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Cardiac Muscle Tissue

*Textus muscularis striatus cardiacus*

For medical students2 min readUpdated 2026-10-10

Cardiac muscle tissue is a specialized type of striated muscle forming the myocardium. It is composed of cardiomyocytes organized into functional fibers that ensure the continuous pumping action of the heart.

Embryonic originMyoepicardial mantle (from the visceral layer of the splanchnotome)
Structural unitTypical cardiomyocyte (diameter ~20 µm, length ~100 µm)
FibersCells form functional fibers with intercalated discs and anastomoses
RegenerationIn the postnatal period — predominantly hypertrophy, without an increase in cell number

Organization and Structure of Cardiomyocytes

The structural unit of the tissue is the typical cardiomyocyte, a cylindrical cell. By joining end-to-end, cardiomyocytes form long strands known as functional fibers.

To ensure the heart functions as a single unit, lateral branches (anastomoses) connect parallel fibers, forming an extensive three-dimensional network.

Unlike skeletal muscle fibers (myosymplasts), the nucleus of a cardiomyocyte (often polyploid) is located strictly in the center of the cell. This is because myofibrils occupy only about 40% of the cytoplasm volume and do not displace the nucleus to the periphery. Externally, each fiber is covered by a basal lamina that envelops only the lateral surfaces of the cells.

Intercalated Discs: A Key Feature

The sites of end-to-end junctions between adjacent cardiomyocytes are called intercalated discs. Under light microscopy, they appear as thick transverse dark lines. Under electron microscopy, the disc is a complex junctional complex consisting of:

  1. Interdigitations — finger-like membrane folds that increase the cell contact area.
  2. Desmosomes — provide mechanical strength, securely anchoring cells together so they do not tear during systole.
  3. Gap junctions (nexuses) — contain ion channels. Through these channels, excitation is instantly transmitted from cell to cell, forcing them to contract synchronously.

Cell Populations of Cardiac Muscle

Although the myocardium is primarily composed of typical (contractile) cells, the tissue is heterogeneous. There are three types of cardiomyocytes:

Energy and Metabolism

The heart works continuously; therefore, cardiomyocytes contain a vast number of mitochondria, lipid droplets, and myoglobin.

Their metabolism is exclusively aerobic. At rest, the heart utilizes fatty acids and ketone bodies (derived from the liver). However, during intense physical exertion, the myocardium switches to utilizing lactate (produced by working skeletal muscles) and glucose.

With age, cells accumulate lipofuscin — an aging pigment formed in telolysosomes from incompletely digested structures.

Regeneration

For a long time, it was believed that cardiomyocytes do not divide after birth. It is now proven that cardiac stem cells exist (predominantly in the atria and apex), which function during the active growth phase up to adolescence.

Nevertheless, in the adult (postnatal) state, their role is negligible. The main mechanism of adaptation to increased loads (such as in athletes) or compensation for damage (after a myocardial infarction) is intracellular hypertrophy. This process increases the volume of existing cells, but hyperplasia (an increase in their number) does not occur.

Frequently asked questions

What functions do secretory cardiomyocytes perform, and in which part of the heart are they predominantly located?

Secretory cardiomyocytes perform an endocrine function and are located predominantly in the atria (more abundant in the right atrium).

They synthesize and store protein hormones in secretory granules:

  • Glycoprotein — exhibits anticoagulant properties.
  • Atrial natriuretic peptide (ANP) — enhances renal excretion of sodium ions and water, leading to a hypotensive effect (lower blood pressure).

Simultaneously with secretion, these cells retain the ability to contract, although their contractile apparatus is less developed than that of working cardiomyocytes.

Which structures (nodes and bundles) make up the cardiac conduction system formed by atypical cardiomyocytes?

The cardiac conduction system consists of two nodes and their emanating bundles:

  • Sinoatrial (SA) node (Keith-Flack node) — located in the upper wall of the right atrium. Three bundles extend from it into the atrial walls.
  • Atrioventricular (AV) node (Aschoff-Tawara node) — located in the lower wall of the right atrium near the septum.
  • Bundle of His — originates from the AV node, enters the interventricular septum, and divides into right and left bundle branches.
  • Purkinje fibers — terminal branches of the bundle branches.
What are the structural features of the sarcotubular system (T-tubules and sarcoplasmic reticulum) in cardiomyocytes compared to skeletal muscle?

The following elements of the sarcotubular system are described in cardiomyocytes:

  • T-tubules — deep invaginations of the sarcolemma surrounding the myofibrils.
  • L-system / sarcoplasmic reticulum — a derivative of the smooth endoplasmic reticulum; includes L-tubules and terminal cisternae.
  • Sarcoplasmic network in muscle tissues participates in the sequestration of Ca²⁺ ions: upon excitation, the signal travels along the sarcolemma and T-tubules, then Ca²⁺ channels of the terminal cisternae open, and the release of Ca²⁺ into the sarcoplasm triggers myofibril contraction.

Standard histological sources do not specify major differences in the sarcotubular system between typical cardiomyocytes and skeletal muscle.

Where is the nucleus located in a cardiomyocyte?

Unlike skeletal muscle tissue, the polyploid nucleus in a typical cardiomyocyte is located strictly in the center of the cell.

What is the function of gap junctions in intercalated discs?

Gap junctions (nexuses) provide electrical coupling. Ions pass through their hydrophilic channels, allowing rapid transmission of excitation and synchronous cell contraction.

Can cardiac muscle regenerate after a myocardial infarction?

Full regeneration with the formation of new fibers does not occur in adults. Replacement occurs via scarring, while the remaining cardiomyocytes adapt through hypertrophy (increase in volume).

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