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.