The path and rate of growth of regenerating motor neurons in the cockroach

Abstract

Nerve root 5 that supplies the coxal depressor muscles from the metathoracic ganglion in the cockroach was crushed. Regeneration of the motor neurons was studied by cutting the nerve at several distances from the crush point and introducing cobalt chloride into the cut end. The operation was followed by a lag period of about 13 days after which the axons regenerated at a rate of 0.9 mm per day. After regeneration had been completed a pattern of axon distribution was established among the nerve branches that was very similar to that found in intact, normal cockroaches. This pattern was established through an apparent directed growth of certain axons from identified cells into branch 5rl, their normal pathway. However, at the same time, random or increased branching of other unidentified motor neurons produced some errors in the distribution of axons among the nerve branches. Eventually these errors were corrected by the degeneration of neuronal processes that did not synapse with correct target muscles. These results demonstrate the requirement for a highly specific intercellular recognition process between individual, identified motor neurons and the appropriate muscles they innervate in order to reform the original innervation pattern during regeneration.