The Synapse

by Jason Amores Sumpter
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what happens when the action potential reaches the end of its journey, When it makes its way to the ax on terminal and is ready to be converted into a chemical signal? Well, the ax on terminal will have formed a junction with another sell. This junction is called a synapse. It's that connection between neurons that allows them to pass signals along signals are almost always going to be traveling from the pre synaptic sell to the post synaptic cell. That is the cell that had the action potential moving through it to the cell on the other side of the synapse. Now, I say almost always, because there are some very notable exceptions, including the Endo Cannabinoid system, which throws these rules out the window. Also, the gas nitric oxide, which can act as a neurotransmitter, basically diffuses in any direction it pleases. It is not bound by these restrictions, however. We're not gonna be bothering really with those exceptions. So you can basically safely assume that synaptic transmission will, you know, basically always go from the pre synaptic sell to the post synaptic cell. It's not until you get into like, more advanced neuroscience stuff where you have to actually worry about those exceptions. So signals, as we've said, can be chemical neural transmitters. Right? The neuron will release neurotransmitters into the synapse. However, not all synapses are chemical synapses, some are electrical and we've actually seen these in other places. These air gap junctions, right? Those protein channels that connect cells together actually will allow the action potential to pass directly from one cell into another cell. Those are electrical synapses. However, we're going to be looking at chemical synapses, and these chemical synapses will contain voltage Gated calcium channels, Thes voltage. Gated calcium channels are critical to neurotransmitter release. So how does this actually all go down? Well, of course you're gonna begin with the action potential, finally making it to the acts on terminal of the pre synaptic cell. That is so here we have our sodium and potassium channels that allow the action potential to move its way along the ax on until it finally reaches the terminal. Here it's going to cause deep polarization and that deep polarization opens thes voltage. Gated calcium channels, thes voltage gated calcium channels will allow calcium ions into the cell. Thes calcium ions act as a signal to synaptic vesicles. Now, in the acts on terminal, you will have lots of these synaptic vesicles, and they are basically going to just be hanging around storing neural transmitters nt. So what? I mean, they're neural transmitters. When they get the calcium signal, they actually bind to the membrane of the acts on terminal and fuse with it. And in this process, they release. They're nure Otranto emitters into what's called the synaptic cleft, this space between the acts on terminal of the pre synaptic cell and the membrane of the post synaptic cell. So these neuro transmitters will diffuse across that gap that synaptic, cleft and bind to receptors on the post synaptic membrane. And in binding, these receptors will actually see the signal be transducer it to the other cell. Now there are going to basically be two kinds of receptors will see on that post synaptic membrane, their eye on a tropic receptors and Motaba tropic receptors, and you don't really need to worry about knowing these terms. I'm throwing them out because it just makes it easier to describe two different categories of post synaptic membrane receptors. So these I on a tropic receptors basically are just membrane receptors that act by opening an ion channel. Pretty simple. So by that, by virtue of that, essentially they are going to be ligand gated ion channels which we talked about before they open in response toe ligand binding like narrow transmitters. Narrow transmitters are a ligand. So here we have a neurotransmitter called acetylcholine. It will bind to this I on a tropic receptor which will open it and allow these ions to move in and out of the cell. Now, on the flip side, you have Motaba tropic receptors. These act through second messengers and they're often going to be G protein coupled receptors. So the neuro transmitter will bind, and then a bunch of stuff is gonna happen in the cell, and they can have a wide variety of effects. For example, they can lead Thio Uh, you know, increasing numbers of receptors on the membrane or they can actually also lead thio ions moving in and out of the cell. They're much more varied, whereas the eye on a tropic receptors are very cut and dry. Like in Bynes ions. Either come in or out of the cell with that, let's go ahead and flip the page