Function of the Giant Fibers of the Central Nervous System of the Crayfish

Abstract

Johnson 1 has made studies about the course, the connections and the function of the giant fibers of the central nervous system of a few crustaceans, especially of Cambarus. Without actually isolating these fibers, he came to the conclusion that both sets of giants, described as the median and the lateral giants, have the function of flipping the tail. The median giants seem to connect with the motor fibers of each abdominal segment by a very simple contact mechanism; the connection of the lateral giants with the motor fibers is not as well known.

With the same method as used in the studies on the peripheral nerve-muscle system of crustaceans 2 it was possible to isolate in the oesophageal commissures these 2 sets of giant fibers of Cambarus clarkii and to stimulate them separately. The median giant fibers are considerably thicker than the lateral ones, the former have a diameter which varies between 200μ and 250μ, whereas the latter measure 60μ to 90μ in the animals used.

It was found that single induction shocks applied to the unprepared oesophageal commissure or to any of the 4 giants caused a twitch-like contraction of the flexor muscles of the tail, a stretching of the legs and an inward movement of the antenna. The other fibers in the oesophageal commissures do not give a noticeable reaction to single shocks (with faradic stimulation various effects may be obtained from them). The tail contractions were registered either isotonically with load or isometrically and it was found that the effect of single shocks applied to any of the 4 fibers did not vary with the strength of the stimulus within wide limits. The threshold contraction is of a considerable strength and only with very strong shocks is a higher contraction obtained, which is due either to stimulation of the other giant fiber in the same commissure by current escape, or to a repeated discharge in the stimulated fiber. There can thus be no doubt about the validity of the all-or-none relation of this system, which is the more convincing as the series of connections which each giant makes in the row of ganglia would be an ideal opportunity for decremental conduction to show, as the number of ganglia reached would be then proportional to the strength of the stimulus.