Axonal endings terminating on dendrites of identified large trigeminospinal projection neurons in rat trigeminal nucleus oralis

Early frequency-dependent information processing and cortical control in the whisker pathway of the rat: electrophysiological study of brainstem nuclei principalis and interpolaris

The rat facial whiskers form a high-resolution sensory apparatus for tactile information coding and are used by these animals for the exploration and perception of their environment. Previous work on the rat vibrissae system obtained evidence for vibration-based feature extraction by the whiskers, texture classification by the cortical neurons, and "low-pass", "high-pass", and "band-pass" filtering properties in both thalamic and cortical neurons. However, no data are available for frequency-dependent information processing in the brainstem sensory trigeminal complex (STC), the first relay station of the vibrissae pathway. In the present paper, we studied the frequency-dependent processing characteristics of the STC nuclei that mainly project to the thalamus, nuclei principalis, and interpolaris. This is the first time that STC nuclei have been studied together via a wide range of stimulation frequencies (1-40 Hz), four different and complementary metrics, and the same experimental protocol. Moreover, the role of corticofugal projection to these nuclei as well as the influence of input from the whiskers has been analyzed. We show that both nuclei perform frequency-dependent coding of tactile information: low pass and band-pass filtering occurs for the spiking rate in short post-stimuli time intervals, high-pass and band-pass filtering occurs for the spiking rate in long trains of stimuli, and an increase of response latencies and low pass filtering occurs for phase-locked stimuli. These information-processing characteristics are neither imposed by the sensorimotor cortex nor introduced by the afferent fibres. The sensorimotor cortex exerts a distinct modulatory effect on each nucleus.

Upper cervical afferents to the motor trigeminal nucleus and the subnucleus oralis of the spinal trigeminal nucleus in the rat: an anterograde and retrograde tracing study

Upper cervical afferents to the motor trigeminal nucleus (Vmo) and the subnucleus oralis (Vo) neurons projecting contralaterally to the cervical cord were demonstrated in the rat. Axon-terminals were labeled with biotinylated dextran and neurons with cholera toxin subunit B. Axons from the C2 and C3 segments terminated ipsilaterally on the somata and proximal dendrites of Vmo neurons. In the Vo, terminals of axons from the C2 and C3 segments were densely distributed on the somata, and proximal to distal dendrites of neurons projecting contralaterally to the cervical cord. The ipsilateral cervical input to the Vmo would modulate the activity of motoneurons of masticatory muscles while that to the Vo neurons subserves the feedback control of the trigemino-spinal reflex.

Trigemino-cervical reflexes in normal subjects

Trigemino-cervical reflexes, recorded from the semispinalis capitis muscle (SCM) in the posterior neck, were studied in 35 healthy volunteers, in response to electrical stimulation of the supraorbital trigeminal nerve and glabellar tapping. Simultaneous responses evoked from the ipsilateral orbicularis oculi muscle (OOM) were also recorded i.e. blink reflexes. Electrical stimulation of the supraorbital nerve elicited a reflex response with a latency of about 50 ms from the ipsilateral SCM which was called C3. An early reflex response, which sometimes had two components with latencies of 18 ms and 35 ms, was elicited with glabellar taps. They were called C1 and C2 respectively. When C1 and C2 were elicited with usual glabellar taps, C3 was suppressed. With electrical stimulation, suppression of C1 and C2 was noted, though C3 could easily be obtained. Electrophysiological characteristics of C1 (and C2) were compatible with an oligosynaptic, innocuous reflex, whereas C3 seemed to be multisynaptic and nociceptive in nature. A negative interaction between these two reflexes was observed.

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 spinotrigeminal pathway and its spatial relationship to the origin of trigeminospinal projections in the rat

