intercalated discs

Meaning: Intercalated discs are the specialized junctions where the ends of neighboring cardiac muscle cells (cardiomyocytes) meet. Each disc contains desmosomes, which anchor cells together against the pull of repeated contraction, and gap junctions, channels that let ions pass directly from one cell's cytoplasm to the next. Ions carry depolarization through gap junctions, so action potentials spread rapidly and the heart contracts as a functional syncytium.

On the MCAT: Expect tissue comparisons: cardiac muscle is striated, branched, usually has one nucleus per cell, and has intercalated discs; skeletal muscle is striated and multinucleate; smooth muscle is nonstriated. A passage may describe blocked gap junction proteins (connexins) and ask you to predict slower, less synchronized spread of excitation, or weakened desmosomes and cells separating under mechanical load.

Common confusion: Cardiac muscle is often called a syncytium, but it is not a true one. A true syncytium, such as a skeletal muscle fiber, forms by fusion of many cells into one cytoplasm. Cardiac cells stay separate, each with its own membrane, and are linked electrically through gap junctions, so the term is functional syncytium. Desmosomes give mechanical strength; gap junctions give electrical coupling.