gap junctions
Summary
Gap junctions are protein channels that join the cytoplasm of adjacent animal cells, letting ions and small molecules pass directly between them. The testable point is electrical coupling: they let depolarization spread cell to cell, as in cardiac muscle.
Detail
Meaning: A gap junction is a channel formed when a connexon (six connexin proteins) in one cell's membrane docks with a connexon in the adjacent cell. The pore joins the two cytoplasms, so ions and small molecules such as cAMP and Ca²⁺ diffuse between cells, while proteins and organelles are too large to pass. Because ions cross, depolarization can spread directly from cell to cell (electrical coupling).
On the MCAT: Expect gap junctions in cardiac muscle at intercalated discs, where they spread depolarization so the heart contracts in a coordinated way; in smooth muscle such as the gut and uterus; and in electrical synapses, which are fast and use no neurotransmitter. A typical passage has a mutation or blocker disrupting gap junctions and asks what happens to conduction: spread slows and cells act more independently. The plant counterpart is the plasmodesma.
Common confusion: Gap junctions get mixed up with the other cell junctions. Tight junctions seal the space between epithelial cells to limit leakage, desmosomes anchor cells to each other and resist mechanical stress, and gap junctions neither seal nor anchor; they provide communication.
Sources
- Campbell Biology
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.