BISC 307L 2nd Edition Lecture 7 Current Lecture Action Potential Conduction In 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 the axon 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 you get 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 it out 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 back to 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 closes them Opening happens immediately closing also starts immediately upon depolarization but it builds 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 are flowing out but once you get further out it gets back to resting potential Look behind the AP it is 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 will take 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 Conduction Vertebrates 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 membrane to depolarize it and trigger an AP In the internodal region the membrane is wrapped so the resistance of that whole structure to current flowing out across
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