A comparison of the distribution and morphology of thalamic, cerebellar and spinal projection neurons in rat trigeminal nucleus interpolaris

The Cerebellar Cortex Receives Orofacial Proprioceptive Signals from the Supratrigeminal Nucleus via the Mossy Fiber Pathway in Rats

Proprioceptive sensory information from muscle spindles is essential for the regulation of motor functions. However, little is known about the motor control regions in the cerebellar cortex that receive proprioceptive signals from muscle spindles distributed throughout the body, including the orofacial muscles. Therefore, in this study, we investigated the pattern of projections in the rat cerebellar cortex derived from the supratrigeminal nucleus (Su5), which conveys orofacial proprioceptive information from jaw-closing muscle spindles (JCMSs). Injections of an anterograde tracer into the Su5 revealed that many bilateral axon terminals (rosettes) were distributed in the granular layer of the cerebellar cortex (including the simple lobule B, crus II and flocculus) in a various sized, multiple patchy pattern. We could also detect JCMS proprioceptive signals in these cerebellar cortical regions, revealing for the first time that they receive muscle proprioceptive inputs in rats. Retrograde tracer injections confirmed that the Su5 directly sends outputs to the cerebellar cortical areas. Furthermore, we injected an anterograde tracer into the external cuneate nucleus (ECu), which receives proprioceptive signals from the forelimb and neck muscle spindles, to distinguish between the Su5- and ECu-derived projections in the cerebellar cortex. The labeled terminals from the ECu were distributed predominantly in the vermis of the cerebellar cortex. Almost no overlap was seen in the terminal distributions of the Su5 and ECu projections. Our findings demonstrate that the rat cerebellar cortex receives orofacial proprioceptive input that is processed differently from the proprioceptive signals from the other regions of the body.

A vibrissa pathway that activates the limbic system

Vibrissa sensory inputs play a central role in driving rodent behavior. These inputs transit through the sensory trigeminal nuclei, which give rise to the ascending lemniscal and paralemniscal pathways. While lemniscal projections are somatotopically mapped from brainstem to cortex, those of the paralemniscal pathway are more widely distributed. Yet the extent and topography of paralemniscal projections are unknown, along with the potential role of these projections in controlling behavior. Here, we used viral tracers to map paralemniscal projections. We find that this pathway broadcasts vibrissa-based sensory signals to brainstem regions that are involved in the regulation of autonomic functions and to forebrain regions that are involved in the expression of emotional reactions. We further provide evidence that GABAergic cells of the Kölliker-Fuse nucleus gate trigeminal sensory input in the paralemniscal pathway via a mechanism of presynaptic or extrasynaptic inhibition.

Gi protein functions in thalamic neurons to decrease orofacial nociceptive response

Orofacial pain includes neuronal pathways that project from the trigeminal nucleus to and through the thalamus. What role the ventroposterior thalamic complex (VP) has on orofacial pain transmission is not understood. To begin to address this question an inhibitory G protein (Gi) designer receptor exclusively activated by a designer drug (DREADD) was transfected in cells of the VP using adeno-associated virus isotype 8. Virus infected cells were identified by a fluorescent tag and immunostaining. Cells were silenced after injecting the designer drug clozapine-n-oxide, which binds the designer receptor activating Gi. Facial rubbing and local field potentials (LFP) in the VP were then recorded in awake, free moving Sprague Dawley rats after formalin injection of the masseter muscle to induce nociception. Formalin injection significantly increased LFP and the nociceptive behavioral response. Activation of DREADD Gi with clozapine-n-oxide significantly reduced LFP in the VP and reduced the orofacial nociceptive response. Because DREADD silencing can result from Gi-coupled inwardly-rectifying potassium channels (GIRK), the GIRK channel blocker tertiapin-Q was injected. Injection of GIRK blocker resulted in an increase in the nociceptive response and increased LFP activity. Immunostaining of the VP for glutamate vesicular transporter (VGLUT2) and gamma-aminobutyric acid vesicular transporter (VGAT) indicated a majority of the virally transfected cells were excitatory (VGLUT2 positive) and a minority were inhibitory (VGAT positive). We conclude first, that inhibition of the excitatory neurons within the VP reduced electrical activity and the orofacial nociceptive response and that the effect on excitatory neurons overwhelmed any change resulting from inhibitor neurons. Second, inhibition of LFP and nociception was due, in part, to GIRK activation.

