Fibers of trigeminal mesencephalic neurons in the maxillary nerve of the rat

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

Selective expression of histamine receptors in rat mesencephalic trigeminal neurons

The perikarya of sensory neurons of the mesencephalic trigeminal nucleus (MTN) receive dense histaminergic hypothalamic innervation. In this study, we examine the yet unknown expression and localization of histamine receptors in the rat MTN using immunohistochemistry with subtype-specific antibodies. Same as the masticatory muscle spindle somata H1 receptors were located along the entire MTN, whereas H3 receptors were detected in the caudal pontine part of the nucleus, which receives input from periodontal afferents. Most of the immunostained cell bodies were surrounded by histidine decarboxylase-, histamine- or vesicular monoamine transporter 2-containing pericellular varicose fibers and terminals in a basket-like manner. Our results suggest that rat MTN neurons are directly influenced by histaminergic descending projections from the hypothalamus. It can be inferred that processing of proprioceptive information at the level of the MTN is controlled via histamine H1 and H3 receptors through different postsynaptic mechanisms.

Localization of orexin-A-immunoreactive fibers in the mesencephalic trigeminal nucleus of the rat

Orexin A is a neuropeptide located exclusively in neurons in the hypothalamic nuclei involved in the central regulation of many brain functions, related to motor activity and state-dependent processes. Orexins modulate behavioral state via actions across multiple terminal fields. In order to determine whether the mesencephalic trigeminal neurons may receive a direct hypothalamic orexinergic input, the distribution of orexin A immunoreactivity was examined in the rat mesencephalic trigeminal nucleus (MTN), using orexin A immunohistochemistry. Orexin-A-immunostained nerve fibers and terminals were found in a close apposition to the perikarya of primary afferent neurons in the MTN with a marked rostrocaudal gradient in their density. In the caudal pontine MTN, only scattered orexin-A-immunoreactive fibers were found, while more rostrally in the pons, and in the midbrain-pontine junction part of the nucleus, orexin-A-immunopositive varicosities were relatively more abundant, located in close proximity to or often surrounding the neuronal profiles. At the level of the inferior or superior colliculi, a large number of orexin-A-containing neuronal processes and terminal arborizations were observed traveling toward and contacting mesencephalic trigeminal neurons, some of which were multipolar. The results of this study show that MTN neurons receive orexin A hypothalamic innervation with a somatotopic arrangement of the projections in the nucleus. The central orexinergic system may exert direct influence upon jaw movements at the level of the MTN and thus to participate in the control of feeding behavior.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Localization of D1 and D2 dopamine receptors in the rat mesencephalic trigeminal nucleus by immunocytochemistry and in situ hybridization

The rat mesencephalic trigeminal nucleus (MTN) receives a dopaminergic innervation. In order to identify and localize dopaminoceptive cells within this nucleus, expression of D1 and D2 dopamine (DA) receptors was examined by immunocytochemistry with subtype-specific antibodies and in situ hybridization with digoxigenin-labeled cRNA probes. Immunocytochemical labeling was restricted to neuronal perikarya and proximal processes whereas the hybridization signal was confined to MTN cell bodies. Cells immunopositive for D1 were located throughout the entire MTN whereas cells labeled with D2 antibodies were concentrated in its caudal portion. D1 receptor message was found in relatively low levels. In contrast, high levels of mRNA for D2 were seen in MTN neurons. The distribution of DA receptor mRNA-containing cells were very similar to those of DA receptor immunoreactivity. Neurons expressing the D1 receptor gene were localized in both rostral and caudal portions, which receive inputs from masticatory muscle spindles and from spindles and periodontal ligament receptors, respectively. D2 receptors were limited to ventrocaudally located cells. These results suggest that processing of proprioceptive information in the MTN is controlled by the DAergic input through different postsynaptic mechanisms.

