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The squid giant axon is the very large (up to 1.5 mm in diameter; typically around 0.5 mm)
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
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 The Stazione Zoologica Anton Dohrn is a research institute in Naples, Italy, devoted to basic research in biology. Research is largely interdisciplinary involving the fields of evolution, biochemistry, molecular biology, neurobiology, cell biol ...
in Naples, the Marine Biological Association in Plymouth and the Marine Biological Laboratory in
Woods Hole Woods Hole is a census-designated place in the town of Falmouth in Barnstable County, Massachusetts, United States. It lies at the extreme southwest corner of Cape Cod, near Martha's Vineyard and the Elizabeth Islands. The population was 781 at ...
. 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 potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
s 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 Escape response, escape reaction, or escape behavior is a mechanism by which animals avoid potential predation. It consists of a rapid sequence of movements, or lack of movement, that position the animal in such a way that allows it to hide, freeze ...
. The increased radius of the squid axon decreases the
internal resistance A practical electrical power source which is a linear electric circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resistance of the source. ...
of the axon, as resistance is inversely proportional to the cross sectional area of the object. This increases the space constant (\lambda = \sqrt), leading to faster local depolarization and a faster action potential conduction (E = E_o e^). In their Nobel Prize-winning work uncovering ionic mechanism of action potentials,
Alan Hodgkin Sir Alan Lloyd Hodgkin (5 February 1914 – 20 December 1998) was an English physiologist and biophysicist who shared the 1963 Nobel Prize in Physiology or Medicine with Andrew Huxley and John Eccles. Early life and education Hodgkin was ...
and
Andrew Huxley Sir Andrew Fielding Huxley (22 November 191730 May 2012) was an English physiologist and biophysicist. He was born into the prominent Huxley family. After leaving Westminster School in central London, he went to Trinity College, Cambridge on ...
performed experiments on the squid giant axon, using the
longfin inshore squid The longfin inshore squid (''Doryteuthis pealeii'') is a species of squid of the family Loliginidae. Description This species of squid is often seen with a reddish hue, but like many types of squid can manipulate its color, varying from a deep ...
as the model organism. The prize was shared with John Eccles. 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 Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
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
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control o ...
'' 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.


See also

*
Lateral giant neuron The lateral giant interneuron (LG) is an interneuron in the abdominal nerve cord of crayfish, lobsters, shrimp of the order Decapoda and their relatives in the crustacean class Malacostraca. It is part of the system that controls a special kind ...
*
Squid giant synapse The squid giant synapse is a chemical synapse found in squid. It is the largest chemical junction in nature. Anatomy The squid giant synapse (Fig 1) was first recognized by John Zachary Young in 1939. It lies in the stellate ganglion on each side ...
* Hodgkin–Huxley model


References

{{DEFAULTSORT:Squid Giant Axon Giant axon Cephalopod zootomy Biology experiments