The anterograde transport of horseradish peroxidase and tritiated amino acids was used to examine the distribution and morphology of spinal afferent fibers terminating in the rat spinal trigeminal complex. The results confirm the existence of a direct, ipsilateral projection from the spinal cord which is distributed exclusively to the deepest layers of the medullary dorsal horn narrow regions subjacent to the spinal trigeminal tract in trigeminal nucleus interpolaris, trigeminal nucleus oralis and the trigeminal main sensory nucleus. Spinal inputs also terminated in the insular trigeminal-cuneatus lateralis nucleus which is a distinct component of the interstitial system of the spinal trigeminal tract. The spinal afferent fibers which terminated in the dorsolateral parts of the spinal trigeminal complex arose from the dorsal column funiculi, while those that terminated in ventral parts of the complex arose from both the dorsal column and lateral funiculi. The tritiated amino acid experiments indicate that at least part of the spinotrigeminal pathway originates from cells located in the cervical spinal dorsal horn. The present findings also document a complex spatial relationship between the spinotrigeminal and trigeminospinal pathways which includes an extensive overlap between spinotrigeminal fibers and spinal projecting neurons in each of the lateralmost regions of the complex. This spatial overlap supports the existence of anatomical substrates which may underlie functional reciprocal loops between the spinal trigeminal complex and cervical spinal cord. Since these regions are primarily concerned with the processing of sensory information from lateral and posterior parts of the face, it follows that the spinotrigeminal pathway may be primarily concerned with the integration of head and neck functions. In addition, the spatial convergence of spinal inputs and the distribution of other trigeminal efferent neurons suggests that part of the spinotrigeminal pathway may be involved in spino-trigemino-thalamic and spino-trigemino-cerebellar pathways in parallel with other spinobulbar pathways in the medulla.

Cell structure and response properties in the trigeminal subnucleus oralis

Extra- and intracellular recording, electrical stimulation, receptive field mapping, and horseradish peroxidase injection techniques were used to study the structure of functionally identified neurons in trigeminal (V) brainstem subnucleus oralis of the rat. Of 15 heavily labeled cells located within oralis, 4 were local-circuit neurons with receptive fields restricted to either an incisor, guard hairs, one vibrissa, or deep facial tissue (nociceptors). Their morphologies were highly varied, with expansive and spiny dendritic trees and recurrent and intersubnuclear axon collaterals. Oralis local-circuit neurons therefore most closely resembled non-vibrissa-sensitive local-circuit cells in adjacent subnucleus interpolaris. Six other stained cells projected to contralateral thalamus, and two others projected to ipsilateral cerebellum. They typically had intramodality convergent receptive fields (i.e., spanning more than one receptor organ, such as multiple vibrissae or teeth) with widespread dendritic trees, and were therefore indistinguishable from similarly projecting cells in interpolaris. Two other cells projected to the ipsilateral spinal cord, as well as other V brainstem subnuclei. One of these responded to high-threshold mechanical stimulation of teeth; the other was discharged by deflection of one mystacial vibrissa. Their dendrites were very widespread and ended in spiny and bulbous appendages. Local axon collaterals were also extensive. The remaining oralis cell had two axons, one projecting to the thalamus, the other to the spinal cord. Its receptive field expressed convergence from multiple receptor organs, including vibrissae, guard hairs, and skin. Its somadendritic morphology was similar to that of oralis cells projecting only to thalamus. We conclude that, with some exceptions, the extensive dendritic trees, axon branching, convergence, and functional diversity of oralis cells approximate those described previously for functionally equivalent neurons in interpolaris (Jacquin et al., 1989a,b). Such anatomical and physiological properties are rarely seen, however, in nucleus principalis (Jacquin et al., 1988a). The structure and function of three atypical principalis cells with structural and functional characteristics typical of oralis cells are also described. It is argued that such cells are rostrally displaced oralis cells.

An analysis of the cyto- and myeloarchitectonic organization of trigeminal nucleus interpolaris in the rat