Projection of jaw-muscle spindle afferents to the caudal brainstem in rats demonstrated using intracellular biotinamide

Intracellular staining with biotinamide was used to study the axonal projection and synaptic morphology of rat jaw-muscle spindle afferents. Intracellular recordings in the mesencephalic trigeminal nucleus (Vme) were identified as spindle afferent responses by their increased firing during stretching of the jaw-elevator muscles. Biotinamide-stained axon collaterals with boutons were found in the trigeminal motor nucleus (Vmo), Vme, the region dorsal to Vmo including the supratrigeminal region, the dorsomedial portion of the trigeminal principal sensory nucleus, and the dorsomedial part of the rostral spinal trigeminal subnucleus oralis. Additional, previously undescribed projections of jaw-muscle spindle afferents were found to the dorsomedial portion of the caudal spinal trigeminal subnucleus oralis (Vodm), the dorsomedial part of the spinal trigeminal subnucleus interpolaris (Vidm), the caudal parvicellular reticular formation, laminae IV and V of the spinal trigeminal subnucleus caudalis (Vc), and the dorsal division of the medullary reticular field. Labeled spindle boutons in Vodm formed predominately axodendritic synapses. Some of these boutons received presynaptic inputs from unlabeled P-type boutons containing clear, spherical, or flattened vesicles. In Vidm, labeled collaterals and boutons were densely clustered into glomerular-like structures. Labeled boutons in Vidm made axodendritic, axosomatic, and axoaxonic synapses and received synaptic contacts from unlabeled boutons containing clear, spherical, or flat and pleomorphic vesicles. Unlabeled presynaptic boutons in Vidm occasionally contained dense core vesicles. Labeled boutons in Vc mainly formed synaptic contacts with large diameter dendrites. This projection of jaw-muscle spindle afferents to caudal brainstem regions may play a significant role in masticatory-muscle stretch reflexes and in the integration of trigeminal proprioceptive information and its transmission to higher centers.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

The reaction of mesencephalic trigeminal neurons to peripheral nerve transection in the adult rat

The effects of peripheral nerve transection on mesencephalic trigeminal (MeV) neurons have been studied qualitatively and quantitatively in the rat. In the qualitative part of the study the brain stem was studied in Fink-Heimer stained sections 3-30 days after a masseteric nerve transection. Degeneration argyrophilia was observed both in the MeV tract and in the supratrigeminal and trigeminal motor nuclei, as well as in the lateral part of the brain stem reticular formation. The first signs of transganglionic degeneration (TGD) were seen 7 days postoperatively, and the amount of degeneration increased considerably with longer survival times. A quantitative analysis of the MeV nucleus was made 60 days after transection of the left masseteric nerve. This analysis showed a 10.5-22.7% reduction of cells on the side that had undergone masseteric nerve transection. The mean difference (left vs right side) was -2.4% in animals that had not been operated on. These findings show that mesencephalic trigeminal neurons with proprioceptive functions are very sensitive to peripheral nerve injury with a substantial cell loss and TGD as the result.

The anatomical substrate for cat extraocular muscle proprioception

The localization of cell bodies and of the central terminal projections of extraocular muscle afferent neurons was examined in adult cats using transport of horseradish peroxidase. The results confirm that primary afferent cell somata subserving extraocular muscle proprioception are located within the medial portion of the ipsilateral trigeminal ganglion. Occasional labeling of cell bodies in the mesencephalic nucleus of the trigeminal nerve occurred only in association with evidence of spread of tracer beyond the eye muscles. These results, taken together with work of others, make it unlikely that the trigeminal mesencephalic nucleus participates significantly in eye muscle proprioception. The central projections of extraocular muscle afferent neurons were found consistently in a restricted area in the ventral portion of the pars interpolaris of the spinal trigeminal nucleus. This corresponds exactly with their site of termination in the monkey [Porter (1986) J. comp. Neurol. 247, 133-143]. Terminal labeling was restricted to this area in cases in which there was no evidence of spread of the tracer beyond the extraocular muscles. In contrast to previous findings in the monkey, the cat did not exhibit a second muscle afferent representation in the cuneate nucleus. Though it is known that extraocular muscle afferent signals interact with both retinal and vestibular signals, and thus probably are involved in both visual processing and oculomotor control, the details of their roles in these processes are not yet clear.(ABSTRACT TRUNCATED AT 250 WORDS)

HRP study of the central components of the trigeminal nerve in the larval sea lamprey: organization and homology of the primary medullary and spinal nucleus of the trigeminus

The medullary and spinal connections of the trigeminal nerve of larval sea lampreys Petromyzon marinus were studied by anterograde and retrograde HRP transport after application into the orbit. Three components were found, all of them ipsilateral: 1) The motor nucleus was undivided in the larva, and its neurons possessed a rich dendritic tree. The single motor root was well separated from the sensory root. 2) The descending root was laterally located, and its fibers ran compactly to spinal levels. 3) Most medullary and many rostral spinal dorsal cells were labeled. Dorsal cells, which were mostly multipolar, had numerous mutual contacts. Some dorsal cell processes contacted the fourth ventricle. The name "primary medullary and spinal nucleus of the trigeminal nerve" (PMSV) is proposed for these dorsal cells. Medullary dorsal cells were not labeled by applying HRP at the level of spinal nerves, but application to the vagus nerve did label some. The possible relationship of this nucleus with the mesencephalic trigeminal nucleus of jawed vertebrates is discussed.

