Monday, July 02, 2007

Sarin Gas

I first heard about Sarin gas in The Rock. It's a nerve gas that melts your skin off and stops your heart after making you spasm so hard you break your back and puke your own guts out, unless you stab yourself in the heart with a gigantic needle full of atropine. Naturally, I thought hollywood probably exaggerated. But exactly what happens?

This I know: neurons communicate with neurotransmitters and propagate through voltage-gated channels. So propagation, signal travel along a single neuron, happens almost like dominoes. There's a whole bunch of "gates" on the neuron that will flip in polarity. When they do so, they cause the next gate along the line to also "flip", creating a cascade effect. The cascade is started at one end of the neuron and continued to the other end, where there is a bulb (a "bouton"). The bouton will flood the synaptic cleft (the space between the pre and post-synaptic neuron boutons) with neurotransmitters. The flood out into the space and bind to the post-synaptic bouton, which will then start a gate cascade, which will continue the signal. One of several neurotransmitters is called acetylcholine. But of course, you need to control these transmitters too because once they flood the post synaptic bouton, they need to be removed so that the process can start again. There's an enzyme which breaks down acetylcholine and its name is derived from that which it controls, like most enzyme names: acetylcholinesterase.

Oh one more thing, the parasympathetic and the sympathetic nervous system regulates nervous response (often called the fight or flight system). The parasympathetic is the system which is responsible for autonomic functions like breathing and your heart beating.

So, with the help of Wikipedia: Sarin gas, like most (all?) nerve gas, is an acetylcholinsterase inhibitor. This means that when nerves fire, there is no clean up afterwards. The synaptic cleft gets flooded with neurotransmitters. This probably has two consequences: 1) an initial overflow of too many nerve signals and 2) once the synaptic cleft is saturized, no more signals can be transmitted. I'm speculating on 1. Anyway, this happens in various ways for various nerve toxins but that's the general idea. Sarin is an acetylcholinesterase inhibitor, so it needs to be counteracted in two ways.

1) Atropine is a competitive antagonist against certain kinds of acetylcholine receptors, the kinds that are dominant in the parasympathetic nervous system. Suppression of the parasympathetic nervous system kicks your heart into overdrive because it is no longer suppressing heart speed. This counteracts the effects of Sarin, which mutes your entire nervous system, including the sympathetic portion which is responsible for your heart beating at all.

2) Atropine is usually used with Pralidoxime, which binds to certain kinds of (organophospate-)inactivated acetylcholinesterase. Presumably, Sarin inhibits acetylcholinesterase in this way and so Pralidoxime reactivates acetylcholinesterase, leading to the clearing of the synpatic cleft, leading to nerve function again, in this case the rather important sympathetic nervous system.

So Atropine + Pralidoxime works a bit like combining psychotropic drugs: Atropine suppressed the parasympathetic so that it combats the slowing sympathetic system suppressed by Sarin and also so you don't die in the time it takes Pralidoxime to run through your system liberating your acetycholinesterase and therefore bringing the sympathetic nervous system back up to normal levels.

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