Category: Cardiac Histology & Cell Biology

1. Introduction The heart is not only a mechanical pump but also a highly specialized electrical organ. Its rhythmic contraction depends on the synchronized propagation of action potentials through specialized conduction structures. These include the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers. While the ventricular myocardium consists of…

Introduction For decades, the human heart was considered a terminally differentiated organ, incapable of significant regeneration after injury. The prevailing view was that cardiomyocytes, once lost to ischemia, infarction, or other insults, could not be replaced, leading to permanent functional impairment. Recent research, however, has challenged this dogma, revealing the existence of resident cardiac progenitor…

Introduction The heart is a remarkably adaptive organ capable of responding to mechanical stress and hemodynamic changes. When subjected to chronic pressure or volume overload, the myocardium undergoes remodeling, a process encompassing cellular hypertrophy, extracellular matrix (ECM) reorganization, and structural changes. While initially adaptive to maintain cardiac output, prolonged remodeling can lead to heart failure,…

1. Introduction The heart’s rhythmic contraction depends on the precise function of ion channels that regulate the flow of sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) across the cardiac cell membrane. These channels govern the action potential in pacemaker and contractile cells, enabling excitation–contraction coupling and synchrony. Ion channelopathies are inherited or acquired disorders caused…

Introduction The heart possesses an intrinsic ability to generate rhythmic action potentials, primarily through the sinoatrial (SA) node. However, this intrinsic rate is not fixed; it is finely tuned by the autonomic nervous system (ANS). Sympathetic and parasympathetic inputs modulate heart rate, conduction velocity, and pacemaker activity to meet the body’s dynamic physiological demands. Understanding…

Introduction The heart’s ability to pump blood efficiently depends on the precise conversion of electrical signals into mechanical contraction, a process known as excitation–contraction (E–C) coupling. In cardiac muscle, this process integrates: This post provides a detailed overview of cardiac E–C coupling, explores the Frank–Starling mechanism at the cellular level, and discusses lusitropy—the control of…

1. Introduction The heart’s ability to contract rhythmically and forcefully relies on precise regulation of intracellular calcium (Ca²⁺). Calcium is the key signaling molecule that links electrical excitation of the cardiac membrane to mechanical contraction of the myocardium—a process known as excitation–contraction coupling (ECC). In cardiac muscle, the primary mechanism driving cytosolic calcium increase is…

Introduction The heart’s ability to contract as a coordinated unit depends not only on the intrinsic pacemaker activity of nodal cells but also on direct electrical communication between cardiac myocytes. This intercellular communication is mediated by gap junctions, specialized membrane structures that allow ions and small molecules to pass between adjacent cells. Gap junctions are…

Introduction The coordinated contraction of the heart relies on the precise electrical activity of ventricular myocytes, which are specialized contractile cardiomyocytes designed to generate force while maintaining rhythmicity. Unlike pacemaker cells, ventricular myocytes do not spontaneously depolarize; they rely on impulses from the conduction system (Purkinje fibers and gap junctions) to initiate action potentials. The…

1. Introduction The sinoatrial (SA) node is the primary pacemaker of the heart, located in the superior part of the right atrium near the junction with the superior vena cava. Its cells possess the unique ability to spontaneously generate action potentials, a property called automaticity. This intrinsic pacemaking activity initiates the heartbeat and sets the…