Morphology of jaw-muscle spindle afferents in the rat

The morphology of jaw-muscle spindle afferents in the rat has been studied by intra-axonal injection of horseradish peroxidase. All stained axons were located in the motor root of the trigeminal nerve and could be traced dorsomedially to the vicinity of the trigeminal motor nucleus, where they divided into an ascending branch in the tract of the mesencephalic nucleus and a descending branch in the tract of Probst. Axon collaterals and swellings on fine collateral branches presumed to be synaptic boutons were located in the following regions: the trigeminal motor nucleus, the region dorsal to the trigeminal motor nucleus including the supratrigeminal nucleus, the parvicellular reticular formation immediately caudal to the trigeminal motor nucleus, the reticular formation at the level of the facial nucleus, and the caudal portion of the mesencephalic nucleus. No evidence of a projection to the cerebellum was observed. Boutons were most numerous in the region surrounding the trigeminal motor nucleus, especially dorsally. Here they were not demonstrated in close proximity to counterstained cells, and therefore it was not possible to determine how many of these contacts are located on cells in this region and how many are on the distal dendrites of trigeminal motorneurons. Boutons located within the trigeminal motor nucleus were always confined to a small portion of the nucleus and were significantly larger than those located dorsally. Some boutons were found in close apposition to trigeminal motorneurons and presumably make somatic contacts. These results suggest that jaw-muscle spindle afferents make somatic and proximal dendritic contacts with only a limited number of trigeminal motorneurons and also project to masticatory interneuronal regions dorsal and caudal to the motor nucleus.

Localisation of the first-order neurone of the jaw opening reflex elicited by periodontal stimulation

The localisation of the first-order neurone of the jaw-opening reflex (JOR), provoked by periodontal stimulation, was investigated in the rat. A section of the mandibular part of the trigeminal ganglion was carried out without impairing the motor root. It suppressed the reflex triggered by the stimulation of the lower incisor. Bilateral destruction of the mesencephalic nucleus and tract does not modify the JOR. These results suggest that the first-order neurone of the reflex is located in the trigeminal ganglion.

Neuronal development in the trigeminal mesencephalic nucleus of the duck under normal and hypothyroid states: I. A light microscopic morphometric analysis

Light microscopic morphometric procedures were used in order to examine the effects of propylthiouracil (PTU) on the development of the mesencephalic nucleus of the trigeminal nerve in the duck. A single vascular injection of a 0.2% solution of PTU was administered at a dosage of 2 microliter/gm embryo weight on embryonic day nine (E9). Control embryos received a similar dose of Ringer's solution. The following parameters of cytodifferentiation of cells of the mesencephalic nucleus of V were studied: somal area profiles, nuclear area, and nuclear cytoplasmic ratios. In addition, the frequency of beak clapping was recorded from E16. Significant differences were observed in somal area profiles in the experimental group at E16 and E18 and in nuclear area profiles from E16 through hatching. Beak activity in the experimental embryos was drastically reduced. It is concluded that PTU induces a retardation in the differentiation of cells of the mesencephalic nucleus of V which may lead to behavior deficits as evidenced by reduction of beak activity. These observations provide a basis for the study of interactions between thyroid hormone and specific neuronal systems in the emergence of an adaptive function.

The motor complex and primary projections of the trigeminal nerve in the monitor lizard, Varanus exanthematicus

The sensory projections and the motor complex of the trigeminal nerve of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The primary afferent fibers diverge in the brainstem into a short ascending and a long descending tract. The former distributes its fibers to the principal sensory trigeminal nucleus, where nerves V1, V2, and V3 are represented along a lateromedial axis. The fibers of the descending tract enter the nucleus of this tract and the reticular formation. Both in the tract and its nucleus, nerves V1, V2 and V3 occupy successively more dorsal positions. A small contingent of nerve V1 fibers course to the accessory abducens nucleus. The descending tract extends caudally into the first and second cervical segments of the spinal cord. The trigeminal motor complex consists of dorsal, ventral, and dorsomedial nuclei. The m. adductor mandibulae externus (the main jaw closer) is represented in the dorsal nucleus, predominantly in its rostral part. The muscles innervated by nerve V3 are represented in the ventral nucleus, mainly in its caudal part. All three divisions of the trigeminal nerve contain peripheral branches of the mesencephalic trigeminal system. Collaterals of the central branches of this system were traced to the ventral motor and the principal sensory trigeminal nuclei.

