Cell junctions in humans, as well as in all multicellular organisms, serve two essential but opposite purposes. First, they allow materials to travel freely from one cell to another, as with gap junctions. In contrast, another set of structures called desmosomes are found in desmosomal junctions as well as tight junctions. In both cases, desmosomes create a largely impermeable barrier between certain tissue types, e.g. the epidermis and dermis of the skin or the blood-brain barrier.
Gap junctions, desmosomes, and tight junctions are the most well characterized cell junctions in humans (aside from cell-cell adhesion proteins), and this article will focus on these three in turn.
Gap junctions consist of proteins called connexons. These proteins form hexameric (six subunit) complexes which align on the plasma membranes of the two adjacent cells. When these channels open (often in reponse to an electrical discharge) any molecule large enough to fit through the central pore of the gap junction can move from one cell to the other. Aligned gap junction proteins form structures aptly called electrical synapses, which have two conformations: open, corresponding to the on state, or closed, corresponding to the off state. The tail twitch reflex seen in goldfish is mediated by electrical synapses .
In humans, gap junctions are found mainly in cardiac muscle as well as parts of the central nervous system. This makes perfect sense from the stanpoint of cardiac physiology. Because cells in the heart are connected by gap junctions, calcium can reach all parts of the ventricles within a few milliseconds. This virtually guarantees synchronous muscle contraction, even in the case of cell to cell excitation (as with bundle branch block). In the brain, glial cells, especially astrocytes, are connected by electrical synapses. Some CNS neurons may be connected by gap junctions as well.
Although the functions of gap junctions in the brain are less obvious than in the heart, neurobiologists speculate that they may contribute to the high speed of signal propagation in the CNS. Alternatively, the major role of CNS gap junctions may be to allow astrocytes to efficiently recycle neurotransmitters like glutamate and maintain extracellular ionic gradients. The debate continues.
In contrast to gap junctions, desmosomes anchor cells together closely enough to create a water resistant barrier. The best understood example of a desmosomal junction in humans is found in the epidermal layer of the skin. Here, desmosomes bind to keratin and other intermediate filament proteins, which contributes to the mechanical strength of human skin.
The third type of cell junction, the tight junction, is exemplified by the so called blood-brain barrier. Tight junctions consist of a series of adherens proteins (consisting of desmosomes bound to each other), which essentially staple the two cells together along these seams. In humans, tight junctions are found between the enterocytes of the small intestine and, most notably, at the blood-brain barrier.
The term blood-brain barrier arose when scientists noticed that fresh human brain tissue failed to take up most stains that other tissues absorbed quite easily. With the advent of electron microscopy, the basis of the blood-brain barrier was elucidated. Capillary endothelial cells in the brain and spinal cord are connected by tight junctions and surrounded by the podocytes (foot processes) of astrocytes. In most body tissues, capillaries are relatively leaky. Unlike the liver or kidneys, however, the brain lacks lymphatics for fluid drainage and is off limits to most cells of the immune system. As such, it makes sense to confine water, microbes, and most other chemicals to the blood circulation. Nonetheless, the brain needs a steady supply glucose and amino acids. To circumvent the blood-brain barrier, cells in the brain contain specialized membrane channels to transport acidic, neutral, and basic amino acids as well as glucose transport proteins that work independently of the hormone insulin.
The blood-brain barrier is permeable to several small molecules, notably ethanol; lipophilic compounds such as organophosphate insecticides and nitrosoureas (mustard gas); as well as toxic doses of heavy metals, e.g. lead, mercury, or platinum.