Corticofugal projections to trigeminal motoneurons innervating antagonistic jaw muscles in rats as demonstrated by anterograde and retrograde tract tracing.

Little is known about the organization of corticofugal projections controlling antagonistic jaw muscles. To address this issue, we employed retrograde (Fluorogold; FG) and anterograde (biotinylated dextran amine; BDA) tracing techniques in rats. Three groups of premotoneurons were identified by injecting FG into the jaw-closing (JC) and -opening (JO) subdivisions of the trigeminal motor nucleus (Vmo). These were 1) the intertrigeminal region (Vint) and principal trigeminal sensory nucleus for JC nucleus; 2) the reticular region medial to JO nucleus (RmJO) for JO nucleus; and 3) the parabrachial (Pb) and supratrigeminal (Vsup) nuclei, reticular regions medial and ventral to JC nucleus, rostrodorsomedial oralis (Vor), and juxtatrigeminal region (Vjuxt) containing a mixture of premotoneurons to both the nuclei. Subsequently, FG was injected into the representative premotoneuron structures. The JC and JO premotoneurons received main afferents from the lateral and medial agranular fields of motor cortex (Agl and Agm), respectively, whereas afferents to the nuclei with both JC and JO premotoneurons arose from Agl also and from primary somatosensory cortex (S1). Finally, BDA was injected into each of the three cortical areas representing the premotoneuron structures to complement the FG data. The Agl and Agm projected to reticular regions around the Vmo, whereas the Pb, Vsup, Vor, and Vjuxt received input from Agl. The S1 projected to the trigeminal sensory nuclei as well as to the Pb, Vsup, and Vjuxt. These results suggest that corticofugal projections to Vmo via premotoneuron structures consist of multiple pathways, which influence distinct patterns of jaw movements.

Biomechanical analysis on platform switching: is there any biomechanical rationale?

OBJECTIVES: The purpose of this study was to examine the biomechanical advantages of platform switching using three-dimensional (3D) finite element models. MATERIAL AND METHODS: 3D finite element models simulating an external hex implant (4 x 15 mm) and the surrounding bone were constructed. One model was the simulation of a 4 mm diameter abutment connection and the other was the simulation of a narrower 3.25 mm diameter abutment connection, assuming a platform-switching configuration. RESULTS: The stress level in the cervical bone area at the implant was greatly reduced when the narrow diameter abutment was connected compared with the regular-sized one. CONCLUSION: Within the limitations of this study, it was suggested that the platform switching configuration has the biomechanical advantage of shifting the stress concentration area away from the cervical bone-implant interface. It also has the disadvantage of increasing stress in the abutment or abutment screw.

The somatotopic organization of trigeminal premotoneurons in the cat brainstem.

This study was performed to complement the results of prior intracellular recording and labeling studies by investigating the general distribution pattern of trigeminal premotoneurons in the cat brainstem using the retrograde tracing methods. The results of the present study reconfirmed the presence of premotoneurons in the trigeminal principal and oral nuclei following horseradish peroxidase injections into the jaw-opening (JO) or jaw-closing (JC) nucleus. Furthermore, we found that labeled cells from the JO nucleus and JC nucleus located in the reticular regions surrounding the trigeminal motor nucleus (Vmo; Vmo shell region) were arranged in a topographic fashion, while those in the parabrachial nucleus, supratrigeminal nucleus, lateral reticular formation caudal to the shell region and raphe nuclei were intermingled with each other. The labeling in the individual nuclei was bilateral with an ipsilateral predominance to each injection site, with the exception of the mesencephalic trigeminal nucleus, where the labeling was ipsilateral to the injection site in the JC nucleus. These results, combined with the data of the previous intracellular tracing studies, indicate that based on the presence of somatotopic organization, premotoneurons can be largely divided into two groups; those projecting to either the JO or the JC nucleus and those projecting to the two nuclei, and we offer the suggestion that roles of premotoneurons for jaw movements differ among the individual nuclei.