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.

On the innervation of trigeminal mesencephalic primary afferent neurons by adenosine deaminase-containing projections from the hypothalamus in the rat

The localization and sources of adenosine deaminase-containing structures in the mesencephalic nucleus of the trigeminal nerve of the rat was studied using indirect immunofluorescence or immunoperoxidase immunohistochemical staining techniques for adenosine deaminase in combination with retrograde fluorescent tracing or lesion methods. The majority of large mesencephalic neurons were engulfed by a dense adenosine deaminase-immunoreactive plexus. Immunostaining was often punctate surrounding neuronal profiles or sometimes had the appearance of varicose fibers coursing over the neuronal surface. Occasionally, immunostained axons were found travelling towards and contacting mesencephalic neurons. Mesencephalic neuronal somas surrounded by immunofluorescence staining for adenosine deaminase were simultaneously labelled with fast blue after injections of this dye into the temporalis or masseter muscles of mastication. Injections of fast blue into the mesencephalic nucleus resulted in fast blue labelling of adenosine deaminase-immunoreactive neurons in the tuberal, caudal and postmammillary caudal magnocellular nuclei of the hypothalamus. Ablation of these hypothalamic nuclei caused a near total depletion of adenosine deaminase-immunostained fibers in the mesencephalic nucleus including those associated with mesencephalic neurons. It is concluded that adenosine deaminase-containing neurons in the posterior hypothalamus innervate mesencephalic primary sensory neurons, which are known to convey proprioceptive input to trigeminal motor nuclei controlling jaw muscles. The possibility is considered that the hypothalamus, via a direct action on these sensory neurons, may exert automatic control over jaw movements related to aggressive attack, defensive or feeding behavior. In addition, it appears that mesencephalic neurons may provide an ideal model system for electrophysiological investigations of the neurotransmitter(s) utilized by adenosine deaminase-containing hypothalamic projections.

Differential distribution of cell bodies and central axons of mesencephalic trigeminal nucleus neurons supplying the jaw-closing muscles and periodontal tissue: a transganglionic tracer study in the cat

Distribution of cell bodies and central axons of mesencephalic trigeminal nucleus (MTN) neurons were examined in the cat by the method of transganglionic transport of horseradish peroxidase (HRP). Jaw-closing muscle afferent MTN neurons were distributed throughout the whole rostrocaudal extent of the MTN, and sent their axons ipsilaterally to the supratrigeminal and intertrigeminal regions, dorsolateral division of the motor trigeminal nucleus, lateral part of the medullary reticular formation, lamina VI of C1-C3 cord segments, and cerebellum. On the other hand, periodontal receptor afferent MTN neurons were located mainly in the caudal part of the MTN, and sent their axons ipsilaterally to the supratrigeminal region and cerebellum. The existence of multipolar MTN neurons with 1-9 smooth dendrites was also confirmed; most of them were jaw-closing muscle afferent neurons.

On the morphology of the mesencephalic trigeminal cells. New data based on tracer studies

Injections of free horseradish peroxidase in the masticatory muscles of the cat resulted in retrograde labeling of not only large and small so-called pseudounipolar cells but also of multipolar neurons within the ipsilateral mesencephalic trigeminal nucleus. The latter cell type was present only in the pontine part of the nucleus, and usually more faintly labeled than the other cells. Several of the so-called pseudounipolar cells showed cell processes similar to dendrites of other cells, a finding indicating that the afferent connections of these cells are much more complex than hitherto assumed. The observations are discussed also with reference to the problem whether the multipolar cells should be considered as displaced locus coeruleus neurons.

