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McGill University
PHGY 313
Sebastien Breau

Types of Synapses  Electrical Synapses o Electrical synapses are relatively simple in structure and function, and the allow the direct transfer of ionic current from one cell to the next. o They occur at specialized sites called gap junctions  The membrane of two cells is separated by only about 3nm and this narrow gap is spanned by clusters of special proteins called connexins. Six of these combine to form a channel called a connexon and two connexons combine to form a gap junction channel  It allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other.  The pore is relatively large (1-2 nm) and is big enough for all the major cellular ions and many small organic molecules to pass through  Most allow ionic current to pass equally well in both directions (unlike the majority of chemical synapses).  Cells connected by gap junctions are electrically coupled o Transmission at the electrical synapses is very fast and, if the synapse is large, fail-safe.  An action potential in the presynaptic neuron can produce almost instantaneously, and action potential in the postsynaptic neuron. o Also occur in the vertebrae brain and is common in every part of the mammalian CNS o When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a small amount of ionic current to flow across the gap junction channels into the other neuron  Causes a postsynaptic potential (PSP) in the second neuron o Since most electrical synapses are bidirectional, when the second neuron generates an action potential, it will in turn induce a PSP in the first neuron o The PSP generated by a single electrical synapses in the mammalian brain is small and may not by large enough to trigger an action potential in the postsynaptic cell  One neuron usually makes electrical synapses with many other neurons so several PSPs occurring simultaneously may strongly excite a neuron (synaptic integration) o The precise roles of electrical synapses vary from one brain region to another. Chemical Synapses  Most synaptic transmission in the mature human nervous system is chemical  The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 20 – 50 nm wide. o It is filled with a matrix of fibrous extracellular proteins. o One function is to make the pre- and post-synaptic membranes adhere to each other o The presynaptic side of the synapse is usually an axon terminal and typically contains dozens of small membrane-enclosed spheres called synaptic vesicles.  These vesicles store neurotransmitter, the chemical used to communicate with the postsynaptic neuron.  Many axon terminals also contain larger vesicles called secretory granules  They contain soluble protein that appears dark in the electron microscope and sometimes called dense-core vesicles. o Dense a
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