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The Action Potential and Synapses

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1. when an AP reaches the presynaptic nerve terminal, it causes voltage gated Ca2+ channels in the presynaptic membrane to open. these are N-type Ca2+ channels which differ form the L and T type Ca2+ channels. these channels are localized to the membrane of the presynaptic terminal and they open rapidly in response to membrane depolarization
2. when the channels open, Ca2+ enters the presynaptic terminal. intracellular Ca2+ is very low at rest, but locally increases rapidly with influx of calcium from the extracellular fluid
3. this Ca2+ causes synaptic vesicles to fuse with presynaptic membrane.
4. these transmitter molecules then diffuse to nearby postsynaptic membrane where they bind to specific postsynaptic receptors. distance between pre and post synaptic membrane is very small and diffusion rapidly moves the transmitter across the synaptic cleft. space is larger between autonomic neurons and their targets.
5. binding of transmitter molecules to receptors embedded in the postsynaptic membrane cause conformational changes in these receptors that usually directly or indirectly cause the opening of ionic channels in the postsynaptic membrane. These receptor-channels are only open while the transmitter molecule is bound
6. in excitatory chemical synapses, the channels taht open cause a transient depolarization of the postsynaptic membrane. if this is sufficiently large then an AP is triggered in the postsynaptic cell. at the NMJ the postsynaptic potential is normally very large to always produce an AP in the muscle fiber. excitatory synapses between neurons, many presynaptic inputs must arrive to produce postsynaptic AP