Ontogeny of a Simple Locomotor System: Role of the Periphery in the Development of Central Nervous Circuitry

Abstract

The development of locomotor systems in the lobster Homarus americanus is described. The tail—flip escape response is fully developed when the larvae hatch, and occurs without the participation of giant fibers. The abdominal swimmerets are undifferentiated at hatching, but are fully developed two to three weeks later when the animals molt to the fourth larval stage. Forward locomotion in the pelagic larvae is achieved using thoracic swimming appendages until the fourth larval stage, when these degenerate and the swimmerets assume the locomotor role. The hypothesis that peripheral structures specify the central nervous connections of motoneurons during ontogeny was tested in the swimmeret system. Presumptive swimmeret appendages, including prospective muscle and sense organs, were extirpated prior to their differentiation in newly hatched larvae. The corresponding swimmeret motoneurons nevertheless grew and formed normal central connections, as evidenced by the appearance of normal patterns of rhythmic locomotor discharge and normal reflexes at the usual time. Moreover, swimmeret motoneurons retained normal patterns of motor output even when the regeneration of their target appendages was prevented for as long as two months. Therefore, the formation of normal motor output patterns during ontogeny is not dependent upon feedback from differentiated target muscle nor from sense organs which normally monitor the results of the motor activity.