Firing patterns of closer motoneurons during reflex activity in the dimorphic claws of the lobster Homarus americanus.

Abstract

The in vivo firing patterns of the fast closer excitor (FCE) and slow closer excitor (SCE) motoneurons to the dimorphic claws of the lobster Homarus americanus were analyzed during reflex closure activity by identifying postsynaptic potentials in muscle fibers with known motor innervation. Three types of claw activity were observed: slow closure, rapid closure, and maintained closure. Slow closure and rapid closure in both claws were mediated by SCE and FCE, respectively. In the cutter, maintained closure was mediated only by SCE; in the crusher, both FCE and SCE could maintain closure. The homologous SCEs displayed no significant differences in activity; in both claws, they fired at medium-to-high frequency. The homologous FCEs did display different spike frequencies during claw closure. The crusher FCE fired at high frequencies; the cutter FCE fired at much lower frequencies. Such in vivo differences in axon activity between homologous FCEs are correlated with claw dimorphism and with the population of muscle fiber types.