Structural and functional changes in an identified cricket neuron after separation from the soma. I. Structural changes

Abstract

The morphological effects of separation from the soma were examined in isolated arborization and isolated axon segments of an identified motor neuron in the Polynesian field cricket, Teleogryllus oceanicus. The identified neuron, the contralateral dorsal longitudinal motor neuron of the metathoracic ganglion (CDLM), possesses an arborization most of which lies contralateral to its soma within the metathoracic ganglion. Midline surgical lesions in the metathoracic ganglion separated CDLM into a distal segment composed of the axon and most of the arborization, and a proximal segment comprised of the remaining arborization, neurite, and soma. Isolated axonal segments were produced by cutting the nerve root containing the axon of CDLM close to the ganglion. The normal anatomy of CDLM was determined by axonal dye-fills using cobaltous chloride. Morphological changes in the isolated arborization of CDLM were examined by axonal dye-fills at successive time intervals. Changes in the isolated CDLM axon were examined via dissection and histological cross-sections of the distal nerve at graded time intervals. In one example, a remnant of the isolated CDLM arborization survived to 168 days postoperative, a time comparable to the longest previously-reported physiological and morphological survival times of distal axonal segments of invertebrates. In general the isolated arborization does not survive this long. Normally occurring branches of the arborization can be preserved about 0 to 50 days. After this period branches of the arborization seem to be lost in progressive fashion from smaller to larger, leading to complete loss of the arborization and axon in most cases at 100 or more postoperative days. There is evidence for the presence of supernumerary fibers in the isolated CDLM arborization between 0 to 63 days postoperative. Such supernumerary fibers indicate an independent capacity for outgrowth of the isolated arborization without connection to the nucleus. The distal axonal segment of CDLM degenerates physiologically and morphologically within 4 to 15 days after peripheral nerve section. This time course is close to that of Wallerian degeneration of vertebrate peripheral nerve axons.