Neurotransmitter release
Video transcript
Voiceover: 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 have 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 that 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 than inside the neuron, so it will flow in and increase
the concentration of calcium here inside the axon terminal. And you just draw a couple
of these, but there are lots of them, of course. The increase 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 membranes 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 will
be lots of neurotransmitter now in the synaptic cleft
where there wasn't before. And remember, I've drawn this
too large, it's actually a very small distance, so the
neurotransmitters has no problem diffusing across and binding
to its receptor on the postsynaptic membrane of the target cell. Now, recall 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 duration that
neurotransmitter is present in the synaptic cleft and
the way that that happens is that an increase frequency
of action potentials reaching the axon terminal will
cause more openings of these voltage gated calcium
channels, so they usually more calcium will flow into the
axon terminal and an increase concentration 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
the 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 the information
contained in the frequency and duration of a train of
action potentials is converted into the amount and duration
of neurotransmitter present in the synaptic cleft and
that information is passed on to the target cell by a
neurotransmitter binding to the receptors and the number of
receptors that binds to and the duration of time the
neurotransmitter is bound to receptors is related to the
amount and 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 the normal processes that push calcium
out of the neuron 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.