Changes in firing behaviour of an insect motor neurone during development and following experimental surgery

The firing behaviour of an identified neurone in the cricket was studied using extracellular recording from the axon. In the last nympal instar (preadult developmental stage), the contralateral dorsal longitudinal motor neurone (CDLM) showed spontaneous activity and was excited by air puffs to the head and cerci and by single shocks to the anterior nerve cord. In the normal adult the CDLM did not exhibit these properties. However, responses which were characteristic of the last instar appeared in the adults which had been subjected to any one of the following surgical procedures: (1) central nervous system injuries which separated the CDLM arborization and axon from the soma; (2) operations which injured the central nervous system without cutting the CDLM; and (3) operations which damaged the cuticle only. Since cuticle damage alone was as effective as the more extensive operations, it is suggested that a sufficient cause for the appearance of nymphal firing behaviour in the adult CDLM is cuticle damage. The factor associated with cuticle damage which mediates the changes in activity of the CDLM neurone is not known, but its action does not require the mediation of the CDLM soma.