Response characteristics of primary periodontal mechanoreceptive neurons in the trigeminal mesencephalic nucleus to trapezoidal mechanical stimulation of a single tooth in the rat

The response characteristics of primary periodontal mechanoreceptive neurons in the trigeminal mesencephalic nucleus (MeNV) were studied by changing the rate and magnitude of trapezoidal pressure applied to the upper incisor in very lightly anesthetized rats. Using a metal microelectrode in the MeNV either for recording or for stimulation, the projection site of primary afferents in the anterior superior alveolar nerve innervating the upper incisor was determined. Its stereotaxic coordinates were 1.0--2.0 mm posterior to the interaural plane and 1.2--1.3 mm lateral to the mid-sagittal plane, corresponding to the caudal part of the MeNV. From this site of 19 animals, single unitary activity from 41 primary periodontal mechanoreceptive neurons was recorded, which were identified by: (1) the constant, brief latency of 1.0--4.0 ms (2.0 +/- 0.6 ms, mean +/- S.D.) from the onset of single tooth tapping; (2) the wave forms of their unitary spike responses; and (3) the ability to follow faithfully trains of repetitive stimuli applied to the anterior superior alveolar nerve at rates of more than 100 Hz for 2 s. The responses of the identified primary mechanoreceptive neurons in the MeNV were tested for repetitive tapping and trapezoidal pressure to the ipsilateral upper incisor. The highest frequency of one-to-one following was observed in repetitive tapping at 100 Hz. All of the 27 tested neurons exhibited such very rapid adaptation as to show only on-off responses to trapezoidal pressures, and did not respond at all unless the pressures were applied more rapidly than 0.6 Newtons/s (N/s). In most of these neurons, the number of evoked spikes was greater in on-responses than in off-responses, and the number and frequency of spikes were increased with an increase in the rate of pressure application. But in several neurons only one spike was triggered even when the pressures were applied at the rate of more than 83.1 N/s.

Reevaluation of projections from the mesencephalic trigeminal nucleus to the medulla and spinal cord: new projections. a combined retrograde and anterograde horseradish peroxidase study

Microinjection of horseradish peroxidase (HRP) into the medullary parvocellular reticular formation (NPvc) resulted in retrograde labeling of neurons throughout the mesencephalic trigeminal nucleus (Mes V). Labeled cells were large and ovoid and were distributed primarily in the expanded pontine part of the nucleus. However, none of the small neurons in Mes V were labeled. Injections of HRP made into adjacent brainstem structures including the nucleus gigantocellularis, ventrolateral reticular formation, vestibular complex, and the spinal trigeminal nucleus failed to label neurons in Mes V. Injections made into the medullary raphe and into regions reported to receive inputs from Mes V--spinal cord, nucleus tractus solitarius, hypoglossal nucleus, and facial nucleus--were also not followed by transport to Mes V. Anterograde axonal transport of HRP from the region of reticular formation innervated by Mes V also labeled axons projecting to Mes V and to visceral and somatic sensorimotor nuclei in the lower brainstem. Recent reports of afferents from the amygdala to Mes V suggest that reflexes involving the mesencephalic trigeminal nucleus might be modulated by signals from limbic and autonomic as well as somatic centers in the brain.

Localization of morphology of cat extraocular muscle afferent neurones identified by retrograde transport of horseradish peroxidase

Afferent neurones that provide proprioceptive innervation extraocular muscles of the cat have been identified by means of retrograde axonal transport of horseradish peroxidase (HRP). Discrete injections of HRP into the medial rectus, lateral rectus, or retractor bulbi muscles labeled pseudounipolar neurons that were localized exclusively to the ipsilateral semilunar ganglion. The distribution of labeled neurons within the ganglion was consistent with its somatotopic organization with the majority found within the ophthalmic subdivision. Cell counts indicating approximately 90 labeled neurones per horizontal rectus muscle correlated well with earlier quantitative observations regarding the percentage of afferent fibers in oculomotor nerves and the number of proprioceptive terminals in the extraocular muscles. Neither the trigeminal mesencephalic nucleus nor the contralateral semilunar ganglion contained labeled neurones following injections of HRP into extraocular muscles. Consistent with other studies of spinal and cranial ganglia the contingent of pseudounipolar neurones present in the cat semilunar ganglion included both light and dark cell types. Light and electron microscope analysis of HRP-labeled neurones in combination with acetylcolinesterase (AChE) histochemistry revealed that only one of the two neuronal types, the light cell, subserves extraocular muscle proprioception. Our data support the hypothesis that ganglion neurone type and, more specifically, soma diameter, are important determinants of functional status.