The objective of this study was to define further the role of the trigeminal motor nucleus (TMNu) in the postnatal ontogeny of the mammalian craniofacial skeleton. To that end, 42 male Sprague-Dawley rats underwent stereotaxic surgery at 40 days of age; 21 received small electrolytic lesions to their left-side TMNu (lesioned group) while 21 had TMNu stimulation with no actual electrolytic lesion produced (sham group). Seven rats from each group were killed at 28, 56, and 84 days postoperative to analyze trigeminal motoneuron (TMNe) count, masticatory muscle weight, and osteological growth vector data. At all three time periods, lesioned animals showed significant differences 1) between the surgery and nonsurgery sides, and 2) from sham animals. However, sham animals also demonstrated significant between-side differences for medial pterygoid muscle weight (56 days), mandibular height (28 and 56 days), and mandibular length data (84 days); these data suggested that even relatively slight damage to TMNe can create morphological changes within the craniofacial complex. Snout deviation in a lesioned rat towards the opposite side from all other lesioned animals was correlated with unique damage to its pontine reticular formation; this suggested that the observed morphological alterations of the craniofacial complex may have been due not only to TMNu damage, but also to changed expressions of the masticatory central pattern generator (CPG). Morphological alterations of the craniofacial skeleton resulting from lesions to the TMNu were likely due to changed neuromuscular activity patterns of the masticatory muscles and their biomechanical effects upon bone.

Stereotaxic surgery was performed on the trigeminal motor nuclei (TMNu) of 47 actively growing rats in order to test the effects of neuromuscular alterations on TMJ disc morphology. Lesioned animals received a small electrolytic lesion to their left-side TMNu, while shams had their TMNu stimulated without an actual lesion being produced. Both lesioned and sham-lesioned animals demonstrated significant (P less than 0.05; P less than 0.01) morphological alterations of their discs 28, 56, and 84 days postoperatively. Data indicated that (i) TMNu lesions do indeed affect disc morphology, and (ii) even subtle damage to trigeminal motoneurones can influence TMJ disc morphology. It is therefore suggested that at least some TMJ disease processes have a neuromuscular basis.