Chemical stimulation of the annelid nerve cord

Abstract

According to Maxwell's 2 classification, based on work with mammals, there are two classes of excitants for nervous tissue, viz: (1) those which act only upon the medullated fibers, such as the calcium precipitants, (2) those which act on the gray matter only, as creatin and strychnin. In order to determine possible similarities and differences between mammalian nervous tissue and that of one of the annelids, experiments were carried out on the nerve cord of the earthworm, Lumbricus terrestris. In these experiments the worm was decapitated, the anterior end of the preparation pinned down and the nerve cord laid bare. The cord was then dissected free for a distance of about 20 segments and the stimulating substances applied directly to it. Stimulation was indicated by squirming movements of the posterior segments.

Excitants of the first class, KC1, BaC1₂, and Na₃ citrate, each in M/8 concentration, gave marked excitation. Of the excitants of the second class, camphor and strychnin, each in saturated solution, and picrotoxin crystals, all yielded positive results within a minute after application, but phenol, nicotin and creatin had no effect, used either as crystals or in solution. M/64 tetra-ethylammonium chloride gave strong stimulation.

The fact that excitants of the first class act on the annelid nerve cord shows that the nerve processes reacting do not differ in this respect from the axons of the myelinated fibers of mammals. The action of the excitants of the second class exhibits two peculiarities; the action is almost immediate, there is no latent period of several minutes as in mammals and in squid'; the fact that the nerve cells of the earthworm are unaffected by phenol, nicotin and creatin indicates a chemical organization different from that obtaining in the neurones of higher forms in which stimulation by these substances does take place.