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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. Squid 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.
In their Nobel Prize-winning work uncovering ionic mechanism of action potentials, Alan Hodgkin and Andrew Huxley performed experiments on the squid giant axon. The large diameter of the axon provided a great experimental advantage for Hodgkin and Huxley as it allowed them to insert voltage clamp electrodes inside the lumen of the axon.
While the squid axon is very large in diameter it is unmyelinated which decreases the conduction velocity potential substantially. The conduction velocity of a typical 0.5 mm squid axon is about 25 m/s. The sodium gain per impulse is 4 pm/cm2 (picomole per square centimeter) and the potassium gain is also 4pm/cm2.citation needed
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