So how did you do? Did that make sense? Did you have to back up, a few minutes in the tutorial and look back at the data that we just went through? Well, if you did then that's great but if you, found it a bit confusing then hang with me now. Let's look at a similar experiment. only now taking it to the level of the passage of current through a single ion channel. Okay, so let's see what happens when we focus again on the nicotinic acetylcholine receptor and the movement of ions through that receptor channel. So what we're going to do is to consider the flow of current that can be recorded while this membrane is clamped at different membrane potentials. So let's being at minus 100 millivolts. So what can be recorded is a strong, inward current that results in a depolarization of the muscle fiber. So we're still at the end plate so we're going to call this an EPP for an end plate potential But it's just the membrane potential of the muscle fiber. OK, and it's depolarizing relative to rest. So, why is it depolarizing? What explains the strong, inward current? Well, sodium ions are going to, pass through this ion channel. This channel, because of its molecular properties, is permeable to both sodium and potassium, but at minus 100 millivolts, there's a huge driving force for sodium but almost no driving force at all for potassium, because 100 millivolts is very near the Nernst equilibrium potential for potassium. So, making the inside of the cell that negative is going to draw in the positively charged sodium ions which are going to want to flow into the cell anyway because of its concentration gradient. So if we have, slightly, less, polarization minus 90 millivolts maybe now we are just. a little bit less polarized than the neurist equilibrium potential for potassium. What we'll find is still strong sodium influx, so we still record a net inward current and significant depolarization significant end-plate potential. But notice now, that we also might observe a small efflux of potassium. Much less than the stronger influx of sodium, which is why we're still recording an inward current, but it's not quite the same magnitude that we say at minus 100 millivolts. So now let's clamp this membrane at 0 millivolts. And at 0 millivolts what we observe is no end plate current and with no end plate current we have no en plate potential. Now that doesn't mean that there's no movement of ions in fact sodium. Is still being driven into the cell. we are nowhere near the nearest equillibrium potential for sodium within the cell so sodium is still going to be drawn in because of the electrical gradient and also because of its concentration gradient. In a similar fashion but in the opposite direction potassium is going to flow out of the cell. Potassium will be repelled by this depolarization of zero millivolts and it's going to flow down its concentration gradient. So there are ionic fluxes but the key is is that thre is no net ionic flux. So the net current is zero. And in essence, at zero millivolts the influx of sodium ions is exactly counterbalanced by the eflux of potassium ions. That's why, in fact, there is no end plate current recorded. Okay, lastly, let's depolarize the cell to plus 70 millivolts. So, for muscle fiber, +70 millivolts, is getting very close to the nearest equilibria potential for sodium, given the concentration of sodium inside and outside of the muscle fiber. So, at +70 millivolts, the driving force for sodium Is essentially nil because we are very close to that Nernst equilibrium potential even though the conductance of the channel is wide open. However, we are far from the Nernst equilibrium potential for potassium. So there's a huge driving force for potassium. That's going to allow potassium to rush out of the cell It's diffusing down its concentration gradient but it's also being repelled by the positivity that we have imposed upon the inside of this muscle fiber. So, as a consequence, what we can record then is an outward current. And that efflux of positive charge leads to a hyperpolarizing end plate potential. Now, let's return to the brain. And let's consider, the inotropic receptors that are found at glutamatergic synapses. Now, as we've already seen, there are at least three types of glutamate receptors. we'll focus primarily on two of them. The receptor for AMPA or ampa. And the receptor for NMDA. When ampa is placed on a typical neuron in the brain, what we see is a very quick current that flows into the cell. typically leading to an excitatory postsynaptic potential, so we call this an excitatory postsynaptic current. So a very sharp very large inward current that happens very rapidly when when AMPAs are apply, applied to a neuron. If the compound N-methyl-D-aspartate is applied What we find is also an inward current but it's not as large an amplitude and it's a little bit slowly developing and it lasts a bit longer than the very fast, short ampicurrent. So these different profiles reflect the activation of different types of receptors both of which bind the andogynous ligent Glutamate. So, let's look in a bit more detail now at the behavior of one very important ionotropic receptor for glutamate, and that's the NMDA receptor for glutamate. So, here in the upper right, we see an illustration of this receptor. It is a receptor that is formed by the aggregation of typically four subunits. It has a binding site for glutamate, on some of the subunits, and sites that bind other types of molecules. I mentioned that glycine Is a co-transporter in the central nervous system. There's a binding site on these sub-units for glycine. so, in order for the channel pore to open, both glutamate and glycine have to interact with their receptors on this channel complex. But, there are other binding sites as well. In particular, there is a site deep within the pore of this channel where magnesium ions can aggregate. And, this is a problem for the conductance of the channel, because even if the pore of the channel is opened, by the binding of glutamate with the co-agonist glycine, current is not going to flow through this channel if magnesium is present. Magnesium essentially blocks the flow of current through the channel. However it's possible to get rid of that magnesium and the way that happens is by depolarising the membrane, so if the inside of the cell becomes, more positive. That positivity can expel this magnesium ion out from the pore of the channel. When that happens, now the cations that can pass freely through the selectivity filter of the channel are free to do so. And those include sodium and calcium ions, that can enter the cell, and potassium ions that can leave the cell. So this is a ion channel that's permeable to cations but not particularly selective for one kind of cation or another. And importantly The conductance through this channel is influenced by magnesium ions. When magnesium is bound, there's no conductance. When magnesium is expelled, then we see conductance. This relationship is charted below in the graph that relates the excitatory post-synaptic current. To the potential of the post synaptic membrane and what we see is that when there is the presence of the transmitter glutamate and magnesium ions there is little to no excitatory postsynaptic current. Record it at hyper-polarizing potentials. But, as the membrane potential begins to depolarize, when we get into this range here, now we begin to see some excitatory post-synaptic current passes. What happens is, the magnesium block is relieved. And positive cations, sodium and calcium, can enter the cell. But notice that as we begin to depolarize towards zero millivolts the current reverses. Now look at the behavior of the excitatory post-synaptic current if we remove magnesium from the solutions bathing the cell. In that situation, we have a relatively linear relationship between current and membrane potential. Which is what, one might expect, if there were not this blockade effect, due to the binding of magnesium ions to the inside of the pore of the ion channel. Now, before we leave our NMDA receptor channels for the present time. I just want to highlight the fact that the ion channel allows for the permeation of calcium. Now this may not seem very important at this juncture of the course, but it is soon to be extremely important. Because, as we'll see in a couple of tutorials. Calcium is critical for mediating synaptic plasticity and the influx of calcium whether we have a rapid significant rise intracellular calcium or perhaps a slower and less significant rise in intracellular calcium could make all the difference in the world. Between strengthening a synaptic connection and weakening that synaptic connection. And as we now understand the NMDA receptor channel is a key coincidence detector of post synaptic depolarization in pre-synaptic activity, which is critically important for understanding how synapses increase or decrease their synaptic strengths. So we'll come back to the NMDA receptor. When we talk about synaptic plascisity , we're specifically focusing in on the important role of calcium.
