Research reportConvergence of selected inputs from sensory afferents to trigeminal premotor neurons with possible projections to masseter motoneurons in the rabbit
Introduction
It is well known that a neural network for control of jaw movements is found in the brainstem. Electrophysiological studies have shown that a group of neurons responsible for generation of masticatory rhythm called masticatory central pattern generator (mCPG) is located in the medial bulbar reticular formation [33], [34]. However, it is obvious from recent studies that these circuits include more rostral nuclei surrounding the trigeminal motor nucleus (NVmot) in which the neural activity was shown to be modulated in fictive mastication [7], [15], [48]. On the other hand, it has been reported that interneurons in this area including the supratrigeminal area (NVs), intertrigeminal area (NVint), subnucleus Ī³ of the oral nucleus of the spinal trigeminal tract (NVspo-Ī³) and main sensory trigeminal nucleus (NVsnpr) project to the NVmot in electrophysiological [1], [2], [7], [36], [50] and anatomical [9], [17], [20], [21], [22], [30], [44], [52] studies. With regards to characteristics of peripheral input convergences to these neurons, most NVint, NVspo-Ī³ and NVsnpr neurons are found to have convergences from more than one peripheral afferent [35], [36], [50], [51]. Olsson and Westberg reported that input characteristics of NVspo-Ī³ premotor neurons differed from those of unselected NVspo-Ī³ neurons, in that unselected neuron groups had a wider convergence [36], [50]. They suggested that the activity of premotor neurons combined with the rhythmical drive from the mCPG might be a way to optimise the temporal ordering and the relative amplitude of the muscle activation pattern. This was also suggested by Kamogawa et al. who provided evidence that NVs neurons bilaterally projecting to the NVmot had peripheral input convergences from the inferior alveolar nerve (IAN) and infraorbital nerve (ION) [16], [17]. On these bases, it is of interest to investigate the convergence profile on and the projection of trigeminal premotor neurons to the NVmot.
The aim of this work is to identify trigeminal premotor neurons in the vicinity of the NVmot on the basis of input convergences from sensory afferents of intra-oral mechanoreceptors and jaw-closing muscle spindles. We also provide evidence in the present study that groups of interneurons located in the vicinity of the NVmot send their axons into the masseteric subnucleus of the trigeminal motor nucleus (NVmot-mass). Preliminary results have been reported in an abstract form [14].
Section snippets
Surgical procedures
The experiments were carried out on 9 adult male New Zealand white rabbits weighing between 2.0 and 4.1 kg. The animals were pre-medicated with ketamine (50 mg/kg, im). After half an hour, they were anesthetized with a mixture of urethane (0.5 g/kg) and Ī±-chloralose (50 mg/kg) administered intravenously through the marginal ear vein, supplemented with Ī±-chloralose (10 mg/kg/h, iv). Physiological saline was administered by intravenous infusion (10 ml/kg/h). Tracheal cannulation was performed and
Identification
This paper is based on recordings made from 30 neurons which were identified as trigeminal premotor neurons. Six neurons were found to project to the NVmot-mass ipsilaterally, 12 neurons contralaterally, and 6 neurons bilaterally. Although the remaining 6 neurons were identified to project contralaterally, their ipsilateral projection was not tested. Histological location of interneurons recorded in this study is summarized in Fig. 1. Eight neurons were found in the NVint, 7 in the NVs, 6 in
Discussion
Our present results provide a functional profile of premotor neurons which are located in the vicinity of the NVmot and project to the NVmot-mass. Interneurons which received inputs from the IAN and/or ION were found mainly in the NVint, NVsnpr and NVspo-Ī³ and most neurons tested received inputs from both of these nerves. Furthermore, interneurons which received input from the MassN and/or responded to passive jaw opening were found in the NVint, NVs and NVspo-Ī³. Our results indicate that those
Acknowledgements
We thank Dr. H. Crick and Dr. S. Ariyasinghe for their helpful comments on this manuscript.
References (53)
- et al.
An electrophysiological study of trigeminal commissural interneurons in the anaesthetized rabbit
Brain Res.
(1990) - et al.
Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus I masticatory muscle motor systems
Brain Res. Brain Res. Rev.
(1997) Masseter muscle excitation induced by stimulation of periodontal and gingival receptors in man
Brain Res.
(1971)- et al.
Variation of the jaw-opening reflex during spontaneous mastication in rabbits
Brain Res. Bull.
(1994) - et al.
Candidate interneurons mediating peripherally evoked disynaptic inhibition of masseter motoneurons of both sides
Neurosci Lett.
(1988) - et al.
Supra- and juxtatrigeminal inhibitory premotor neurons with bifurcating axons projecting to masseter motoneurons on both sides
Brain Res.
(1994) - et al.
Identification of brainstem interneurons projecting to the trigeminal motor nucleus and adjacent structures in the rabbit
J. Chem. Neuroanat.
(2000) - et al.
Identification of premotor interneurons which project bilaterally to the trigeminal motor, facial or hypoglossal nuclei: a fluorescent retrograde double-labeling study in the rat
Brain Res.
(1993) - et al.
Monosynaptic connections between neurons of trigeminal mesencephalic nucleus and jaw-closing motoneurons in the rat: an intracellular horseradish peroxidase labelling study
Brain Res.
(1991) - et al.
Responses of neurons in nucleus supratrigeminalis to sinusoidal jaw movements in the cat
Exp. Neurol.
(1987)