Cardiac muscle: intercalated discs !

Tissue

Cardiac muscle is the striated, involuntary muscle of the heart; its branching cells are joined end to end so they beat as one.

Updated 2026-11-01

See it in 3D: Left ventricle · Right ventricle

What it is

Cardiac muscle cells (cardiomyocytes) are striated like skeletal muscle but are short, branched cells with one or two central nuclei. They are linked by intercalated discs that hold them together mechanically and couple them electrically, so a wave of depolarization spreads through the whole chamber. Adult cardiomyocytes barely divide, so lost muscle is replaced by scar.

Key facts

Intercalated disc = fascia adherens (anchors actin) + desmosomes (hold cells together) + gap junctions (let ions pass).
Gap junctions make the myocardium a functional syncytium.
Calcium entering from outside the cell during the plateau triggers calcium release from the sarcoplasmic reticulum (calcium-induced calcium release).
Pacemaker and conducting cells (SA node, Purkinje fibres) are modified cardiomyocytes; Purkinje fibres are large and pale with few myofibrils.
Atrial cardiomyocytes store granules of atrial natriuretic peptide.

Structure

Longitudinal section: branching striated cells with a single central oval nucleus and a pale perinuclear zone that collects brown lipofuscin with age.
Intercalated discs: dark, step-like transverse lines where two cells meet end to end.
Cross section: round to irregular cells with central nuclei, a rich capillary network and little connective tissue between them.
T tubules are wide and lie at the Z lines; each meets one sarcoplasmic reticulum cistern, forming a dyad.
Many large mitochondria fill the cytoplasm between myofibrils, matching the heart's constant aerobic demand.

Clinical notes

Myocardial infarction: dead myocytes lose their striations and nuclei within about a day, neutrophils then macrophages arrive, and granulation tissue matures into a collagen scar over weeks.
Because the scar cannot contract or conduct normally, large infarcts lead to heart failure and arrhythmias.
Hypertension and aortic stenosis make the left ventricle hypertrophy: cardiomyocytes get bigger, they do not multiply.
Chagas cardiomyopathy, important in Colombia and Latin America, shows chronic myocarditis and fibrosis from Trypanosoma cruzi.

Test yourself

Think of the answer, then tap the question to see it.

Which three junctions make up the intercalated disc?

The fascia adherens, which anchors actin filaments; desmosomes, which hold the cells together; and gap junctions, which let ions and current pass between cells.

How does cardiac muscle look different from skeletal muscle on a longitudinal section?

Cardiac cells are branched with one or two central nuclei and intercalated discs; skeletal fibres are unbranched with many peripheral nuclei.

How does the left ventricle adapt to chronic hypertension at the cell level?

Cardiomyocytes enlarge (hypertrophy) by adding sarcomeres; they do not increase in number.

Why does a large myocardial infarction leave a permanent loss of function?

Adult cardiomyocytes barely divide, so dead muscle is replaced by collagen scar that cannot contract.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of histology

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: 10.7 Cardiac Muscle Tissue (OpenStax, 2022); 4.4 Muscle Tissue and Motion (OpenStax, 2022); 10.9 Development and Regeneration of Muscle Tissue (OpenStax, 2022).

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Representative model, for educational purposes only.