The central projections of the great auricular nerve primary afferent fibers--an HRP transganglionic tracing method

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

Neuromodulation in drug-resistant epilepsy: A review of current knowledge

Nearly 1% of the global population suffers from epilepsy. Drug-resistant epilepsy (DRE) affects one-third of epileptic patients who are unable to treat their condition with existing drugs. For the treatment of DRE, neuromodulation offers a lot of potential. The background, mechanism, indication, application, efficacy, and safety of each technique are briefly described in this narrative review, with an emphasis on three approved neuromodulation therapies: vagus nerve stimulation (VNS), deep brain stimulation of the anterior nucleus of the thalamus (ANT-DBS), and closed-loop responsive neurostimulation (RNS). Neuromodulatory approaches involving direct or induced electrical currents have been developed to lessen seizure frequency and duration in patients with DRE since the notion of electrical stimulation as a therapy for neurologic diseases originated in the early nineteenth century. Although few people have attained total seizure independence for more than 12 months using these treatments, more than half have benefitted from a 50% drop in seizure frequency over time. Although promising outcomes in adults and children with DRE have been achieved, challenges such as heterogeneity among epilepsy types and etiologies, optimization of stimulation parameters, a lack of biomarkers to predict response to neuromodulation therapies, high-level evidence to aid decision-making, and direct comparisons between neuromodulatory approaches remain. To solve these existing gaps, authorize new kinds of neuromodulation, and develop personalized closed-loop treatments, further research is needed. Finally, both invasive and non-invasive neuromodulation seems to be safe. Implantation-related adverse events for invasive stimulation primarily include infection and pain at the implant site. Intracranial hemorrhage is a frequent adverse event for DBS and RNS. Other stimulation-specific side-effects are mild with non-invasive stimulation.

“The Wandering Nerve Linking Heart and Mind” – The Complementary Role of Transcutaneous Vagus Nerve Stimulation in Modulating Neuro-Cardiovascular and Cognitive Performance

The vagus nerve is the longest nerve in the human body, providing afferent information about visceral sensation, integrity and somatic sensations to the CNS via brainstem nuclei to subcortical and cortical structures. Its efferent arm influences GI motility and secretion, cardiac ionotropy, chonotropy and heart rate variability, blood pressure responses, bronchoconstriction and modulates gag and cough responses via palatine and pharyngeal innervation. Vagus nerve stimulation has been utilized as a successful treatment for intractable epilepsy and treatment-resistant depression, and new non-invasive transcutaneous (t-VNS) devices offer equivalent therapeutic potential as invasive devices without the surgical risks. t-VNS offers exciting potential as a therapeutic intervention in cognitive decline and aging populations, classically affected by reduced cerebral perfusion by modulating both limbic and frontal cortical structures, regulating cerebral perfusion and improving parasympathetic modulation of the cardiovascular system. In this narrative review we summarize the research to date investigating the cognitive effects of VNS therapy, and its effects on neurocardiovascular stability.

Non-invasive vagus nerve stimulation modulates trigeminal but not somatosensory perception: functional evidence for a trigemino-vagal system in humans

ABSTRACT

Non-invasive vagus nerve stimulation (nVNS) is effective in several types of headache disorders. We sought to unravel the mechanism of how nVNS exhibits this efficacy. This study used a randomized, single-blind, sham-controlled, crossover-design, and comprised three projects with three independent cohorts of healthy participants. Project I (n=15) was explorative. Six quantitative sensory test (QST) parameters, including mechanical pain threshold (MPT), were measured over the left V1 dermatome and forearm, and compared before and after unilateral nVNS. Projects II (n=20) and III (n=21) were online pre-registered . QST parameters were compared over the left (Project II) or bilateral V1 and V3 dermatomes (Project III), respectively, in addition to the left forearm as a control. A secondary analysis of heart rate variability (HRV) using a historical control group was used to control for systemic effects of nVNS. Verum-nVNS induced trigeminal-specific modulation of pain threshold (i.e., MPT) over the left V1 in Project I, left V1 and V3 in Project II, and bilateral V1 and V3 in Project III. Data pooled from Project II and III demonstrated greater increase of MPT in the V1 vs. V3 dermatome. There were no differences associated with sham-nVNS in any projects. HRV parameters did not change after nVNS. Our results provide functional evidence of a long hypothesized functional trigemino-vagal system in humans and may explain why nVNS is effective in some headache but not in somatic pain disorders. Since unilateral nVNS modulated the trigeminal thresholds bilaterally, this effect is probably indirect through a central top-down mechanism.

