Organization and morphology of masticatory neurons in the rat: A retrograde HRP study

Abstract

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 pn and 7 to 30 q, respectively, while their somal diameters (D = 2Jarealn) 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 motoneuron subnucleus innervating the anterior digastric and mylohyoid muscles (two main jaw openers) was also identified by HRP application to the mylohyoid 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 mylohyoid 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 nucleus (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 mandibular 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 pn and 5 to 25 pn, 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 dorsolateral parvicellular reticular formation in whole mandibular cases. XI1 motoneurons 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 pn and 9 to 29 pn, respectively.

Expression of Group I and II Metabotropic Glutamate Receptors in Trigeminal Neurons during Postnatal Development

“…They demonstrated that immunoreactivity was correlated with cell size, such that small to medium size neurons associated with nociceptive processing were labeled, while large diameter dorsal root ganglion neurons were not labeled. A topographical study in rats demonstrated that Me5 neurons that innervate periodontal mechanoreceptors are located in caudal regions of the nucleus, while those which innervate jaw muscle spindles are localized throughout the rostral-caudal extent of the nucleus [Jacquin et al, 1983]. This finding, in conjunction with our data, makes it difficult to correlate mGluR expression with functional properties in Me5 neurons.…”

The <i>Krox-20</i> Null Mutation Differentially Affects the Development of Masticatory Muscles

“…The first possible mechanism is associated with the particular loss of AD and MY motoneurons as a result of the mutation. This mechanism is based on data regarding the caudal position of AD and MY motoneurons within the trigeminal motor nucleus and their localization in the accessory trigeminal motor nucleus adjacent to the caudal pole of the motor nucleus [Jacquin et al, 1983;Meesson and Olszewski, 1949;Matsuda et al, 1978;Szekely and Matesz, 1982]. Our localization of trigeminal motoneurons adjacent to the trigeminal motor nucleus in the fetus is remarkably similar to findings originally described by Meesson and Olszewski [1949] as the ·-division of the trigeminal motor nucleus and later described by Szekely and Matesz [1982] as the accessory trigeminal motor nucleus.…”

“…This conclusion better links the loss of r3 with the observed muscle loss, when considering the caudal localization of AD and MY motoneurons within the trigeminal motor nucleus [Jacquin et al, 1983;Meesson and Olszewski, 1949;Matsuda et al, 1978]. The loss of other masticatory muscles, associated with motoneurons likely derived from r1 and/or r2, is not likely since Krox-20 is not expressed in r1 or r2.…”

Identification of Alpha and Gamma Trigeminal Motoneurons by the Vibratome Paraplast Technique for HRP Histochemistry

“…1979: Rokx et al. 1984, The sizes of the rat anterior digastric motoneurons, on the other hand, are unimodally distributed within a small range despite the presence of relatively many slow fibres [Jacquin et al. 1983;Limwongse and DeSantis.…”