In this video, I want to talk about the structure of the synapse. Synapses are where neurons contact and communicate with their target cells, and the word "synapse"
comes from Greek words meaning "to clasp together." So let me start by showing where synapses happen, so first I'll draw the soma of a neuron in red, and a few dendrites, these branching processes
of neurons in blue, one dendrite and here's another dendrite. And then I'll draw the axon
of the neuron in green. It's this long unbranched
process, until it reaches the end, and then it branches into multiple terminals, and
I'll just draw a few here, but it can have many, many terminals. And then let me just
draw some target shapes to represent the types
of target cells that a neuron may contact with the synapse. And these cells may be another neuron, they might be a muscle
cell, or they may be a gland cell, and some neurons even have axon terminals that end on blood vessels to secrete substances
into the bloodstream, called hormones. So synapses are these spots, where the axon terminals of a neuron are contacting their target
cell, and there are a couple of types of
synapses, one of which has a gap like I've drawn here, although it's actually a much
smaller gap than I've drawn, and the other one doesn't have a gap. They are physically connected. The type of synapse
that has a gap is called a chemical synapse, because
it releases molecules, or chemicals, at the synapse that cross from the axon terminal to the membrane of the target cell. The other type of synapse is called an electrical synapse. And with this type of synapse, the cells are actually physically connected, so that the axon terminal physically connects with the membrane of the target cell, and there are special channels
called "gap junctions" that actually let the inside of the neuron communicate with the
inside of the target cell. The cytoplasm of the two
are really connected, and ions can flow directly from the neuron into the target cell. In this set of videos,
I'm just gonna talk about the chemical synapses,
where there is a gap, because they are far and away more common than electrical synapses, which are fairly rare, at least in humans. Now a typical human
neuron can have a massive number of connections through synapses. It can connect thousands
of different target cells through its axon
terminals, and the typical neuron receives information through thousands of synapses. Most of those synapses
come in to the dendrites, so that the axon of another
neuron is contacting and communicating with the
dendrite of this neuron, and there could be many, many synapses on a dendrite, and part of
the reason it's branched, is just so that it can
have more surface area, to form more synapses. So that for most neurons,
most of the synapses are actually coming in
through their dendrites. However most neurons also do get a smaller number of synapses coming in to the soma. And there are even synapses onto the axon, but usually not just
anywhere onto the axon, it's usually onto the
axon terminal, so that this axon comes in, and
its axon terminal is synapsing on the axon
terminal of this neuron. And it could be the same
with this axon terminal, it could have synapses, and this axon terminal right here. And so try to imagine
thousands of synapses just literally covering this whole neuron, and that it in turn is
connecting with thousands of target cells through synapses. And you can just imagine how complex the information is that's flowing into and out of the neuron from other parts of the nervous system. But now let's zoom in,
and let's look at the structure of an individual synapse, like for instance this synapse right here. Let me start by drawing
a big axon terminal, so I'll just blow up
that axon terminal and make it really big here in green, and then I'll just draw
a target type of shape to represent the target cell, which again could be another neuron,
could be a muscle cell, or it could be a gland cell. And then in the central nervous system, covering most of the synapses are the end feet of astrocytes. Now let me just draw that in purple, and I'll actually just label that, let me just write "astrocyte,"
and this would be one of the end feet of the astrocyte that, in the central nervous
system are just plastered all over synapses. So for a chemical synapse like this, there is a gap here, between the membrane of the axon terminal of the neuron, and the membrane of the target cell. And the gap is actually
much smaller than this, but I just needed a little room to draw. It's a very, very small
gap, but the cells are not actually physically touching each other. There is a gap. And we call this gap the synaptic cleft. Let me just write that
out - synaptic cleft. And that's the space between the neuron and the target cell,
and then we have names for this piece of membrane over here, and this piece of membrane. This membrane facing the synaptic cleft, on the axon terminal
of the neuron, we call the presynaptic membrane, presynaptic membrane is right here, because it's before the synaptic cleft. And then this piece of
membrane on the target cell facing the synaptic cleft is the postsynaptic membrane, postsynaptic membrane, and
that's right over here. And so it's presynaptic
because it's before the synaptic cleft, and it's postsynaptic because it's after the synaptic cleft. Just on the inside of
the presynaptic membrane are vesicles, which are little membrane-enclosed bubbles inside the cytoplasm of the neuron, and these are called synaptic vesicles. Synaptic vesicles - that's each of these bubble-like structures just inside the presynaptic membrane of the neuron. And these synaptic vesicles are full of molecules called neurotransmitter. Let's draw a couple of dots in here to represent these molecules,
and they are called collectively neurotransmitter,
because they transmit information from the
neuron to the target cells, and all of these
molecules are collectively called neurotransmitters. And there are different
types of neurotransmitters, that we'll get into on other videos. On the postsynaptic membrane are receptors that are specific for the neurotransmitter in the synaptic vesicles. The neurotransmitter will
fit like a key in a lock to these neurotransmitter receptors on the postsynaptic membrane. And in the next video we'll talk about how the neurotransmitter
and the synaptic vesicles is released from the presynaptic membrane to cross the synaptic
cleft, and bind to its receptors on the postsynaptic membrane.