Squid axons and their non-myelination

Apr 19, 2010
Myelinated axons conduct action potentials at 120 meters per second on average.

Unmyelinated axons conduct action potentials at 5 meters per second on average.

I want to formulate a coherent question about how a giant squid and colossal squid and for that matter any other large squid manage to get through life, but sadly I can only ask how the heck large invertebrates get around this.

I know they divert a lot of control of their arms and possibly tentacles to ganglia around their limbs, but even still, there's considerable signal delay inherent in that.
Conduction velocity also depends on axon diameter, which is one reason squid have their famous giant axons (much of what is known about action potential dynamics and synaptic physiology come from studies on the squid giant axon and its giant synapse) for generating their explosive escape jetting motor response. Check out some of the older papers on squid giant axons to get an idea of the axon diameters involved in L. pealei (I think this is the standard species used in those studies), and what conduction velocities are standard for axons of that size. A quick look indicates that for an axon of 0.5mm (yes, that's a single axon!) the CV is about 25m/s. Fast enough for doing what they need to do. Even in very large animals, assuming some allometric increase in axon diameter, AP travel time is probably not that slow.

The giant size of many invertebrate neurons is often cited as a parallel path that invert evolution took to increase CV, whereas vertebrates increase their CVs with myelin. Squid are certainly as fast as fish when it comes to swimming, so I think it worked out OK!

BTW, if you are ever in Massachussets (I don't know where you are normally), try to visit the MBL, where much of the squid stuff was originally worked out. Its worth a visit!

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