October 08, 2008

International Cephalopod Appreciation and Awareness Day: Let's celebrate with a re-post

It's the 2nd Annual (Unofficial) International Cephalopod Appreciation and Awareness Day! Since I didn't have time to write anything for this year, I'm just shamelessly going to dig up what I posted last year and put it back online.

Here's my tribute to cephalopods everywhere, the story of how a humble squid helped revolutionize the field of neuroscience.

>>Re-post from October 8, 2007.

Today, October 8 (get it?) is the unofficial cephalopod awareness day. At least according to this website. Say no more says I! I needn't be coerced into writing about our beautiful cephalopod overlords, even though it's late in the day.

I would like to acknowledge the contribution of cephalopods to the field of neuroscience, which I work in. Octopuses and squid are known for having advanced sensory systems and brains for being invertebrate, as well as for showing some aspects of learning, advanced memory and object recognition. But that's not what I want to talk about. I'm going a bit more old-school. I want to talk about cephalopods' contribution to the field of electrophysiology. One particular cephalopod's contribution to be precise. That's her on the top of this post: The atlantic squid or common squid, Loligo pealei.

There are two major tactics in the natural world when organisms have evolved very fast nerve signaling. One very efficient is myelinization, the covering of the nerve fibers with layers of very fatty cells that act as the insulation around an electrical cable, preventing the electrical signal from leaking and scattering away from the original direction. This is what we have in our nervous systems. The other tactic is what cephalopods have. It's usually seen as less efficient and more clunky, but that's just to say that us humans would never have been able to evolve into what we are had we gone in that direction. Evidently it works just dandy for our cephalopod friends (and who are we to judge them anyway?). I'm talking of course about huge freaking nerve fibers, the notoriously awesome giant squid axons. These nerve fibers can have a diameter of 0,5mm to 1mm while our puny little human nerve fibers are up to a thousand times smaller. A thousand times!

Like myelinization, having a greater diameter of the nerve fiber (which basically is a membrane tube projecting from the nerve cell) makes the conduction of the nerve signal faster. I won't confuse you with the physics behind why this is, basically because I would get lost in formulas and get confused myself no doubt, it's enough to say that increasing the diameter of the nerve fiber makes the electrical resistance smaller. Squids use these giant axons to mediate their escape response. The nerve cells that have giant axons go to the squid's mantle muscles which contract with an amazingly fast response time to any possible threat, forcing a jet of water through the squid's siphon and propelling it the hell out of there.

Now, the lovely squid on the top of this post didn't jump-start the field of electrophysiology all by itself, it had some help from the brilliant scientists at the Plymouth Marine Laboratory in England, most notably A. L. Hodgkin, A. Huxley and B. Katz. Since the giant squid axon from the common squid is so huge, they were able not only to put little wires inside it and make electric measurements, but also to take a little roller and squish out its contents and replace it with more controlled solutions to check exactly what is was that made the conduction of electrical signals possible. By doing this they found out that it is the flow of positively charged sodium and potassium ions (charged atoms) across the nerve fiber's membrane through special proteins called ion channels that constitutes the electric signal. The difference in concentration of these two ions between the inside and outside of the nerve fiber create a difference in charge, a potential, a voltage, across the membrane. This resting voltage changes when some of the ion channels open and ions rush across the membrane, this change is very local but it causes the ion channels right next-door on the nerve fiber to open as well which spreads the change in voltage and in turn opens some more ion channels a bit further down the nerve fiber and presto, you have a propagating electric signal - an action potential - that spreads down the fiber at amazing speed. These results were published in the 1940s and 50s in a series of classical biological articles by Hodgkin, Katz and others. What they found out back then is as true for us as it is for the common squid and it is perhaps the most important discovery in all of neuroscience. It was awarded with the Nobel prize in physiology or medicine in 1963.

But that's not all. The nerve cells with giant axons also have giant synapses, the chemical communication sites between nerve cells, which taught us a lot about how those are built. The squid giant axons also provided us with a way of studying how different molecules are actively transported within cells through the so called cellular skeleton, a web of protein tubes and wires inside the cell. Giant axons need giant transportation highways and those are easier to see than teeny-tiny alleyways.

All this thanks to some scientists that just happened to be on the right place at the right time and an awesome squid that had the evolutionary smarts to evolve some truly amazing neuroanatomical features.

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