Synaptic Transmission

Voltage-gated Ca2H channels in the membrane of the bouton regulate transmitter release

When a nerve impulse reaches the bouton, the voltage reversal across the membrane produced by the impulse (called depolarization) causes voltage-gated Caz+ channels to open in the plasma membrane of the bouton. The influx of Ca2+ from the extracellular space causes the synaptic vesicles to migrate to, and fuse with, the presynaptic membrane, thereby releasing the neurotransmitter into the synaptic cleft by exocytosis. The neurotransmitter then diffuses across the synaptic cleft. At the same time, the presynaptic membrane of the bouton that released the neurotransmitter quickly forms endocytotic vesicles that return to the endosomal compartment of the bouton for reloading with neurotransmitter. Meanwhile, specific re ceptors on the postsynaptic membrane bind neurotransmitter, causing ligand-gated Na+ channels in that membrane to open, allowing Na+ to enter the neuron. This ion flux causes local depolarization in the postsynaptic membrane, which in turn, in favorable conditions (sufficient amount and duration of neurotransmitter release), can cause voltage-gated Na+ channels that are present in the same area to open, thereby generating a nerve impulse. The firing of impulses in the postsynaptic neuron is due to the summative action of hundreds of synapses.

The chemical nature of the neurotransmitter determines the type of response at that synapse in the generation of neuronal impulses

The release of neurotransmitter by the presynaptic component can cause either excitation or inhibition at the postsynaptic membrane.

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