Events at the Synapse

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We have examined the anatomy of a synapse. Now let’s look at the details of synaptic transmission. [Sparking sound] An action potential in the axon terminal causes voltage-gated calcium channels to open and calcium to enter the terminal. Now let’s watch the calcium channel in action. [Sparking sound] The presence of calcium inside the cell causes the synaptic vesicles to fuse with the membrane. Each vesicle releases a fixed amount of neurotransmitter into the synaptic cleft. Neurotransmitter diffuses across the synaptic cleft. Neurotransmitter binds to a receptor on the postsynaptic neuron where it can act directly or indirectly. Chemically gated ion channels remain open as long as the neurotransmitter is bound to the receptor and are not sensitive to changes in the membrane potential. Synaptic current, or ion movement through chemically gated channels, may depolarize or hyperpolarize the neuron. This example illustrates depolarization of the postsynaptic neuron. Synaptic transmission ends when the neurotransmitter dissociates from the receptor and is removed from the synaptic cleft. Most often, the neurotransmitter is pumped back into the presynaptic terminal or into nearby glial cells. Here we illustrate the neurotransmitter glutamate being pumped back into the presynaptic terminal. In some cases, the neurotransmitter is broken down by enzymes and the break down products are taken up by the presynaptic cell. The neurotransmitter acetylcholine is an example of this process. When break down products are transported into the presynaptic terminal, they are used to resynthesize neurotransmitter. The neurotransmitter which is being returned to the terminal is repackaged into vesicles for storage and subsequent release. The mechanism by which neurotransmitters are returned to the terminal is specific for each neurotransmitter and can be selectively affected by drugs.