Discussion in 'Physiology and Biology' started by tonmo, Apr 30, 2006.
I received the following email, and wanted to share it here, as requested.
Squid do not have a spinal cord. Their giant axon is not myelinated. If I can I will find a diagram of where their giant axon is situated in their body. They actually have two giant axons, one for each side of their body. I believe the axon was named because of its diameter.
Here is a link for a reference about giant squid axon. It is a little old, but I think most of the information is accurate.
Arnold, J. M., W. C. Summers, D. L. Gilbert, R. S. Manalis, N. W. Daw and R. J. Lasek. 1974. A Guide to Laboratory Use of the Squid Loligo pealii Woods Hole, MA: Marine Biological Laboratory. 74 pp.
Only vertebrates have some of their nerves myelinated. As I understand, myelin increases the speed of propagation along the nerve without having to increase the diameter.
I am not sure how accurate Herman Melville was in his facts about Sperm Whales but when the animal is 50 feet long, its spinal column would be large too.
I am sure there are other members who could give you a more detailed (and perhaps accurate) description.
Augmenting Cuttlegirl's responses, there is an article by JZ Young, I think in Proceedings of the Royal Society B, in which he says that giant squid giant axons are not particularly larger in diameter than the giant axons of smaller squids. In general, squid giant axons are quite different from spinal cords, since a squid giant axon is a single nerve, while a spinal cord is a bundle of many, many nerves.
I think the whale's spinal cord is much more like typical vertebrate spinal cords. However, in terms of "do all creatures of the deep have such a thing," at TONMOcon, William Gilly mentioned that there is a similar mechanism to the squid giant axon (and its associated stellate ganglion) in certain fish that seems to serve the same function: they both need to send a signal rapidly from the head to the tail in order to execute a rapid burst of speed as an escape mechanism. So there is some commonality.
I'm not aware of microtubules playing much of a role, quantum mechanical or otherwise, in axons... the propogations of action potentials along axons depends largely on the voltage gated ion channels in the axon's membrane, and the ion pumps that keep a difference in concentration of the sodium and potassium ions inside and outside the axon. Most of the tricky stuff in neurons happens in the dendrites, and a bit in the soma...
Microtubules are very important when it comes to extending axons, which is crucial because they don't work very well if they conduct action potentials to nowhere.
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