The cyto- and myeloarchitectonic organization of trigeminal nucleus interpolaris (Vi) was examined in the rat using correlated Nissl- and myelin-stained sections. The caudal boundary of Vi is marked by a spatial overlap with the rostral pole of the medullary dorsal horn (MDH), where there is a dorsal and medial displacement of the substantia gelatinosa (SG, lamina II) layer of MDH. This spatial displacement was further documented using cytochrome-oxidase-reacted sections through the periobex region (POR) of the medulla, where the relatively unstained SG contrasts sharply with the intensely stained Vi neuropil. The rostral boundary of Vi is characterized partly by a distinct overlap with the caudal pole of the dorsomedial region (DM) of trigeminal nucleus oralis (Vo), and partly by a more gradual transition with ventral and lateral regions of Vo. The presence of the distinct MDH-Vi overlap is discussed in terms of its impact on the widespread contention that Vi is involved in the processing of dental pain afferents in the POR. Six separate and distinct regions of rat Vi can be distinguished on the basis of differences in their overall cyto- and myeloarchitecture: (1) a ventrolateral parvocellular region (vlVipc), which occupies the ventrolateral caudal half of Vi; (2) a ventrolateral magnocellular region (vlVimc), which occupies a similar region in the rostral half of the nucleus; (3) a border region (brVi), interposed between the spinal trigeminal tract (SVT) and vlVipc and vlVimc; (4) a dorsolateral region (dlVi), which lies predominantly in the rostral two-thirds of Vi subjacent to the dorsal half of SVT; (5) a dorsal cap region (dcVi), occupying the dorsomedial aspect of the nucleus throughout its entire rostrocaudal extent; and (6) an intermediate region (irVi), which lies immediately ventral to dcVi within the concavity formed by the medial borders of vlVipc and vlVimc. It is proposed that these cyto- and myeloarchitecturally distinct regions of Vi may largely represent functionally distinct regions, based on reported differences in the organization of afferent and efferent projections within the nucleus.

Synaptic organization of primary axons in trigeminal nucleus oralis

This report examines the morphology and synaptic connections of small-diameter primary trigeminal axons that terminate in the border zone (BZ) and ventrolateral (VL) subdivisions of rat trigeminal nucleus oralis (Vo). Primary axons were made visible for light and electron microscopic analysis by utilizing the method of anterograde transport of horseradish peroxidase. BZ receives the terminal arborizations of two different populations of small-diameter primary axons. One of these arises from unmyelinated parent fibers and terminates in the dorsal one-half of BZ, while the other has small myelinated parent branches that arborize throughout the subdivision. Terminating within VL are the arbors of a second population of small myelinated primary axons. The endings of all three populations of primary axons lie in synaptic glomeruli. Endings in both subdivisions derived from small myelinated parent fibers lie centrally in glomeruli. Those in VL form axodendritic synapses on numerous dendritic shafts and spines, while endings in BZ glomeruli make at least one axodendritic synapse on one or two dendritic shafts. Endings of unmyelinated primary axons in BZ lie at the periphery of glomeruli where each forms a single axodendritic synapse on a central dendrite. It is at these asymmetrical axodendritic synapses that these three populations of primary axons are thought to transfer their inputs directly to the dendritic arbors of second-order BZ and VL neurons. Common to all three glomeruli is one or more small axonal endings filled with flattened synaptic vesicles that establish axoaxonic synapses on the primary ending as well as axodendritic synapses on the dendritic element(s) receiving primary input. In view of their symmetrical to intermediate synaptic contacts, these endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of second-order BZ and VL neurons in response to primary input.

Common fur and mystacial vibrissae parallel sensory pathways: 14 C 2-deoxyglucose and WGA-HRP studies in the rat

Stimulation of mystacial vibrissae in rows A,B, and C increased (14C) 2-deoxyglucose (2DG) uptake in spinal trigeminal nucleus pars caudalis (Sp5c) mostly in ventral portions of laminae III-IV with less activation of II and V. Stimulation of common fur above the whiskers mainly activated lamina II, with less activation in deeper layers. The patterns of activation were compatible with an inverted head, onion skin Sp5c somatotopy. Wheatgerm Agglutinin-Horseradish Peroxidase (WGA-HRP) injections into common fur between mystacial vibrissae rows A-B and B-C led to anterograde transganglionic labeling only of Sp5c, mainly of lamina II with less label in layer V, and very sparse label in III and IV. WGA-HRP skin injections appear to primarily label small fibers, which along with larger fibers, were metabolically activated during common fur stimulation. Mystacial vibrissae stimulation increased 2DG uptake in ventral ipsilateral spinal trigeminal nuclei pars interpolaris (Sp5i) and oralis (Sp5o) and principal trigeminal sensory nucleus (Pr5). Common fur stimulation above the whiskers slightly increased 2DG uptake in ventral Sp5i, Sp5o, and possibly Pr5. The most dorsal aspect of the ventroposteromedial (VPM) nucleus of thalamus was activated contralateral to whisker stimulation. Stimulation of the common fur dorsal to the whiskers activated a region of dorsal VPM caudal to the VPM region activated during whisker stimulation. This is consistent with previous data showing that ventral whiskers and portions of the face are represented rostrally in VPM, and more dorsal whiskers and dorsal portions of the face are represented progressively more caudally in VPM. Mystacial vibrissae stimulation activated the contralateral primary sensory SI barrelfield cortex and a separate region in the second somatosensory SII cortex. Common fur stimulation above the whiskers activated a cortical region between the SI and SII whisker activated regions of cortex. It is proposed that this region represented the combined SI and SII common fur regions of somatosensory neocortex. Both whisker and common fur stimulation activated all layers of cortex, with layer IV being most activated followed by II-III, V, and VI. These data indicate that sensory input from the mystacial vibrissae in the adult rat is processed in brainstem, thalamic, and cortical pathways which are predominantly parallel to those which process information from the neighboring common fur sensory receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology and synaptic connections of myelinated primary axons in the ventrolateral region of rat trigeminal nucleus oralis

