Abstract
The mesencephalic trigeminal nucleus (Me5) is mostly composed of a unique population of large pseudounipolar cells which are considered centrally displaced ganglion cells. It is known that mesencephalic trigeminal neurons are very sensitive to injury of their peripheral axonal branches that may lead to degeneration of the central axons. Therefore, Me5 could serve a suitable paradigm for studying in a chronological manner the plastic structural alterations in primary sensory neurons after peripheral axotomy. We set it as a task to follow up the structural adaptive changes in axotomized Me5 neurons of the rat. Adult Wistar rats of both sexes were subjected to unilateral transection of the masseteric nerve trunk. The morphology and number of survived neurons were chronologically recorded and statistically analyzed. We found that one week after peripheral axotomy the number of Me5 neurons ipsilateral to the nerve cut was somewhat smaller when compared with the contralateral intact side. At that time, the affected neurons showed initial chromatolytic changes. Two weeks after the intervention, the number of axotomized neurons further decreased and clear signs of degeneration could be observed in their perikarya and axonal processes. The level of degeneration increased in the days to follow until the fourth postoperative week. Eight weeks after the axotomy, the number of counted Me5 neurons ipsilateral to the nerve cut was still statistically significantly reduced than the one on the contralateral control side. Our results show that the process of adaptation after peripheral axotomy is characterized by a substantial cell loss and the adaptive morphological response to injury of survived Me5 neurons comprises of qualitative and quantitative alterations, thus implying their remarkable structural plasticity.
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