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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 Atlantic squid's (Loligo pealei) water jet propulsion system. Squid use this system primarily for making brief but very fast movements through the water usually when escaping predators. 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. This has obvious adaptive advantage when escaping from predators.
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.
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