Animal models for investigating benign essential blepharospasm

The focal dystonia benign essential blepharospasm (BEB) affects as many as 40,000 individuals in the United States. This dystonia is characterized by trigeminal hyperexcitability, photophobia, and most disabling of the symptoms, involuntary spasms of lid closure that can produce functional blindness. Like many focal dystonias, BEB appears to develop from the interaction between a predisposing condition and an environmental trigger. The primary treatment for blepharospasm is to weaken the eyelid-closing orbicularis oculi muscle to reduce lid spasms. There are several animal models of blepharospasm that recreate the spasms of lid closure in order to investigate pharmacological treatments to prevent spasms of lid closure. One animal model attempts to mimic the predisposing condition and environmental trigger that give rise to BEB. This model indicates that abnormal interactions among trigeminal blink circuits, basal ganglia, and the cerebellum are the neural basis for BEB.

Neuronal circuits with whisker-related patterns

Neuronal circuits with whisker-related patterns, such as those observed in the rodent somatosensory barrel cortex, have been widely used as a model system for investigating the anatomical organization, development and physiological roles of functional neuronal circuits. Whisker-related patterns exist not only in the barrel cortex but also in subcortical structures along the trigeminal neuraxis from the brainstem to the cortex. Here, we briefly summarize the organization, formation, and function of each neuronal circuit with whisker-related patterns, including the novel axonal trajectories that we recently found with the aid of in utero electroporation. We also discuss their biological implications as model systems for the studies of functional neuronal circuits.

Responses to tactile stimulation in deep cerebellar nucleus neurons result from recurrent activation in multiple pathways

In a previous study, we found that neurons in the deep cerebellar nuclei (DCN) respond to 5-ms brief facial tactile stimulation in rats anesthetized with ketamine-xylazine with multiphasic response patterns lasting over 200 ms. It remained unclear, however, to what extent these responses were shaped not only by ascending sensory input from the trigeminal nuclei but also by interactions with other major cerebellar afferent systems, in particular the inferior olive (IO) and cerebral cortex. In the present study, we recorded from the IO, cerebral cortex, cerebellar granule cell layer (GCL), and DCN during the presentation of 5-ms facial tactile stimuli to elucidate potential mechanisms of how extended DCN response patterns are generated. We found that tactile stimulation resulted in robust multiphasic local field potentials responses in the IO as well as in the activation of a wide region of the somatosensory cortex (SI) and the primary motor cortex (MI). DCN neurons responded to electrical stimulation of any of these structures (IO, SI, and MI) with complex temporal patterns strikingly similar to air-puff lip stimulation responses. Simultaneous recordings from multiple structures revealed that long-lasting activation patterns elicited in DCN neurons were based on recurrent network activation in particular between the IO and the DCN with a potential contribution of DCN rebound properties. These results are consistent with the hypothesis that sensory stimulation triggers a feedback network activation of cerebellum, IO, and cerebral cortex to generate temporal patterns of activity that may control the timing of behavior.

Splenius muscle activities induced by temporomandibular joint stimulation in rats

Recent studies show that temporomandibular joint disorders cause hyperalgesia and deficits in the postural control of cervical region. However, the effects of specific modalities of receptors in the temporomandibular joint area on these phenomena are still unclear. In this study, we investigated the neck muscle activities while natural mechanical stimulation was applied to the temporomandibular joint. Single motor unit activities were recorded bilaterally from the splenius muscles in 22 Wistar rats. Mechanical stimulation applied to the left temporomandibular joint elicited tonic discharges in the left or right splenius muscle. The mean threshold values for mechanical stimulation were 48.1 (+/-16.2 S.E.M.) and 54.1 mN (+/-16.3 S.E.M.) for left and right sides, respectively. It is suggested that the temporomandibular joint mechanoreceptors not only affect the motor unit activities of neck muscles, but also are concerned in the regulation of postural control of the head.

Angular tuning bias of vibrissa-responsive cells in the paralemniscal pathway

One of the most salient features of primary vibrissal afferents is their sensitivity to the direction in which the vibrissae move. Directional sensitivity is also well conserved in brainstem, thalamic, and cortical neurons of the lemniscal pathway, indicating that this property plays a key role in the organization of the vibrissal system. Here, we show that directional tuning is also a fundamental feature of second-order interpolaris neurons that give rise to the paralemniscal pathway. Quantitative assessment of responses to vibrissa deflection revealed an anisotropic organization of receptive fields with regard to topography, response magnitude, and the degree of angular tuning. Responses evoked by all vibrissae within the receptive field of each cell exhibited a high consistency of direction preference, but a striking difference in angular tuning preference was found among cells that reside in the rostral and caudal divisions of the interpolaris nucleus. Although in caudal interpolaris vectors of angular preference pointed in all directions, in rostral interpolaris virtually all vectors pointed upward, revealing a strong preference for this direction. Control experiments showed that the upward bias did not rely on a preferential innervation of rostral cells by upwardly tuned primary vibrissa afferents, nor did it rely on a direction-selective recruitment of feedforward inhibition. We thus propose that the upward preference bias of rostral cells, which project to the posterior group of the thalamus, emerges from use-dependent synaptic processes that relate to the kinematics of whisking.

