Histologically, the SA node is identifiable because its cells are characteristically different from surrounding myocardium. Which histological feature is specific to the SA node cells?

• A Abundant intercalated discs and gap junctions
• B Large binucleate cells with centrally placed nuclei
• C Small, poorly striated cells with sparse myofibrils set in a fibrous matrix ✓
• D High mitochondrial density and regular sarcomeric structure

Explanation

The SA node (and AV node) are composed of specialized pacemaker cells that are smaller than ordinary cardiomyocytes, have fewer and irregularly arranged myofibrils (hence poorly striated), and are embedded in a dense fibrous connective tissue matrix rich in collagen. This fibrous insulation electrically isolates the node from the surrounding atrial myocardium, allowing controlled propagation. The sparser myofibrillar content reflects their primary role in impulse generation rather than contractile force generation.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.