Neuromuscular Physiology of the Closer Muscles in the Pedipalp of the Scorpion Leiurus Quinquestriatus

Abstract

1. The closing system of the pedipalp claw in the scorpion is composed of two agonistic muscles situated in both the patella (the longer closer muscle) and tibia (the short closer muscle). 2. Membrane electrical constants of the muscle fibres were found to be: λ = 0.6 mm.; ri = 1 x 106 Ωmm.-1; Ri = 280 Ωcm.; rm = 4.4 x 105 Ωmm.; Rm = 830 Ωcm.2; Cm. = 3.6 µF. cm.-2; τ = 3.0 msec.; input resistance = 3.3 x 105 Ω. 3. Each muscle fibre is multiterminally innervated by two motor axons, one initiating a junction potential and the second inducing a post-synaptic spike-like response. 4. The long closer muscle is composed of two anatomically distinct motor units, the short closer muscle of three motor units. Each unit is innervated by its own pair of specific motor axons. 5. No distinction between ‘slow’ and ‘fast’ axons or muscle fibres could be observed. Both muscles responded with a twitch to a single stimulus applied to the nerve. 6. No peripheral inhibition was observed.