Auricular transcutaneous vagus nerve stimulation modulates the heart-evoked potential

BACKGROUND

There is active interest in biomarker discovery for transcutaneous auricular vagus nerve stimulation (taVNS). However, greater understanding of the neurobiological mechanisms is needed to identify candidate markers. Accumulating evidence suggests that taVNS influences activity in solitary and parabrachial nuclei, the primary brainstem relays for the transmission of visceral sensory afferents to the insula. The insula mediates interoception, which concerns the representation and regulation of homeostatic bodily states. Consequently, interoceptive pathways may be relevant to taVNS mechanisms of action.

HYPOTHESES

We hypothesized that taVNS would modulate an EEG-derived marker of interoceptive processing known as the heart-evoked potential (HEP). We also hypothesized that taVNS-induced HEP effects would be localizable to the insula.

METHODS

Using a within-subject, sham-controlled design in 43 healthy adults, we recorded EEG and ECG concurrent to taVNS. Using ECG and EEG data, we extracted HEPs. Estimation of the cortical sources of the taVNS-dependent HEP responses observed at the scalp were computed using the Boundary Element Method and weighted Minimum Norm Estimation. Statistics were calculated using cluster-based permutation methods.

RESULTS

taVNS altered HEP amplitudes at frontocentral and centroparietal electrode sites at various latencies. The taVNS-dependent HEP effect was localized to the insula, operculum, somatosensory cortex, and orbital and ventromedial prefrontal regions.

CONCLUSION

The results support the hypothesis that taVNS can access the insula as well as functionally and anatomically connected ventral prefrontal regions. HEPs may serve as an objective, non-invasive outcome parameter for the cortical effects of taVNS.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

Tympanic Resonance Hypothesis

Seemingly unrelated symptoms in the head and neck region are eliminated when a patch is applied on specific locations on the Tympanic Membrane. Clinically, two distinct patient populations can be distinguished; cervical and masticatory muscle tensions are involved, and mental moods of anxiety or need. Clinical observations lead to the hypothesis of a “Tympanic Resonance Regulating System.” Its controller, the Trigeminocervical complex, integrates external auditory, somatosensory, and central impulses. It modulates auditory attention, and directs it toward unpredictable external or expected domestic and internal sounds: peripherally by shifting the resonance frequencies of the Tympanic Membrane; centrally by influencing the throughput of auditory information to the neural attention networks that toggle between scanning and focusing; and thus altering the perception of auditory information. The hypothesis leads to the assumption that the Trigeminocervical complex is composed of a dorsal component, and a ventral one which may overlap with the concept of “Trigeminovagal complex.” “Tympanic Dissonance” results in a host of local and distant symptoms, most of which can be attributed to activation of the Trigeminocervical complex. Diagnostic and therapeutic measures for this “Tympanic Dissonance Syndrome” are suggested.

The anatomical basis for transcutaneous auricular vagus nerve stimulation

The array of end organ innervations of the vagus nerve, coupled with increased basic science evidence, has led to vagus nerve stimulation (VNS) being explored as a management option in a number of clinical disorders, such as heart failure, migraine and inflammatory bowel disease. Both invasive (surgically implanted) and non-invasive (transcutaneous) techniques of VNS exist. Transcutaneous VNS (tVNS) delivery systems rely on the cutaneous distribution of vagal afferents, either at the external ear (auricular branch of the vagus nerve) or at the neck (cervical branch of the vagus nerve), thus obviating the need for surgical implantation of a VNS delivery device and facilitating further investigations across a wide range of uses. The concept of electrically stimulating the auricular branch of the vagus nerve (ABVN), which provides somatosensory innervation to several aspects of the external ear, is relatively more recent compared with cervical VNS; thus, there is a relative paucity of literature surrounding its operation and functionality. Despite the increasing body of research exploring the therapeutic uses of auricular transcutaneous VNS (tVNS), a comprehensive review of the cutaneous, intracranial and central distribution of ABVN fibres has not been conducted to date. A review of the literature exploring the neuroanatomical basis of this neuromodulatory therapy is therefore timely. Our review article explores the neuroanatomy of the ABVN with reference to (1) clinical surveys examining Arnold's reflex, (2) cadaveric studies, (3) fMRI studies, (4) electrophysiological studies, (5) acupuncture studies, (6) retrograde tracing studies and (7) studies measuring changes in autonomic (cardiovascular) parameters in response to auricular tVNS. We also provide an overview of the fibre composition of the ABVN and the effects of auricular tVNS on the central nervous system. Cadaveric studies, of which a limited number exist in the literature, would be the 'gold-standard' approach to studying the cutaneous map of the ABVN; thus, there is a need for more such studies to be conducted. Functional magnetic resonance imaging (fMRI) represents a useful surrogate modality for discerning the auricular sites most likely innervated by the ABVN and the most promising locations for auricular tVNS. However, given the heterogeneity in the results of such investigations and the various limitations of using fMRI, the current literature lacks a clear consensus on the auricular sites that are most densely innervated by the ABVN and whether the brain regions secondarily activated by electrical auricular tVNS depend on specific parameters. At present, it is reasonable to surmise that the concha and inner tragus are suitable locations for vagal modulation. Given the therapeutic potential of auricular tVNS, there remains a need for the cutaneous map of the ABVN to be further refined and the effects of various stimulation parameters and stimulation sites to be determined.