Trigeminovestibular and trigeminospinal pathways in rats: retrograde tracing compared with glutamic acid decarboxylase and glutamate immunohistochemistry

This study identified neurons in the sensory trigeminal complex with connections to the medial (MVN), inferior (IVN), lateral (LVN), and superior (SVN) vestibular nuclei or the spinal cord. Trigeminovestibular and trigeminospinal neurons were localized by injection of retrograde tracers. Immunohistochemical processing revealed gamma-aminobutyric acid (GABA)- and glutamate-containing neurons in these two populations. Trigeminovestibular neurons projecting to the MVN and the IVN were in the caudal principal nucleus (5P), pars oralis (5o), interpolaris (5i), and caudalis (5c) and scattered throughout the rostral 5P. Projections were bilateral to the IVN, with an ipsilateral dominance to the MVN, except from the rostral 5P, which was contralateral. Neurons projecting to the LVN were numerous in the ventral caudal 5P and the 5o and less abundant in the rostral 5P, 5i, and 5c. Our results suggested that only 5P and 5o project to the dorsal LVN. Neurons projecting to the SVN were in the dorsal 5P, 5o, and 5i but not in 5c. Trigeminospinal neurons were mainly in the ventral 5o and 5i and in the lateral 5c, rarely or never in 5P. Among trigeminovestibular neurons, most of the somas were immunoreactive for glutamate, but some reacted for GABA. Among trigeminospinal neurons, the number of somas immunoreactive for each of the two amino acids was similar. Trigeminal terminals were observed in contact with vestibulospinal neurons in the IVN and LVN, giving evidence of a trigeminovestibulospinal pathway. Therefore, inhibitory and excitatory facial inputs may contribute through trigeminospinal or trigeminovestibulospinal pathways to the control of head/neck movements.

Nature of neck motor unit activities evoked by different trigeminal inputs in rats

Recent reports have suggested functional interactions between the masticatory and head motor systems. However, the neurophysiological mechanisms behind these phenomena have not been thoroughly investigated. In this study, the motor unit activity was recorded bilaterally from the dorsal neck splenius muscles (SPL) while the cell bodies of trigeminal afferent fibers were stimulated at either the trigeminal mesencephalic nucleus (Me5) or the Gasserian ganglion (5Gn) in 13 barbiturate-anesthetized Wistar rats. After Me5 electrical stimulation, SPL motor unit responses were recorded ipsilaterally (n = 24), with an onset latency distribution between 1.3 and 1.8 ms. After 5Gn stimulation, SPL responses were recorded bilaterally from 18 single units, with latencies ranging from 2.2 to 2.4 ms. Our results suggest that there are at least two distinct pathways from the trigeminal afferents to the neck muscles, with significant functional differences.

Neck motor unit activities induced by inputs from periodontal mechanoreceptors in rats

Clinical evidence suggests that head movements may be coupled with oro-facial functions, which are predominantly controlled by somatosensory inputs from the oro-facial area. However, the effects of specific modalities of sensory inputs on the neck muscles' motor activity are still unclear. In the present study, natural pressure stimulation was applied to the rat's upper first molars, while motor unit electromyographic activity was recorded from the dorsal neck splenius muscle. During the hold phase of pressure stimulation, clear tonic discharges were elicited in the splenius muscles on both sides. Mean threshold values were 622.3 mN (+/- 19.6 SEM, n = 39) and 496.8 mN (+/- 26.4 SEM, n = 43) for ipsi- and contralateral sides, respectively (p < 0.001, Mann-Whitney U test). Analysis of our data suggests that periodontal inputs may play an important role in controlling the motor activity of neck muscles, in addition to its well-known coordination of the masticatory function.

