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Video transcript
In this video, I wanna talk about how neurotransmitters are removed from the synapse. In other videos, we've talked about how action potentials travel down axons, and the information that's contained in action potentials, is really contained in the frequency of action potential firing, and the duration of a train of action potentials. And when the action potential reaches the axon terminal, at the end of the axon, neurotransmitters are released into the synaptic cleft to bind to neurotransmitter recepters on the target cell. But if a neuron is firing action potentials very frequently, if there are large numbers of action potentials reaching the axon terminal, then the rate of neurotransmitter release into the synapse, may exceed the rate that neurotransmitter can just passively diffuse out of the synapse, so that diffusion is the first method by which neurotransmitter can be removed from a synapse, "Diffusion." But, that only works if the neuron is firing action potentials at a slow frequency. At a fast frequency, diffusion won't be enough, and there'll be a build up of neurotransmitter in the synapse. And this would be a problem, because if the neurotransmitter is just lingering in the synapse, then neurotransmitters bound the neurotransmitter receptor, most of the time, and the information contained in the frequency, and the duration of trains of action potentials, wont' be able to be transmitted to the target cell. The synapse will basically not be functional, to communicate additional information. Therefore, neurotransmitter may need to be actively removed, instead of just through passive diffusion, to clear out the neurotransmitter from the synaptic cleft. And it turns out that there are several ways that this happens. The first of these active methods, or the second method to remove neurotransmitter from the synapse, are enzymes that can break down the neurotransmitter in the synapse. So certain synapses contain enzymes that'll actually break down the neurotransmitter into its component parts, which are no longer able to stimulate the neurotransmitter receptor. So they're removing active neurotransmitter from the synapse. The next active method, is that some pre-synaptic membranes contain special pumps, special active transport channels, that actively pump back in the neurotransmitter, into the axon terminal, where it's often recycled, to be used for a subsequent round of neurotransmission, by being released back into the synapse. So these pumps are called "Re-uptake Pumps," because they take the neurotransmitter back into the axon terminal where it came from in the first place. By doing so, they remove the neurotransmitter from the synaptic cleft. Another big method of actively removing neurotransmitter from the synapse, is by "Astrocyte Endfeet." So the astrocytes in the central nervous system put their endfeet on lots of the synapses in the central nervous system, and they also have pumps at a lot of these synapses, that can actively pump the neurotransmitter out of the synapse, into the astrocyte. And sometimes it'll just be broken down, or used in the astrocyte, or sometimes the astrocyte will actually transfer some of the substances of the neurotransmitters back into the axon terminal of the neuron, where it will be recycled, and used again for neurotransmission. So all of these different methods allow the synapse to basically be rapidly turned on and off. 'Cause neurotransmitter can be rapidly released into the synaptic cleft, and then it can be rapidly cleared out, so that the synapse is capable of conveying more information from the neuron, to the target cell.