Neurotransmitter release

in this video I want to talk about how neurotransmitter is released at the synapse in the last video we went over the structure of a typical chemical synapse with an axon terminal like I've drawn here in green that has synaptic vesicles full of neurotransmitter and we talked about how on the postsynaptic membrane of the target cell there are receptors for those neurotransmitter molecules but the question is how do the neurotransmitter molecules get out of these synaptic vesicles in the axon terminal to cross the synaptic cleft and bind to their receptors to understand neurotransmitter release we need to talk about this new type of ion channel this is a voltage-gated calcium channel we talked about voltage-gated sodium and potassium channels when we talked about the action potential but at the axon terminal there are these voltage-gated calcium channels that play a big role in neurotransmitter release when the action potential comes down the axon and reaches the axon terminal the action potential will change the membrane potential at the axon terminal and it will open these voltage-gated calcium channels when these voltage-gated calcium channels open calcium will flow in to the axon terminal because that's at a much higher concentration outside the neuron and inside the neuron so it will flow in and increase the concentration of calcium here inside the axon terminal let me just draw a couple of these but there are lots of them of course the increased concentration of calcium inside the axon terminal when these voltage-gated calcium channels are open are going to cause changes to proteins on the synaptic vesicles and proteins on the presynaptic membrane of the axon terminal and they're going to cause them to interact and fuse so let me just erase these little bits of membrane here and draw how these are actually fusing together so that now the inside of the synaptic vesicle is actually in communication with the outside of the neuron with the synaptic cleft and then by diffusion the neurotransmitter molecules will exit the axon terminal and they'll flow out into the synaptic cleft and there'll be lots of neurotransmitter now in the synaptic cleft where there wasn't before and remember that I've drawn this too large it's actually a very small distance so that neurotransmitter is no problem diffusing across and binding to its receptor on the postsynaptic membrane of the target cell now recall the we talked about the information contained in action potentials is really contained in the frequency and the duration of a train or a series of action potentials being fired down the axon of the neuron well that information is now going to be converted into the amount and the duration that neurotransmitter is present in the synaptic cleft and the way that that happens is that an increased frequency of action potentials reaching the axon terminal will cause more opening of these voltage-gated calcium channels so that usually more calcium will flow into the axon terminal and an increased concentration of calcium in the axon terminal will cause more synaptic vesicles to fuse with the presynaptic membrane so that a greater amount of neurotransmitter is released into the synaptic cleft and a longer duration of a train of action potentials will cause neurotransmitter release to occur over a longer period of time so there's a longer duration of neurotransmitter being present in the synaptic cleft so this is the way that the information contained in the frequency and duration of an train of action potentials is converted into the amount in the duration of neurotransmitter present in the synaptic cleft and then that information is passed on to the target cell by a neurotransmitter binding to the receptors and the number of receptors that it binds to and the duration of time that neurotransmitters bound to receptors is related to the amount in the duration of neurotransmitter in the synaptic cleft and when the train of action potentials stops firing the voltage-gated calcium channels will close calcium will stop flowing into the axon terminal and then the normal processes that push calcium out of the neuron will will quickly lower that concentration of calcium in the axon terminal and synaptic vesicles will stop fusing with the presynaptic membrane and neurotransmitter will stop being released