Neurons in the ventrolateral (VL) subdivision of rat trigeminal nucleus oralis (Vo) have most of their dendritic arbors confined within this region. This study examines the morphology and synaptic connections of a population of myelinated primary trigeminal axons that arborize within VL and are in a position to provide input directly to VL neurons. Primary axons were visualized for light and electron microscopic analysis by injecting 30% horseradish peroxidase (HRP) in 2% dimethylsulfoxide (DMSO) into the sensory root of the trigeminal nerve and allowing 24-36 hours for the anterograde transport of HRP into the terminal axonal arbors. This population is characterized by its cone-shaped terminal arbors, which generate many axonal endings (2-8 micron in diameter) along unmyelinated terminal strands. These arbors arise from collaterals emanating from thinly myelinated (2-5 micron in diameter) parent branches descending in the spinal V tract, which, on the basis of their size, are considered to be small myelinated (A sigma) primary trigeminal axons. HRP-labeled P endings belonging to this population of primary axons are scalloped, filled with spherical to ovoid (40-70 nm in diameter) synaptic vesicles, and lie centrally in glomeruli where they make asymmetrical axodendritic synapses on dendritic shafts and spine heads. It is at these synapses that this population of primary trigeminal axons is probably transferring its input directly to the dendritic arbors of VL neurons. The dendritic shafts and spine heads also receive symmetrical to intermediate axodendritic synapses from endings containing flattened (70 X 29 nm) synaptic vesicles. These terminals also establish axo-axonic synapses on the P ending. Other synaptic components found less often in the glomeruli include small terminals containing oval (14-23 nm) synaptic vesicles that establish symmetrical to intermediate synapses on the P ending, boutons containing pleomorphic (35-80 nm) synaptic vesicles that form symmetrical to intermediate synapses on the P ending as well as on dendritic shafts, and small peripheral endings containing round (20-40 nm) synaptic vesicles that establish asymmetrical synapses on dendritic shafts.

Morphology and synaptic connections of small myelinated primary trigeminal axons arborizing among neurons in the border zone of rat trigeminal nucleus oralis

The anterograde transport of horseradish peroxidase (HRP) was used to examine the morphology and synaptic connections of a morphologically distinct group of small-diameter primary trigeminal axons that arborize throughout the border zone (BZ) of rat trigeminal nucleus oralis. Thinly myelinated parent branches (0.75-1.5 micron in diameter) descending in the spinal V tract (SVT) were seen to issue medially directed collaterals that entered BZ, where they branched and eventually terminated by giving rise to thin terminal strands characterized by several relatively widely spaced axonal endings. Based on the size and morphology of the parent branches in SVT, in the root entry zone, and in the sensory root of the trigeminal nerve, these primary axonal (P) endings are considered to be derived from small-diameter myelinated primary trigeminal axons (SDMA). The P endings measured 1-2 micron in diameter and contained numerous agranular spherical (40-60 nm) synaptic vesicles. In the BZ neuropil, most P endings lay in glomeruli, where each formed at least one asymmetrical axodendritic synapse on a dendritic shaft. It is at these synapses that this group of primary axons is thought to transfer its input directly to the dendritic arbors of BZ neurons. A small (0.5-1.5 micron) axonal (F) ending filled with flattened synaptic vesicles (29 X 60 nm) was observed to form at least one symmetrical to intermediate axoaxonic synapse on the P ending, as well as at least one axodendritic synapse on the same dendritic shaft receiving the primary input. Some F endings only contacted dendritic shafts. In view of their symmetrical to intermediate synaptic contacts, F endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of BZ neurons in response to SDMA input. This would be accomplished either postsynaptically through the axodendritic synapses on the dendritic shafts, and/or presynaptically through the axoaxonic synapses on the P endings.

