What passes through a gap junction?

Ions and small water-soluble molecules up to roughly one kilodalton, including current-carrying ions and second messengers such as calcium and inositol trisphosphate, can move directly between coupled cells.

What are gap junctions made of?

They are built from connexin proteins; six connexins form a hemichannel (connexon) in one cell, and two connexons from adjacent cells dock end-to-end to create a continuous channel between the two cytoplasms.

Gap Junctions and Cell Communication

Gap junctions are communicating junctions that connect the cytoplasm of adjacent cells through clusters of channels, allowing ions and small molecules to pass directly from one cell to the next. They provide a route for electrical and metabolic coupling, synchronising the activity of cells in tissues such as cardiac muscle, smooth muscle, and many epithelia.

Definition

A gap junction is a communicating cell junction formed by paired hemichannels (connexons) in the membranes of adjacent cells, each connexon assembled from connexin proteins, that together create an aqueous channel allowing direct cytoplasmic exchange of ions and small molecules below roughly one kilodalton.

Scope

This topic covers the structure of gap junctions as channels built from connexins, the kinds of molecules they permit to pass, their role in electrical and metabolic intercellular communication, and the regulation of channel gating. It is treated as a reference and educational entry, not as clinical guidance.

Key concepts

Communicating junction
Connexin proteins
Connexon (hemichannel)
Direct cytoplasmic coupling
Electrical coupling
Metabolic coupling
Channel gating and regulation
Intercellular calcium waves

Mechanisms

Each gap-junction channel is formed by the docking of two connexons, one contributed by each cell, and each connexon is a ring of six connexin subunits surrounding a central pore. Where many such channels cluster, they form a plaque that bridges the narrow gap between the apposed membranes. The pore admits ions and small water-soluble molecules up to about one kilodalton—including ions that carry electrical current and second messengers such as calcium and inositol trisphosphate—so coupled cells share both electrical signals and metabolites. Channel permeability is not constant: gating is regulated by factors such as intracellular calcium concentration, pH, and connexin phosphorylation, allowing cells to open or close communication. This coupling supports coordinated behaviour, including the propagation of intercellular calcium waves across tissues.

Clinical relevance

Gap-junction coupling underlies coordinated functions such as the spread of electrical excitation in the heart and the synchronisation of secretory and contractile cells, so connexin biology is widely studied in physiology and pathology. This entry describes mechanism for reference and education and is not a basis for diagnosis or treatment.

History

The communicating junction was recognised among the elements distinguished by Farquhar and Palade (1963) and was later resolved by freeze-fracture and physiological studies as a site of direct cell-to-cell coupling. Identification of the connexin gene family established the molecular composition of the channels and allowed their gating and permeability to be analysed in detail.

Key figures

Daniel Goodenough
David Paul
Werner Loewenstein
Juan Saez

Seminal works

farquhar-palade-1963
goodenough-paul-2009
saez-2003

Frequently asked questions

What passes through a gap junction?: Ions and small water-soluble molecules up to roughly one kilodalton, including current-carrying ions and second messengers such as calcium and inositol trisphosphate, can move directly between coupled cells.
What are gap junctions made of?: They are built from connexin proteins; six connexins form a hemichannel (connexon) in one cell, and two connexons from adjacent cells dock end-to-end to create a continuous channel between the two cytoplasms.