Comparative study of the neuronal plasticity along the neuraxis of the vibrissal sensory system of adult rat following unilateral infraorbital nerve damage and subsequent regeneration

The aim of the present study was to examine the physiological consequences of a unilateral infraorbital nerve lesion and its regeneration at different levels of the somatosensory neuraxis. In animals whose right infraorbital nerve had been crushed, a large unresponsive area was found in the main bra instem trigeminal nucleus (Pr5). Responses evoked by ipsilateral vibrissal deflection in the middle of Pr5 reappeared only on days 22-35 after the ner ve had been transected, whereas recovery from the nerve crush took only 7-9 days. However, no sign of short-term neuronal plasticity was observed in P r5 after peripheral nerve injury. An enlargement of the receptive fields in two-thirds of the units and a lengthening in the delay of the evoked respo nses were observed as long-term plastic changes in Pr5 neurons after periph eral-nerve regeneration. In the ventral posteromedial nucleus of the thalam us (VPM) of partly denervated animals, however, only minutes or hours after the nerve crush, certain units were found to respond in some cases not onl y to the vibrissae, but also to mechanical stimulation of the face over the eye (two units), the nose (one unit), and the midline (one unit). Apart fr om the experiments involving incomplete denervation, the vibrissal represen tation areas of the VPM were unresponsive to stimulation of both the vibris sae and other parts of the face until nerve regeneration had occurred. In t he somatosensory cortex, an infraorbital nerve crush immediately resulted i n a large cortical area being unresponsive to vibrissal deflection. It was noteworthy, however, that shortly after the nerve crush, this large unrespo nsive whisker representation cortical area was invaded from the rostromedia l direction by responses evoked by stimulation of the forepaw digits. In sp ite of the reappearance of vibrissa-evoked responses 7-10 days after the ne rve crush, an expanded digital representation could still be observed 3 wee ks after the nerve crush, resulting in an overlapping area of digital and v ibrissal representations. The withdrawal of the expanded representation of forepaw digits was completed by 60 days after the nerve crush. The results obtained in Pr5, the VPM, and the cortex strongly suggest that the higher t he station in the neuraxis, the greater the degree of plasticity after infr aorbital nerve injury.