Reflex conduction in the giant fibres of the earthworm

Abstract

The present work confirms the conclusion of Friedlander and others that the giant fibres mediate the end-to-end shortening reaction in the earthworm. The chief concern has been to investigate Stough's claim that the median giant fibre conducts impulses only in the direction from head to tail and the lateral giants only in the reverse direction. Two methods have been employed. (a) The nerve cord was exposed at each end of the worm, and electrical records taken simultaneously from the two extremities when the surface of the worm was touched at different places. The results were usually a train of impulses in one or other giant fibre, and it was found that whenever an impulse appeared at one end of a given fibre, it always appeared at the other end of the same fibre. Each fibre, therefore, when it conducted at all, always conducted in both directions. Sensory nerves from the head appeared only connected to the median giant, since stimulation anterior to the clitellum never resulted in lateral fibre activity. Similarly, the tail appeared only to join with the lateral giant fibres. (b) Stough's own method was used, and his observations confirmed, extended and re-interpreted. Either the median or both lateral fibres were divided in one segment. The success of this operation could be judged by leading off the giant fibre responses from the undissected worm (figure 5). Next day, when the worm had recovered, the shortening reflex was observed when the worm was touched at the head, the tail, or in the middle. The shortening was either throughout, or was arrested at the operation site, depending upon whether the active giant fibre was the intact or the damaged one. The results are summarized on p. 119. From both the head and the tail Stough's observations are confirmed, and it is agreed that impulses from the head are conducted back by the median giant alone. The absence of impulses in the laterals might be due to contrary one-way conduction as Stough assumes, or to the absence of their sensory connexion with the head. But (a) above shows that the latter is correct, and the same must be concluded from touching the middle region of the worm, which apparently Stough did not do, for this part connects with the lateral giants, and thus affords a demonstration that these fibres may also conduct antero-posteriorly. The difference in function of the separate giant fibres, therefore, is probably related to their difference in sensory distribution.