Squid axon

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Basic Information

Squids are marine invertebrates, they belong to the phlylum mollusca. They serve as a powerful model system to study the electrophysiology of the neurons. The giant axon is a nerve fiber which radiates from the stellate ganglion, the axon is about 1mm in diameter. The large size of the axon provides an experimental adavntage to insert voltage clamp electrode and perform other electrophysiological experiments.[1]

Anatomy

Near the mantle cavity of the squid a large ganglion is present. Nerves radiate from it to span the entire mantle in a star like pattern. Each ganglion has 10 nerves and each nerve has a large tubular structure about 1mm in diameter. This large nerve is called the giant squid axon. These nerve fibers were mistaken for an artery initially, but the absence of amoebocytes and blood in its transverse section confirmed that they were not blood vessels. 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.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.

Picture shows a single axon of squid (on the left) and collection of nerve fiber of rat squidaxon.org

Physiology

The giant axon of the squid has been the preparation of choice for recording the electrical events associated with the opening and closing of the ion channels,its large size allows the introduction of electrodes and exchange of solutions giving almost complete control of the chemical environment and voltage across axolemma[2]

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 as it allowed them to insert voltage clamp electrodes inside the lumen of the axon. Based on a number of experiments in giant squid axon Hodgkin and Huxley constructed a first quantitative model of the electrical excitability of neurons. This model describes the three ionic currents: fast inward sodium,time-dependent outward potassium current and time-independent leak current[3]

Links

http://www.science.smith.edu/departments/NeuroSci/courses/bio330/squid.html

References

1)Squid as Experimental Animals By Daniel L. Gilbert, William J. Adelman, John M. Arnold

2)Adelman and Gilbert,1990;Mullins and Brinley,1990

3)Principles of Neural Science by Eric Kandel