Synaptic Transmission (continued)

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Synaptic Transmission (continued)

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 8 Current Lecture Steps in Chemical Synaptic Transmission 5 steps: On the left is the presynaptic terminal. The space between the structures is the synaptic cleft. On the right is the postsynaptic terminal. 1. Synthesis of transmitter T from precursor P. Enzymatic synthesis. The enzyme that catalyze this (can be one or more than one) are made in the cell body of the neuron and transported down the axon via the axoplasmic transport system of cytoskeleton, microtubules, etc. Only in the cell body are there organelles necessary for protein synthesis like the nucleus, ribosome, golgi apparatus, etc. Things that get transported include proteins and organelles like mitochondria. Transmitter synthesis is local(usually, except for peptide transmitters). Keep in mind that each of these steps is a place where transmitter release can be modulated (potential targets of therapeutic intervention when you want to mess with cellular function) 2. Storage into vesicles. Precursor of transmitter is free at first, then synthesis, then packaged into vesicle. Green dot on vesicle is an active transporter. Concentration inside is super, super high so you need an active transporter to get it in. 3. Release of the neurotransmitter. Release occurs when action potential comes down the axon and invades the terminal. Neurotransmitter release The action potentials are coming downwards. Do they actively invade the nerve terminal and all of tis branches? It depends. In some synapses, especially when the presynaptic terminal is huge, there are vg-Na channels all the way into the branch of the terminal to ensure that the AP invades the entire terminal. If the presynaptic terminal is small, it doesn’t matter much because even if the AP stops actively conducting before the branch, you only need a local depolarization and the passive currents ahead of the AP is enough to open the bottom vg Na channels. If the length constant of this axon is long relative to the distance between end of axon and presynaptic membrane, you don’t need an AP. Presynaptic membrane – at synapse, the membrane directly opposite the postsynaptic receptor is highly specialized for transmitter release. It is different from the membranes on the sides. It has blue blocks which are apparatus necessary for vesicles to dock at the membrane, so that with the proper signal they can fuse and release their contents. There are also voltage gated Ca channels shown in green that are right next to the release sites. As depolarization invades terminal, VG Ca channels open and Aa enters the cell down a strong gradient. Rises in internal Ca con is toxic to cell, so we want a localized increase in Ca conc. But the increase causes exocytosis of the NT, which diffuses out rapidly. Right = vesicle. Upper half = traditional classical understanding of exocytosis. It fuses with membrane and becomes part of the membrane, and collapses into it and retains its integrity as an island of vesicle membrane, and it is internalized once again and recycled. Insertion of new membrane pushes ...

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