The very large nerve fiber that controls part of the water jet propulsion system in squid
The squid giant axon is the very large (up to 1.5 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[1] in 1909,[2] 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.[3][4] Squids use this system primarily for making brief but very fast movements through the water.
On the underside of the squid's body, between the head and the mantle, 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,[5] and squid have evolved the giant axon to improve the speed of their escape response. The increased radius 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 (), leading to faster local depolarization and a faster action potential conduction ().[6]
While the squid axon is very large in diameter it is unmyelinated which decreases the conduction velocity substantially. The conduction velocity of a typical 0.5 mm squid axon is about 25 m/s. During a typical action potential in the cuttlefishSepia giant axon, an influx of 3.7 pmol/cm2 (picomoles per centimeter2) of sodium is offset by a subsequent efflux of 4.3 pmol/cm2 of potassium.[8]
^Purves, Dale; Augustine, George J.; Fitzpatrick, David; Katz, Lawrence C.; LaMantia, Anthony-Samuel; McNamara, James O.; Williams, S. Mark (2001). "Increased Conduction Velocity as a Result of Myelination". Neuroscience. 2nd edition. Sunderland, MA.{{cite book}}: CS1 maint: location missing publisher (link)