Morphological characteristics and terminating patterns of masseteric neurons of the mesencephalic trigeminal nucleus in the rat: An intracellular horseradish peroxidase labeling study

Abstract

In order to study the morphological characteristics and terminating patterns of the neurons of the trigeminal mesencephalic nucleus (Vme), 55 masseteric neurons in Vme in the rat were stained by intracellular injection of horseradish peroxidase (HRP). Labeled cells were distributed throughout the nucleus. These neurons were divided into three types: uni- or pseudounipolar (type A, n = 43), bipolar (type B, n = 5), and multipolar cells (type C, n = 7). Each type was further divided into two subtypes according to the largest diameter of the perikarya (type a greater than or equal to 30 microns, type b less than 30 microns). The central processes of type Aa neurons projected to the following three groups of target nuclei: 1) nuclei functioning as interneurons, including supratrigeminal nucleus (Vsup), intertrigeminal nucleus (Vint), juxta-trigeminal region (Vjux), and parvicellular nucleus of the pontomedullary reticular formation (PcRF); 2) motor nuclei, including the trigeminal motor nucleus (Vmo), accessory facial nucleus (NVIIacs), accessory abducens nucleus (NVIacs), and a small number of labeled axons in the oculomotor nucleus and trochlear nucleus; 3) sensory nuclei, including the dorsomedial part of the principal trigeminal sensory nucleus (Vpdm) and the dorsomedial part of subnucleus oralis of the trigeminal spinal nucleus (Vodm). Labeled processes were dense in the Vsup, Vmo, and Vpdm. The proprioceptive pathway of the fifth nerve is discussed. Direct projections from type Aa neurons of Vme to the Vpdm and dorsolateral part of the Vsup contribute to conduction of the proprioceptive information from spindles of masticatory muscle to the contralateral thalamus in the rat. Different axon morphology, distribution, terminal branch density, and terminating patterns of type Aa neurons were noted in different functional groups of the projecting nuclei, especially in the Vsup, Vmo, and Vpdm. The highest terminal branching density, the most extensive distribution, and two different types of branching patterns (claw-like and comb-like) were observed in Vsup. Selective distribution and single-beaded or "Y"-shaped terminal branches were observed in Vmo. In the Vppdm the axonal branches were sparser than in the Vsup or Vmo, and had an arrangement like the branches of a weeping willow tree. These characteristics of anatomical organization might be related to the function of each projecting nucleus.

Mesencephalic trigeminal nucleus neurons supplying the jaw closing muscles have no spinal projection: A fluorescent double-labeling study in birds and mammals

“…These results indicate that two morphologically distinct cell populations can be identified in the MeV of duck, rat, and rabbit and that primary sensory cells in the MeV for jaw closing muscles do not send collaterals to the cervical spinal cord. In the light of these findings, the existence of major differences related to MeV among animal species, such as, for instance, rat, rabbit, and cat, as already suggested by Walberg et al (1984) and Luo et al (1991), can be confirmed. With respect to the morphology of the MeV cells related to the jaw closing muscle afferents in particular, discrepancies are noticed between the above-mentioned species.…”

“…In the literature, the findings concerning the central projections of the MeV neurons are controversial, since RECORD 249:255-258 (1997) they largely depend on the species studied and methods used (Luo et al, 1991;Raappana and Arvidsson, 1993). The present study, carried out in an avian and two mammalian species, verified, by means of fluorescent retrograde double-labeling technique, the possibility that the MeV neurons that innervate the muscle spindles of the jaw closing muscles could also have collaterals projecting to the cervical level of the spinal cord.…”

Electrophysiological Features of Neurons in the Mesencephalic Trigeminal Nuclei

“…The Mes V has other roles including transferring information of vibrissa movements to other central nervous system (CNS) areas [2] as well as regulating feeding and exploratory behavior [3]. According to the morphological characteristics and terminating patterns, three types of neurons could be classified in Mes V: (1) jaw muscle spindle afferent Mes V neurons, (2) periodontal ligament mechanoreceptor afferent Mes V neurons and (3) interneurons [4,5]. However, some investigators think that the small multipolar interneurons are not necessarily Mes V neurons.…”

“…As multipolar Mes V neurons may exhibit different properties from pseudounipolar ones, the electrophysiological features reviewed in this paper are mainly referring to common Mes V neurons which receive projections from muscle spindles in the jaw-closing muscles and periodontal mechanoreceptors [6]. Mes V neurons were identified on the basis of their morphological characteristics: a spherical or oval cell body, a large nucleus, a prominent single nucleolus and a pseudounipolar process [5]. The electrophysiological features, reflecting the input-output relation of a single cell, are important for exploring the basic function of a cell within a circuit.…”

VGluT1- and GAD-immunoreactive terminals in synaptic contact with PAG-immunopositive neurons in principal sensory trigeminal nucleus of rat

“…Vp comprises a ventral division and a dorsal division (Vpd), which are cytoarchitectonically different and have been proposed to serve the different functions of relaying and processing of sensory signals from the head, such as tactility and trigeminal proprioception [2][3][4] . Such functions of the Vpd can be supported by the findings that this structure receives direct primary input from intraoral structures and possesses diverse afferent and efferent fiber connections with other brain areas [5][6][7][8] .…”