Squid giant axon
Encyclopedia
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
. It was discovered by English zoologist and neurophysiologist John Zachary Young
in 1936. 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 potential
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
. The increased diameter of the squid axon decreases the internal resistance of the axon, as resistivity is inversely proportional to the cross sectional area of the object. This increases the space constant, λ=sqrt(rm/ri). The increased space constant propagates a given local depolarization further, which speeds up the action potential, according to the equation E=Eoe(-x/λ).
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 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
which decreases the conduction velocity potential 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
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.
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
that controls part of the water jet propulsion system in squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
. It was discovered by English zoologist and neurophysiologist John Zachary Young
John Zachary Young
John Zachary Young FRS , generally known as "JZ" or "JZY", was an English zoologist and neurophysiologist, described as "one of the most influential biologists of the 20th century .....
in 1936. 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 potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
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 response, escape reaction, or escape behaviour is a possible reaction in response to stimuli indicative of danger, in particular, it initiates an escape motion of an animal...
. The increased diameter of the squid axon decreases the internal resistance of the axon, as resistivity is inversely proportional to the cross sectional area of the object. This increases the space constant, λ=sqrt(rm/ri). The increased space constant propagates a given local depolarization further, which speeds up the action potential, according to the equation E=Eoe(-x/λ).
In their Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
-winning work uncovering ionic mechanism of action potentials, Alan Hodgkin
Alan Lloyd Hodgkin
Sir Alan Lloyd Hodgkin, OM, KBE, PRS was a British physiologist and biophysicist, who shared the 1963 Nobel Prize in Physiology or Medicine with Andrew Huxley and John Eccles....
and Andrew Huxley
Andrew Huxley
Sir Andrew Fielding Huxley, OM, FRS is an English physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his experimental and mathematical work with Sir Alan Lloyd Hodgkin on the basis of nerve action potentials, the electrical impulses that enable the activity...
performed experiments on the squid giant axon. 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
Electrophysiology
Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
inside the lumen of the axon.
While the squid axon is very large in diameter it is unmyelinated
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
which decreases the conduction velocity potential 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
Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....
Sepia
Sepia (genus)
Sepia is a genus of cuttlefish in the family Sepiidae encompassing some of the best known and most common species. The cuttlebone is relatively ellipsoid in shape...
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.