Chemical synaptic action is always accompanied by small delays associated with NT release, diffusion and binding, and the time it takes ion gate proteins to open and ions to move in or out of the postsynaptic membrane (PSM). The capacitance of the PSM must also be charged or discharged to realize an epsp voltage transient. Electrical synapses allow the depolarization or hyperpolarization of the presynaptic neuron to be directly coupled to the postsynaptic neuron without delays. Electron microscopy and X-ray diffraction studies have shown that an electrical synapse, also called a gap junction, consists of facing areas of the pre- and postsynaptic membranes separated by about 3.5 nm. Penetrating each membrane are curious protein structures called connexons. There are hundreds if not thousands of connexons in each gap junction. Each connexon is made up from six subunits called connexins, spaced around a common center (like the sections of a orange). Each connexin is 7.5 nm in length, and is directly opposite a corresponding connexin in the opposite membrane. The center axes of each pair of pre-and post synaptic connexons are aligned; the opposite pre- and postsynaptic connexons touch in the gap.
When the presynaptic membrane is depolarized, a tube of ~1.5 nm diameter is formed in the centers of each pair of opposite connexons. This tube allows ions, amino acids, and molecules with molecular weights up to 1000 to pass freely, depending on electrostatic and diffusion potential energies. For example, Na+, K+, Cl, Ca++, c-AMP, and ATP could pass.
All gap junctions can be classified as either bidirectional (most electrical synapses are) or rectifying (the classical example is the giant motor synapse of the crayfish). If two neurons, A and B, are connected by a nonrectifying gap junction, depolarization of A will produce an immediate, similar, smaller depolarization in
B, and vice versa. In the case of a rectifying electrical synapse, depolarization of A will produce an immediate, similar, smaller depolarization in B, but depolarization of B has little effect on the membrane potential of A. Thus, the connexons of A cause the connexons of B to open, but not vice versa. Note that nonrectifying gap junctions permit two-way, neural communication. (It is worthy to note that bidirectional chemical synapses have been observed in the brains of crabs. They are quite uncommon, otherwise.)
Gap junctions are found in the vertebrate retina. The horizontal cells (HCs) in the outer plexiform layer form a syncytium, connected to each other by gap junctions. Thus, any electrical activity induced in a central HC will spread rapidly and decre-mentally out from the center on the connected HCs. Neighboring rods and cones are also connected to one another by gap junctions (Kolb et al., 1999). Such electrical synapses allow rapid decremental interneuronal signaling.
In heart muscle, which forms a syncytium, there are extensive arrays of gap functions between muscle cells to facilitate the propagation of the electrical wave synchronizing the coordinated muscle contraction required for pumping. Gap junctions serve a similar role in spreading depolarization-triggered contraction in visceral smooth muscle, such as found in the gut, bile ducts, uterus, and blood vessels. Gap junctions are also found in liver cells, presumably to expedite the passage of low-molecular-weight chemicals between cells. Are there waves of electrical activity in liver cells?
Why do electrical synapses exist? One property of gap junctions is their speed and reliability. There is no synaptic delay, and there is no need for complex metabolic machinery to manufacture a neurotransmitter and package it in vesicles ready for release by depolarization of the bouton. There is also no need for postsynaptic receptors and an enzyme to break down the neurotransmitter, and more metabolic machinery to recycle the transmitter. Chemical synapses are complex; gap junctions are simple. So why are there not more gap junctions in the nervous system?
Gap junctions apparently require a large amount of presynaptic ion current to transmit large excitatory (or inhibitory) signals to a postsynaptic neuron. Thus, in the crayfish giant motor fiber, a very large presynaptic neuron supplies enough presynaptic depolarizing current through a rectifying electrical synapse to cause the giant motor fiber to fire in a 1:1 manner. A CNS motorneuron system that uses both electrical and chemical synapses is the Mauthner neuron system in fish. The electrical coupling is inhibitory (Eccles, 1964). Mauthner neurons are used to synchronize tail motoneurons, providing a strong swimming response.
It is noteworthy that gap junctions are also seen in adjacent glial cells in the CNS. Perhaps they serve here to transmit low-molecular-weight regulatory molecules from cell to cell, or equalize extracellular cation concentrations around neurons.
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