Squid giant axon

The squid giant axon is the very large (up to 1 mm in diameter; typically around 0.5 mm) axon that controls part of the water jet propulsion system in squid. It was first described by L. W. Williams in 1909, but this discovery was forgotten until English zoologist and neurophysiologist J. Z. Young demonstrated the axon's function in the 1930s while working in the Stazione Zoologica in Naples, the Marine Biological Association in Plymouth and the Marine Biological Laboratory in Woods Hole. Squids use this system primarily for making brief but very fast movements through the water.

Between the tentacles of a squid is a siphon through which water can be rapidly expelled by the fast contractions of the body wall muscles of the animal. This contraction is initiated by action potentials in the giant axon. Action potentials travel faster in a larger axon than a smaller one, and squid have evolved the giant axon to improve the speed of their escape response. The increased diameter of the squid axon decreases the internal resistance of the axon, as resistance is inversely proportional to the cross sectional area of the object. This increases the space constant (${\displaystyle \lambda ={\sqrt {r_{m}/r_{i}}}}$), leading to faster local depolarization and a faster action potential (${\displaystyle E=E_{o}e^{-x/\lambda }}$).

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