Neurogenic pacemakers in the legs of Opiliones

Abstract

After autotomy, the legs of all the species of Opiliones examined, and of a Kenyan Pholcid spider, twitched spontaneously at the femoro-patellar and tibio-basitarsal joints, for periods of up to an hour. These joints lack extensor muscles, extension being achieved at the femoro-patellar joint probably by haemolymph pressure, but at the tibio-basitarsal joint of Opiliones by a cuticular spring which can extend the joint fully. Comparable twitching activity could be evoked without autotomy if the central nervous system was burnt, or by asphyxiation. Electromyograms from the femur or tibia of an isolated twitching leg showed regular motor bursts which accompanied flexions, and sensory activity during extension. Forced movements of the joints did not perturb the rhythm of the motor bursts. An isolated proximal half of a femur could still generate the same bursting pattern whereas no other region showed this activity after its isolation. Bursts recorded in the tibia were shown to be dependent on the integrity of the femur. By stimulation of the femur with 1 -ms current pulses it was possible to reset the rhythm. Stimulation with 1-s pulses caused an acceleration or inhibition of the rhythm according to the direction of the current. Spontaneous bursts could be evoked in silent isolated legs, or in intact quiescent legs, by similar 1-s current pulses. It is postulated that the femur contains independent neurogenic pacemakers which are activated by injury current from the damaged leg nerve; they produce regular bursts of motor impulses without the interplay of proprioceptive loops, and are responsible for the movements observed.