Neuromuscular block (1956), pt 2 of 2

so now observe what happens when an impulse comes along the nerve note first that the outside of the nerve is positively charged relative to the inside that is until an impulse passes when the outside briefly negative becomes this way nervous impulses are so quickly down toward the muscle and here that's similar process acts whenever a muscle is caused a contract the only difference in this case is that the change in electrical state is accompanied by physical action namely a contraction these electrical impulses you may recall a very rapid and very small but they've all been measured frequently and here's this set up for you to see in ring a solution of front muscles placed the nerve is traced and laid across two electrodes with wires attached stimulated through these the muscle contracts another lead is added the bath run dry - recording electrodes are placed nearby and the impulses which these receive are visible on the CRT and photographed simultaneously but potential changes on a single input are far less able to demonstrate and so back to our diagram we'll go and here we can show how the acetylcholine a slightly reduces the indicate potential until it produces by this action the muscular impulse and contraction the oscilloscope tube should make it quite clear you see the potentials diminishing it here and when it's reduced to a critical fraction a spike marks the start of both impulse and contraction the basic principle in being made plain let's try our relaxants over again to both curing first now we've said that this acts like a block but in fact this is only partly true for some acetylcholine does get true but not enough to reduce potential to the low-level essential this since acetylcholine is partly stopped is what is termed competitive block so we see on the oscilloscope a potential reduction quite remote from the height at which it could produce a spike succinylcholine and deca muthoni on the other hand act rather differently before both of these can reduce endplate potentials to the critical value and in fact cause the muscle to contract but having done so they simply remain and you cannot excite the muscle gain what's more this reduce potential spreads quite a little beyond the endplates edge you will not be surprised in that what we have shown as depolarization block is known the oscilloscope we know will obviously show first a spike of normal height and consequential reduce potential ah there now from our Labor's to reap the fruits let's put our relaxants into two groups competitors and depolarizes but some don't quite belong to either and one more thing against this plan is that some depolarizes can act as competitors and vice versa just why these compounds act this way it's really very hard to say though if you look at their molecules you may see what one might describe as a sort of family likeness they all a rather complicate the end of a similar size and shape to quaternary nitrogen atoms in each appear known for about a hundred years to be a characteristic of agents producing muscular block acetylcholine follows a similar pattern but with only one quaternary nitrogen atom yeah and it's only half their size but in spite of much surmise and no one knows whether to attach much significance to these facts then our just a word before I go there's one more thing you ought to know about suspected overdose if ever you meet this situation give artificial respiration if after this it's still not plain you did better to see the film again