Drop off of potential on the postsynaptic element

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drop-off of potential on the postsynaptic element; since there are a large number of channels, this is not the case. The action potential is propagated fully down from the pre- to the post-synaptic element. There are a number of types of synapses. There are the rare electrical synapses, where cells are “electrically coupled” and ions flow from cytoplasm to cytoplasm. The transmission is very fast in electrical synapses. This is not very common, and is usually only seen in embryos. The most common type that this class discusses is chemical synapses. Before getting into how they operate, it is important to understand there are all kinds of receptors, channels, etc. that are necessary to propagate an action potential in the post-synaptic element. (See question below). Synapses in the CNS are categorized based on their connections. Axodendritic (the most common), are axon to dendrite connections. Axosomatic is a connection of the axon to the cell body. Axoaxonic is axon to axon, and dendodendritic is dendrite to dendrite. Additionally they can be classified by their shape. Gray’s Type I synapses are asymettrical, excitatory synapses. Gray’s Type II are symmetrical and inhibitory. One final type of synapse is not to another axon, but at the neuromuscular junction. It is interesting to note that studies of the NMJ helped establish in large part the principles of synaptic transmission. 12) Explain the principles of synaptic transmission.
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For the sake of explanation, we will use the chemical synapse at the neuromuscular junction, which utilizes the neurotransmitter acetylcholine (ACh). Synaptic transmissions tarts when the nerve impulse reaches the presynaptic axon terminal. Depolarization of the presynaptic membrane leads to a sequence of events that lead to transmitter release and activation of receptors on the postsynaptic membrane. Synaptic vesicles exist in pools, that are either tethered to the cytoskeleton, or are free in the cytoplasm. Some of the free vesicles travel to the plasma membrane and dock, and a series of priming reactions prepare the vesicular and plasma membranes for fusion. The fusion occurs through the drawing together of the SNARE proteins on both the vesicle and the presynaptic membrane (note: this is an energy intensive process, and is the primary reason there is such a large concentration of mitochondria in the axon terminal.) When the axon terminal is depolarized, voltage gated calcium channels open, and calcium rushes into the axon terminal. Some of the calcium binds to a protein on the synaptic vesicle membrane called synaptotagmin. When calcium binds to synaptotagmin on the synaptic vesicles nearest the active zone, the vesicles are drawn even closer to the presynaptic membrane. The vesciles fuse with the axon terminal membrane and release transmitter cargo into the synaptic cleft. Some of the transmitter molecules bind to receptor molecules in the postsynaptic membrane.
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