Synaptic Transmission is a mechanism of communication between neurons in the nervous system of organisms. This mechanism of transmission is an important as it helps to regulate parasympathetic and sympathetic actions of the peripheral nervous system (PNS) and also underlies the modulation and regulation of cognition, mood, learning, and memory within the central nervous system (CNS). Synaptic transmission involves two types of synaptic modalities called electrical synapses and chemical synapses. While this review will touch on electrical synaptic transmission the focus will largely be on chemical synaptic transmission.
Electrical synapses permit passage of currents, ions, and small molecules from the presynaptic terminal to the postsynaptic terminal through gap junctions. Gap junctions are intercellular specialization complexes that are comprised of connexons. Each connexon are comprised of six connexins, which are individual protein subunits, and together two connexons form a gap junction. There are a wide variety of connexin subunit types which allow gap junction to have a diverse range of functional properties such as pH sensitivity, Ca2+ sensitivity, membrane voltage sensitivity, different cell types, and direction of transmission (unidirectional or bidirectional). Gap junctions differ from other traditional membrane channels in that they cross two membranes which connect the cytoplasm of one cell to the cytoplasm of another, they also require proteins from two different neurons to form cell-cell channels, and the channels of gap junction are much wider in diameter than ion channel pores. Gap junctions permit the transmission of ions, ATP, small metabolites, and second messenger molecules. Due to the direct membrane connections, a result of the gap junction, transmission from one neuron to another is extraordinarily rapid due to instantaneous passive current flow. A general consensus for the importance of these synapses is that they allow electrical activity among many neurons to be synchronized, allowing cells to fire action potentials in unison, resulting in release of molecules to stimulate other cellular mechanisms rapidly and with ease. Electrical synapses are also coincidence detectors since localized changes in membrane potential can be detected be other neighboring cells which in turn allows for coordinated cellular activity. Many interneurons of the nervous system, such as inhibitory interneurons, are coupled together by electrical synapses, since interneurons are important in quickly regulating local networks of neurons. Different structures in the brain that utilize electrical synapses consist of the retina, olfactory bulb, cerebral cortex, hippocampus, suprachiasmatic nucleus, hypothalamus, the inferior olive, the brainstem, and the spinal cord motor and sensory systems. Electrical synapses are shown to be capable of plasticity as they are able to modify their coupling strength in response to different physiological changes and conditions, allowing for neural networks to reconfigure themselves. Though this brief review sheds some light on the functional mechanisms of electrical synapses, the entire scope of the information is still incomplete and poorly understood making it difficult to explain the specific roles these synapses play in the nervous system. Growing evidence now suggests that synaptic transmission involves the coupling of electrical synapses as well as chemical synapses as …
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