Facilitation in the rapid response of the earthworm, Lumbricus terrestris L.

In previous papers (Roberts, 1962 a, b) the earthworm's rapid response, mediated through the giant-fibre reflex, was described and analysed. In the course of these experiments it was noticed that in the early stages of repetitive stimulation successive responses do not remain the same size but show a marked tendency to increase in magnitude. This staircase effect is particularly pronounced in fatigued preparations in which accommodation has occurred at synapses in the giant-fibre reflex and it would appear to involve a process of facilitation in the reflex mechanism. There are few accounts of facilitation in annelids, though there are many references to it in other groups, particularly coelenterates and Crustacea. Horridge (1959) describes spatial summation in fatigued specimens of the polychaete Harmothoe. Budington (1902) and Botsford (1941) both recorded staircase effects in the earthworm but neither of these workers was able to say conclusively where in the neuromuscular machinery facilitation was taking place. The purpose of this paper is to investigate the location in the rapid response reflex where a process of facilitation might occur.