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Gap junctions are channels between animal cells that allow small molecules and ions to pass from one cell to another, aiding in communication and achieving physiological balance. They are made up of hexagonal protein structures called connexons and are essential for proper body function, including electrical and metabolic coupling. Gap junctions are not found in skeletal muscle fibers or free-circulating cells. They are important for isolating damaged cells and transmitting signals between neurons.
Gap junctions are cylindrical channels between animal cells that allow small molecules and ions to pass from within one cell into the adjacent cell. When found in plant cells, similar structures are called plasmodesmata. Without these steps, the material could not pass through the plasma membrane, which separates the inside of the cell from the outside. They help cells communicate chemical and electrical signals rapidly and achieve homeostasis, or physiological balance.
Gap junctions only pass ions or charged particles and small molecules up to about 1,000 daltons into the next cell. Unlike other cell channels, these do not limit the type of material that passes from cell to cell. These channels are found in most animal cell types, with the exception of skeletal muscle fibers and free-circulating cells such as red blood cells and circulating lymphocytes.
Gap junctions form when two opposing connections, or hemichannels, join across the intracellular space or the space between two adjacent cells. Near the canal, the intracellular space narrows to about 30 angstroms (1.2e-7in) from about 200 angstroms (9.8e-7in) or more. Connexons are hexagonal protein structures made up of six proteins called connexins.
The three main functions of gap junctions contain damaged cells, metabolic coupling and electrical coupling. If a cell becomes damaged, it is important to isolate it from other cells or kill the damaged cell so that the defect does not spread. Junctions communicate death signals between cells and shut down in response to increased intracellular calcium levels and low pH. Damage to communication with these steps is proposed to cause cancer, because cells lose their ability to isolate and kill defective cells.
Gap junctions are essential for proper body function due to their role in electrical coupling. Because they allow charged particles, or ions, to pass from one cell to another, ions cause the overall charge of the cell to change. If the cell’s charge becomes more positive, it is called depolarization, and if the cell becomes sufficiently depolarized, or positive enough, it causes an action potential. The action potential, in turn, triggers a rapid surge of signals culminating in a muscle contraction. These steps are used in this capacity in smooth muscle and heart muscle.
Electrical coupling also occurs between adjacent neurons at specialized junctions called electrical synapses. These electrical synapses also transmit neurons across the intracellular space to create a depolarization or more positive charge in the adjacent neuron. Electrical signaling is much faster than chemical signaling and can offer broadcast signals in both directions.
Gap junctions aid in metabolic coupling by allowing chemical second messengers such as calcium ions and cyclic adenosine monophosphate, also known as cAMP or cyclic AMP, to pass into the cytoplasm of the adjacent cell. Cyclic AMP is a second messenger chemical derived from adenosine triphosphate, more commonly known as ATP. Cyclic AMP passes easily across gap junctions, allowing it to carry the message of hormones. Hormones are important messenger chemicals, many of which cannot pass through cell membranes on their own and require the help of secondary messengers and channels such as gap junctions.
