Two groups of 60 day old male Wistar Rats were subjected to right-sided rubro-spinal tractotomy at the fourth cervical and thirteenth thoracic vertebral levels respectively. Four animals in each group were sacrificed at each of the time intervals 1, 3, 4, 7, 14, 21 and 55 days. Counts of chromatolytic neurons from both groups of animals at time intervals ranging from 1–21 days established a time course for the chromatolytic response. Morphological observations showed a more severe central chromatolysis in the cervical group commencing at Day 3, and a much less severe central chromatolysis in the thoracic group starting at Day 4. By Day 21 the majority of the neurons of the cervical group were atrophic and by 55 days all were atrophic. The reaction following thoracic lesions was less intense and the maximum number of reacting neurons was observed at Day 7. Following this stage the neurons gradually returned to a normal morphological state which was complete by Day 55. Measurements of neuronal diameters from semi-thin Epon sections in the cervical group yielded statistically significant alterations in the perikaryal diameters of experimental neurons, demonstrating swelling at Days 1, 3 and 4 followed by shrinkage at Days 7, 14 and 21.

The findings of this study suggest that the severity of the chromatolytic reaction in intrinsic neurons is intimately related to the distance of the lesion from the neuronal soma, while the time of onset of chromatolysis varies with lesion distance but is not directly proportional to it.