Describe the structure of cardiac muscle.

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Accepted Answer

![Figure — Labelled histological diagram of cardiac muscle showing branched, cross-striated cylindrical fibres ](/figures/structural-organisation-in-animals/cardiac-muscle-histology-branched-striated-fibres-nucleus-intercalated-discs.svg)

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> [!TLDR]
> Cardiac muscle is a unique striated, involuntary muscle type restricted to the heart, whose branching fibres are electrically and mechanically joined by intercalated discs to enable coordinated, tireless, rhythmic contraction.
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> **Location:** Cardiac muscle forms the **myocardium**, the muscular middle layer of the heart wall, and is found nowhere else in the body.
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> **Structure:**
> - Fibres are **cylindrical and branched**, forming an interconnected network.
> - They show cross-striations (alternating light and dark bands), similar to skeletal muscle, due to the regular arrangement of actin and myosin filaments in sarcomeres.
> - Each fibre is typically **uninucleate** (a single, centrally placed nucleus), occasionally binucleate.
> - Fibres are joined end-to-end at specialised junctions called **intercalated discs**, which contain:
>   - **Desmosomes** — provide strong mechanical adhesion between adjacent cells.
>   - **Gap junctions** — allow direct electrical continuity, so action potentials spread rapidly from cell to cell.
> - Numerous mitochondria are packed between myofibrils, reflecting the high, continuous energy demand of the heart.
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> **Functional significance:** …

Feature	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Location	Attached to bones	Walls of hollow organs (stomach, blood vessels, bladder, uterus)	Wall of the heart (myocardium)
Control	Voluntary (you decide to move)	Involuntary (autonomic nervous system + hormones)	Involuntary (autonomic + intrinsic pacemaker)
Cell shape	Long, cylindrical, unbranched	Spindle-shaped (tapered ends), no striations	Short, branched, cylindrical
Striations	Present (alternating light/dark bands)	Absent	Present
Nuclei	Multiple, peripheral	Single, central	Usually one or two, central
Intercalated discs	Absent	Absent	Present (unique to cardiac muscle)
Speed of contraction	Fast	Slow	Moderate, rhythmic
Fatigue	Fatigues easily	Does not fatigue	Does not fatigue (beats a lifetime)

Q.Describe the structure of cardiac muscle.

Concept understanding — Muscular Tissue Types

Muscular Tissue Types – From Intuition to Precision

The Three Types at a Glance

Skeletal Muscle – The Voluntary Worker

Smooth Muscle – The Unseen Regulator

Cardiac Muscle – The Tireless Pump

Q.Describe the structure of cardiac muscle.

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