Parallel streams for the relay of vibrissal information through thalamic barreloids

This study investigated the organization of a vibrissal pathway that arises from the interpolar division of the spinal trigeminal complex (SP5i), transits through the ventral posterior medial nucleus (VPM), and innervates the somatosensory cortical areas in the rat. Using Fluoro-Gold and biotinylated dextran amine, respectively, as retrograde and anterograde tracers, the following organization plan was disclosed. The SP5i projection arises from a population of small-sized neurons that selectively innervate the ventral lateral part of VPM. In cytochrome oxidase-stained material, this region does not display any barreloid arrangement, but Fluoro-Gold injections in single barrel columns labeled rods of cells that extend caudally into the ventral lateral division of VPM. Thus, on the basis of retrograde labeling, barreloids were divided into core and tail compartments, which correspond to the rod segments running across the dorsal and ventral lateral parts of VPM, respectively. Double-labeling experiments revealed that SP5i afferents innervate the tail of barreloids. The anterograde labeling of thalamocortical axons show that most "core cells" project to a single barrel column, whereas some "tail cells" give rise to branching axons that innervate the second somatosensory area and the dysgranular zone of the barrel field. Injections that straddled the transition zone between the core and tail regions disclosed cells projecting to a single barrel column and to the surrounding dysgranular zone. These results suggest that the projection of "barreloids cells" to the granular and/or dysgranular zones relates to the class of prethalamic input(s) they receive.

Thalamic projections from the whisker-sensitive regions of the spinal trigeminal complex in the rat

This study investigated the axonal projections of whisker-sensitive cells of the spinal trigeminal subnuclei (SP5) in rat oral, interpolar, and caudal divisions (SP5o, SP5i, and SP5c, respectively). The labeling of small groups of trigeminothalamic axons with biotinylated dextran amine disclosed the following classes of axons. 1) Few SP5o cells project to the thalamus: They innervate the caudal part of the posterior group (Po) and the region intercalated between the anterior pretectal and the medial geniculate nuclei. These fibers also branch profusely in the tectum. 2) Two types of ascending fibers arise from SP5i: Type I fibers are thick and distribute to the Po and to other regions of the midbrain, i.e., the prerubral field, the deep layers of the superior colliculus, the anterior pretectal nucleus, and the ventral part of the zona incerta. Type II fibers are thin; branch sparsely in the tectum; and form small-sized, bushy arbors in the ventral posterior medial nucleus (VPM). Accordingly, a statistical analysis of the distribution of antidromic invasion latencies of 96 SP5i cells to thalamic stimulation disclosed two populations of neurons: fast-conducting cells, which invaded at a mean latency of 1.23 +/- 0. 62 msec, and slow-conducting cells, which invaded at a mean latency of 2.97 +/- 0.62 msec. 3) The rostral part of SP5c contains cells with thalamic projections similar to that of type II SP5i neurons, whereas the caudal part did not label thalamic fibers in this study. A comparison of SP5i projections and PR5 projections in the VPM revealed that the former are restricted to ventral-lateral tier of the nucleus, whereas the latter terminate principally in the upper two tiers of the VPM. These results suggest a functional compartmentation of thalamic barreloids that is defined by the topographic distribution of PR5 and type II SP5i afferents.

Induction of NADPH-diaphorase/nitric oxide synthase in the brainstem trigeminal system resulting from cerebellar lesions

Recent evidence indicates that NADPH-diaphorase (NADPH-d) and nitric oxide synthase (NOS) can be induced in cerebellar afferent neurons following mechanical, thermal, or chemical damage to the cerebellar cortex (Saxon and Beitz [1994] Neuroreport 5:809-812). The present study reports on the induction of NADPH-d/NOS in neurons of the brainstem trigeminal complex (BVC). Three groups of rats were used: Group I received a unilateral glass micropipette lesion into the vermal/paravermal region of the cerebellar cortex, group II received a concurrent injection of fluoro-gold along with the pipette lesion, and in group III the cerebellar cortex on one side was aspirated. Following survival times of 7-120 days, animals were processed for NADPH-d histochemistry. All three groups showed projection-specific induction of NADPH-d in different regions of the brainstem trigeminal complex. Induced neurons were distributed throughout the ipsilateral subnucleus interpolaris, principal trigeminal nucleus, and intertrigeminal nucleus. Subnucleus oralis contained a small number of induced neurons localized to the ipsilateral dorsomedial portion of the subnucleus. Projection-specific induction was confirmed by the presence of neurons double-labeled for NADPH-d and Fluoro-Gold. Although the functional consequences of NADPH-d/NOS induction remain to be elucidated, the induction of these enzymes in precerebellar neurons suggests that nitric oxide may play a role in the neuronal response to target specific lesions.

