Gap junctions are cylindrical channels between animal cells that allow small molecules and ions to pass from the inside of one cell to the inside of the adjacent cell. When found in plant cells, similar structures are called plasmodesmata. Without these passageways, material could not pass through the plasma membrane, which separates the inside of the cell from the outside. They help cells to communicate chemical and electrical signals quickly and achieve homeostasis, or physiological balance.
Gap junctions only let ions, or charged particles, and small molecules up to about 1,000 daltons pass through into the next cell. Unlike other cellular channels, these ones do not restrict what type of material passes from cell to cell. These channels are found in most types of animal cells, with the exception of skeletal muscle fibers and freely circulating cells such as red blood cells and circulating lymphocytes.
Gap junctions form when two opposing connexons, or hemichannels, join across the intracellular space, or the space between two adjacent cells. Near the channel, the intracellular space narrows to roughly 30 angstroms (1.2e-7in) from around 200 angstroms (9.8e-7in) or wider. Connexons are hexagonal protein structures composed of six proteins called connexins.
The three principle functions of gap junctions are containing damaged cells, metabolic coupling, and electrical coupling. If a cell becomes damaged, it is important to isolate it from other cells or kill the bad cell so that the defect doesn’t spread. The junctions communicate death signals between cells and shut down in response to heightened intracellular calcium levels and low pH. It is proposed that damage to communication with these passageways causes cancer, because the cells lose their ability to isolate and kill defective cells.
Gap junctions are essential to the body’s proper functioning due to their role in electrical coupling. As they allow charged particles, or ions, to pass from cell to cell, the ions cause the overall charge of the cell to change. If the cell’s charge becomes more positive, it is called a depolarization, and if the cell becomes sufficiently depolarized, or sufficiently positive, it causes an action potential. The action potential, in turn, sets off a rapid wave of signals that culminates in a muscle contraction. These passageways are used in this capacity in smooth muscle and cardiac 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 a more positive charge in the adjacent neuron. Electrical signaling is much faster than chemical signaling and can offer transmit signals in both directions.
Gap junctions aid metabolic coupling through allowing chemical second messengers such as calcium ions and cyclic adenosine monophosphate — also known as cAMP or cyclic AMP — to pass into the adjacent cell’s cytoplasm. Cyclic AMP is a second messenger chemical derived from adenosine triphosphate, more commonly known as ATP. Cyclic AMP readily passes through gap junctions, allowing it to transmit 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.