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 po ...

that controls part of the water

jet propulsion Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating o ...

system in

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...

. 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 ...

Naples Naples (; it, Napoli ; nap, Napule ), from grc, Νεάπολις, Neápolis, lit=new city. is the regional capital of Campania and the third-largest city of Italy, after Rome and Milan, with a population of 909,048 within the city's adminis ...

, the Marine Biological Association in

Plymouth Plymouth () is a port city and unitary authority in South West England. It is located on the south coast of Devon, approximately south-west of Exeter and south-west of London. It is bordered by Cornwall to the west and south-west. Plymouth ...

and the

Marine Biological Laboratory The Marine Biological Laboratory (MBL) is an international center for research and education in biological and environmental science. Founded in Woods Hole, Massachusetts, in 1888, the MBL is a private, nonprofit institution that was independent ...

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 A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...

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 The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

-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 bo ...

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 ...

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 A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...

. 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 of ...

Sepia Sepia may refer to: Biology * ''Sepia'' (genus), a genus of cuttlefish Color * Sepia (color), a reddish-brown color * Sepia tone, a photography technique Music * ''Sepia'', a 2001 album by Coco Mbassi * ''Sepia'' (album) by Yu Takahashi * " ...

'' 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.

References

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

See also

References