Organization and morphology of masticatory neurons in the rat: A retrograde HRP study
Authors: Jacquin Mark F. , Rhoades Robert W. , Enfiejian Howard L. , Egger M. David 
Published: January 2005

The somatic representations of the trigeminal (V) and hypoglossal (XII) nerves in the rat brainstem were examined by using retrograde horseradish peroxidase (HRP) transport from peripheral nerves. HRP applied to either the mandibular division of V, or the V motor root, revealed the V motor nucleus (Mot V), a largely ellipsoid collection of multipolar motoneurons extending rostrally from the level of the facial genu to the rostral border of the principal sensory V nucleus. The distributions of major and minor axes of Mot V cell bodies were unimodal, ranging from 12 to 60 μm and 7 to 30 μm, respectively, while their somal diameters (D \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {2\,{\rm area}/{\rm \pi}} $\end{document} ) and areas were somewhat bimodally distributed. A smaller collection of cell bodies was also labelled along the medial trajectory of the V motor root in the ventrolateral pons. These cells were, as a rule, smaller than those in Mot V. A V motoneu‐ron subnucleus innervating the anterior digastric and mylohyoid muscles (two main jaw openers) was also identified by HRP application to the mylo‐hyoid nerve. This subnucleus extended the entire length of Mot V; at caudal levels it occupied the ventromedial margin; midway it almost disappeared, reappearing rostrally where the labelled cells were diffuse. Most labelled my‐lohyoid axons coursed dorsomedially and rostrally toward the floor of the fourth ventricle before turning laterally as a V genu to exit the pons with the V motor root.

To delineate the somatotopic organization of the V mesencephalic nu‐cleus (Mes V), HRP was applied to the whole V mandibular ganglion, the V motor root, the lingual, inferior alveolar, mylohyoid, and auriculotemporal nerves, individually or collectively, or the infraorbital nerve. In whole man‐dibular cases, labelled Mes V cells extended from the level of caudal Mot V to the level of the rostral superior colliculus. Mes V had a widened caudal base and a tapered rostral end that occupied the lateral margin of the central gray. Mes V cells were unipolar with unimodal distributions of major and minor axes which ranged from 12 to 42 μm and 5 to 25 μm, respectively. Labelled cells were visible at all levels of Mes V, but primarily caudally, following inferior alveolar nerve applications. Infraorbital nerve applications also labelled Mes V cells, though only caudally. Applications of HRP to the mylohyoid or auriculotemporal nerves labelled few, if any, Mes V cells. None at all were observed following lingual nerve HRP applications. Retrogradely labelled cells were also observed in the supratrigeminal nucleus following either whole mandibular or inferior alveolar applications and in the dorsolat‐eral parvicellular reticular formation in whole mandibular cases. XII moto‐neurons innervating tongue extensor muscles were labelled following HRP application to the ascending ramus of XII. These cells formed a subnucleus largely in the ventromedial central gray of the medulla. The distributions of major and minor axes of these cells were unimodal and ranged from 17 to 45 μm and 9 to 29 μm, respectively.