Trigeminocerebellar and trigemino-olivary projections in rats

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

Transneuronal transport of biotinamide was observed following intracellular injection of biotinamide into rat jaw-muscle spindle afferent axons. Microelectrodes were advanced into the mesencephalic nucleus of the trigeminal nerve where jaw-muscle spindle afferent axons were identified by their increased firing during stretching of the jaw-elevator muscles. Biotinamide (Neurobiotin) was then injected into individual axons and the animals were maintained under anesthesia for 2-6 h. The animals were then killed via an overdose of anesthetic and the brainstem was processed histochemically. Biotinamide-filled axon collaterals and terminals were readily visible in the trigeminal motor nucleus, the trigeminal sensory nuclei, and adjacent reticular formation. In addition to these intracellularly stained axons, two to five neurons per animal (total of 36 in eight rats) were observed with a homogeneous gray reaction product distributed throughout their somata, proximal, and secondary dendrites. These neurons ranged in size from small (8-20 mu m, n - 26) to medium-sized (<30 mu m, n = 10) and were closely apposed by numerous (up to 20) biotinamide-stained spindle afferent boutons. Most of these neurons (n = 22) were located in the dorsomedial portion of the spinal trigeminal subnucleus interpolaris (Vi) 2.5-4.5 mm caudal to the intra-axonal injection site. Electron microscopic analysis in two rats suggests that the transneuronal biotinamide labeling occurred predominantly through asymmetric, axodendritic synapses between biotinamide-filled axon terminals and Vi neuronal dendrites. Although recent in vitro studies have reported that biotinamide permeates through gap junctions, in this study we found no evidence of biotinamide traversing the gap junctions which exist between trigeminal mesencephalic nucleus (Vme) neuronal somata. These results demonstrate that biotinamide can occasionally be transneuronally transported presumably via synapses; further information is needed to explain the seemingly sporadic nature of this transport.

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.

Trigeminal and dorsal column nuclei projections to the anterior pretectal nucleus in the rat

The projections of the trigeminal (V) sensory nuclei (VSN) and the dorsal column nuclei (DCN) to the anterior pretectal nucleus (APT) of the rat were investigated by the use of anterograde and retrograde transport of wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into the APT retrogradely labeled neurons in the contralateral VSN and DCN. The labeled neurons in the VSN were most concentrated in the rostral V subnucleus interpolaris (Vi), but were also found in caudal V subnucleus oralis (Vo). No labeled neurons were seen in V subnucleus caudalis. In the DCN, retrogradely labeled neurons were observed in rostral portions of both the cuneate (Cu) and gracile (Gr) nuclei. Injections of WGA-HRP into the rostral Vi or caudal Vo resulted in dense anterograde terminal labeling in the ventral two-thirds of the APT; the labeling was maximal in the ventromedial part of the caudal half of the APT and did not extend into its most rostral portion. Labeling resulting from injections of tracer into Cu or Gr was located primarily in the ventral half of the APT, was maximal in the mid-levels of the nucleus and extended into its rostral portions. These results indicate the existence of prominent somatosensory projections to the APT and are consistent with recent findings suggesting a role for the APT in sensorimotor integration.

Trigeminocerebellar projections to the posterior lobe in the cat, as studied by anterograde transport of wheat germ agglutinin-horseradish peroxidase

Cerebellar projections of the nucleus interpolaris and oralis of the spinal trigeminal nucleus were studied in the cat by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into these nuclei labeled many mossy fiber terminals mainly ipsilaterally in the rostral folium of lobule IX (IXa or IXa + b), the simple lobule, the anterior part (sublobule A) of the paramedian lobule and the posterior part of crus II. Labeled terminals were also seen in the anterior lobe, lobules VI and VII, the anterior part of crus I, and the paraflocculus dorsalis. Projection fields in the horizontal plane of lobules were reconstructed from a series of transverse sections through each folium of lobule IX, the paramedian lobule, and the posterior part of crus II on the ipsilateral side. In sublobule IXa + b, labeled terminals were distributed in five longitudinal areas extending along the apicobasal axis of the sublobule. These five areas were located in the apical two-thirds of the ipsilateral half of the sublobule. Labeled terminals were distributed in five longitudinal areas in sublobule A (the rostral part) of the paramedian lobule. In the posterior part of crus II, four aggregations of labeled terminals were present in cross sections through a lobule. They were distributed in the apicobasal extent of the lobules. The present study indicates that the projection fields of trigeminocerebellar fibers are longitudinally arranged along the apicobasal axis of the cerebellar lobules.

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.