnormalities. These include electrocardiography (ECG), echocardiography, magnetic resonance imaging (MRI), and cardiac catheterization. ECG measures the electrical activity of the heart, while echocardiography uses ultrasound to visualize cardiac structures and assess function. MRI provides detailed images of the heart's anatomy, and cardiac catheterization allows for the evaluation of coronary artery patency and intracardiac pressures.

1. 1. Therapeutic Interventions

The treatment of cardiac muscle disorders involves a combination of lifestyle modifications, pharmacotherapy, and surgical interventions. Medications such as beta-blockers, ACE inhibitors, and diuretics are commonly used to manage heart failure and hypertension. Surgical options include coronary artery bypass grafting (CABG), percutaneous coronary intervention (PCI), and heart transplantation.