Morphological features of identified trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus oralis

The retrograde transport of horseradish peroxidase (HRP) was used to assess the overall morphology of neurons in the dorsal half of the border zone (BZ) of rat trigeminal nucleus oralis (Vo) that project to ipsilateral orofacial portions of four major tactile areas (crura I and II, the paramedian lobule, and the uvula) of the cerebellar cortex. We wished to answer two important questions: Does this group of cells consist of one or more morphologically distinct types, each projecting to different cerebellar tactile areas? Does the overall morphology or morphologies of these BZ neurons resemble one or more of the five types of identified trigeminocerebellar neurons in the dorsomedial (DM) subdivision of Vo, or do these trigeminocerebellar cells represent an additional morphologically distinct type or types restricted to BZ? The morphology of BZ neurons innervating the orofacial portions of all four cerebellar tactile areas was similar and did not resemble that of any of the five types of DM cells. They were characterized by a pyramidal- to fusiform-shaped cell body measuring 10-13 X 20-25 microns, which emitted three or four primary dendrites; one was directed dorsally, while the others took a more ventral trajectory. The primary dendrites generated a dendritic arbor arranged as a flattened disk oriented parallel to the spinal V tract. The dendritic field was largely restricted to BZ; it measured up to 150 microns in width, and spanned up to 450 microns dorsoventrally and rostrocaudally. An axon arose from the dorsal aspect of the cell body and gave rise to a single short collateral within 10 microns of its origin. This collateral remained unbranched and generated several boutons within BZ, while the parent axon, without branching further, traveled dorsolaterally toward the inferior cerebellar peduncle. Frequently, a second axon arose ventrally from the soma, and after a short unbranched course entered a deep axon bundle, where it assumed a descending trajectory. The intranuclear portion of this second axon was characterized by several boutons en passant.

The dorsomedial portion of trigeminal nucleus oralis (Vo) in the rat: cytology and projections to the cerebellum

Electrophysiological studies have described four major tactile areas in the rat cerebellar cortex. These areas are in crus I, crus II, the paramedian lobule (PML), and the uvula, and a major portion of each is related to the ipsilateral orofacial region. This study demonstrates that neurons in trigeminal nucleus oralis (Vo) that project to the orofacial portions of these four major tactile areas are localized in the dorsomedial (DM) subdivision of the nucleus. The distribution, light-microscopic morphology, and relative densities of trigeminocerebellar neurons within DM, retrogradely labeled with horseradish peroxidase (HRP) following injections into each of the four major tactile areas, were analyzed and compared as well as correlated with the myelo- and cytoarchitecture of DM observed in Nissl sections, 1-micron sections, and Golgi material. On the basis of myelo- and cytoarchitectonic as well as trigeminocerebellar connectional criteria, three portions of DM were identified: caudal DM (CDM), middle DM (MDM), and rostral DM (RDM). The greatest portion of DM is made up of MDM (1.3 mm long), which can be further subdivided into dorsal (MDMd) and ventral (MDMv) zones. CDM forms the caudal 800 microns of DM, while RDM makes up the rostral 280 microns of the subdivision. Longitudinally running deep axon bundles permeate CDM, MDMv, and RDM, but are conspicuously absent from MDMd. The majority of neurons found throughout CDM, MDMv, and RDM have medium-sized (15- to 30-microns) somata and can be divided into two types on the basis of their somatodendritic morphology. CDM, MDMv, and RDM also contain a small neuronal cell type (5- to 15-microns cell body) that is encountered less frequently than either one of the two types of medium-sized cells. A fourth type of neuron with a large (25- to 50-microns) fusiform- to pyramidal-shaped cell body is the least frequently observed neuronal cell type and is located principally in CDM and MDMv. MDMd contains a fifth type of neuron characterized by a small (5- to 15-microns) oval soma. Data from the retrograde HRP experiments show that all five of these neuronal cell types in their respective portions of DM project to one or more of the orofacial portions of the four major tactile areas of the cerebellar cortex. Many medium-sized neurons of both types in CDM, MDMv, and RDM project to crus I, crus II, and/or PML.(ABSTRACT TRUNCATED AT 250 WORDS)