Generation and transmission of neural impulses
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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.