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USC BISC 307L - Action Potentials and Synaptic Transmissions
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BISC 307L 2nd Edition Lecture 7Current LectureAction Potential ConductionIn the segment of axon shown below, the middle is undergoing an AP travelling to the right. The band in the center is the region of the membrane where sodium channels are open, and sodium is entering the cell down a strong electrochemical gradient. So if you look at the plus and minus signs across the membrane, you would notice it is positive inside with respect to the outside, which is expected if you’re considering the zone where the sodium channels are open. Far ahead of the action potential, and far behind it, it is the opposite and negative inside and the membrane is at rest.Loopy arrows going to right = sodium current that came in. Current flows down length of theaxon, and flows out of the resistance and capacitance periodically. The primary open channel is a leakage K channel. Membrane has a capacitance represented by + and – signs. As charge flows down the axon and across the membrane through open potassium channels and across membrane capacitance, it depolarizes the membrane. Initially, there were leakage K channels were the only things open. But there also exist voltage gated sodium channels that, when depolarized enough, start flipping open. If there are only a few, the current coming in leaks back out of the K channels. The open K channels allow for repolarization back to resting potential. But outward flowing current keeps coming because of the AP to the left. More depolarization occurs, more channels open, and youget to a point where the inward sodium current just exceeds the outwards leakage current – that is threshold, the point where current is coming in faster than open channels can carry itout. Have a net inward current, which depolarizes more, speeding things up, and you get an explosive positive feedback cycle. When that happens, the AP has moved (would be off to the right). Since the channels are uniformly distributed down the length of the axon, this happens continually in a wave, depolarizing the membrane ahead of it as it goes, drawing upon the stored energy that the Na/K pump put in the system by pumping out the Na, and that’s why its regenerative. Feeds on itself until it gets to the end.Behind the AP, current is flowing to the left and there are more open channels than normal. Normally, the only open channels are potassium leakage channels. But now there are voltage gated K channels which were opened when you depolarized, and they are just lingering in an open state. (can see according to previous slide that vg-K channels open more slowly and close more slowly). Get a period where you have Na gates closed, but K still closing. That explains the dip underneath resting potential. Can see the K flowing out through the membrane in the picture. Not through capacitance but through open K channels immediately following AP. Current flowing out behind the membrane doesn’t depolarize it, it repolarizes it (sending it backto negative values). Remember that membrane potential is more affected by more permeable ions, in this case it is permeable to K so that explains the decrease of the AP toward negative values after it hits the peak in the previous slide. Way off to the left where AP hasn’t been for a while, everything is back to rest. Also not shown is sodium inactivation – depolarization has two effects on Na channels. It opens, and then closesthem. Opening happens immediately, closing also starts immediately upon depolarization, but itbuilds up more slowly. There is a period of time where Na permeability is high, but the falling of sodium permeability(previous slide) is due to automatic shutoff of these channels. After the peak of the AP, the Na is inactivated, and has closed. So to the left of the band, K channels are still open AND we have inactivated the Na voltage gated channels, a state in which they linger before they reset themselves. As AP travels down the axon, currents travelling ahead of it are continually depolarizing membrane and triggering new AP, and behind it, there is a zone following it where it is in a refractory state (extra K channels are stabilizing the membrane near Ek, and Na gates are inactivated and can’t be opened again for a while) where it is inexcitable for a while. This explains why when the AP gets to the end of the axon, it just stops and doesn’t bounce back (immediately behind it is a stretch of membrane that can’t be excited again). Explains why when you stimulate an axon with a current, it can never fire any faster than the refractory period dictates. So there is an upper limit. If you were to plot membrane potential as a function of distance, you would get the bottom graph. Depolarized where Na is coming in. Depolarized also to the right where currents areflowing out, but once you get further out it gets back to resting potential. Look behind the AP, itis repolarizing, and due to so many open K channels it is even more negative and closer to Ek than resting potential. You get this graph of membrane potential vs. distance at one instant of time. If you compress and flip, you get the graph on the previous slide of voltage as a function of time. Trivial but instructive connection between these two graphs. Top right = Raxoplasm is inversely proportional to the square of the radius. It explains why big axons conduct faster – the bigger the axon, the bigger the radius, the lower the axoplasmic resistance. Current flowing down the axon or spreading across the membrane to depolarize, willtake the path of least resistance and will choose to go down the axon if it has lower resistance, which it will if it is big. When it goes out a further distance before it goes out of the membrane, it depolarizes the membrane farther out to the right, and therefore the action potential travels faster. In the invertebrates, there is a very predictable relationship between diameter/radius of axon and its conduction velocity. The best that invertebrates can do to have AP’s that conduct fast is to make them gigantic. See giant axons in invertebrates who need escape responses. Myelinated Axons – Saltatory ConductionVertebrates evolved a trick to make them conduct fast without being very huge – the myelin sheath.The gap between sheaths is called the Node of Ranvier. Myelin is a glial cell that wraps itself around the axon.Two important electrical consequences - -Increases resistance of the wall of the axon (current would normally flow out across


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