Squid giant axon

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.

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.

[6] In their Nobel Prize-winning work uncovering ionic mechanism of action potentials, Alan Hodgkin and Andrew Huxley performed experiments on the squid giant axon, using the longfin inshore squid as the model organism.

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 substantially.

During a typical action potential in the cuttlefish Sepia 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.