Skilled reaching deterioration contralateral to cervical hemicontusion in rats is reversed by pregabalin treatment conditional upon its early administration

Introduction

Gabapentinoids are first-line treatments for painful traumatic and nontraumatic central nervous system disorders. Evidence from a large human study suggests that early use of gabapentinoids after spinal cord injury improves motor scores. The underlying mechanism is unknown.

Objectives

We sought to examine the effects of early pregabalin (PGB, a gabapentinoid) treatment on performance in a fine motor task (skilled reaching) after cervical hemicontusion. We also asked whether early PGB administration affected PGB responsiveness later on.

Methods

Rats received C4/5 cervical hemicontusions. Injury severities ranged from 80 to 150 kdyn. We monitored evidence of skin irritation (peri-incisional and elsewhere) and quantified food pellet retrieval using the Montoya staircase test. Behaviours were assessed in rats receiving early (for 3 weeks from injury induction) and/or late (resuming or beginning at week 8) PGB treatment in animals with 150-kdyn injuries.

Results

Contralateral skilled reaching waned in control animals with 150-kdyn injuries. This was prevented in animals, which received early PGB as long as treatment continued. Deterioration of skilled reaching was reversed by later (week 8) PGB only in animals that had received early treatment. Ipsilateral reaching impairment was not improved by PGB. Relief of skin irritation verified early PGB efficacy.

Conclusion

Hemicontusive spinal cord injury produces a contralateral motor phenotype evocative of on-going neuropathic pain. Early PGB preserves sensitivity to subsequent PGB treatment, indicating that motor function is impaired by neuropathic pain and can be improved indirectly by early PGB administration. Direct effects of PGB on motor circuitry cannot be excluded but are not supported by our data.

The influence of respiration on brainstem and cardiovagal response to auricular vagus nerve stimulation: A multimodal ultrahigh-field (7T) fMRI study

BACKGROUND

Brainstem-focused mechanisms supporting transcutaneous auricular VNS (taVNS) effects are not well understood, particularly in humans. We employed ultrahigh field (7T) fMRI and evaluated the influence of respiratory phase for optimal targeting, applying our respiratory-gated auricular vagal afferent nerve stimulation (RAVANS) technique.

HYPOTHESIS

We proposed that targeting of nucleus tractus solitarii (NTS) and cardiovagal modulation in response to taVNS stimuli would be enhanced when stimulation is delivered during a more receptive state, i.e. exhalation.

METHODS

Brainstem fMRI response to auricular taVNS (cymba conchae) was assessed for stimulation delivered during exhalation (eRAVANS) or inhalation (iRAVANS), while exhalation-gated stimulation over the greater auricular nerve (GANctrl, i.e. earlobe) was included as control. Furthermore, we evaluated cardiovagal response to stimulation by calculating instantaneous HF-HRV from cardiac data recorded during fMRI.

RESULTS

Our findings demonstrated that eRAVANS evoked fMRI signal increase in ipsilateral pontomedullary junction in a cluster including purported NTS. Brainstem response to GANctrl localized a partially-overlapping cluster, more ventrolateral, consistent with spinal trigeminal nucleus. A region-of-interest analysis also found eRAVANS activation in monoaminergic source nuclei including locus coeruleus (LC, noradrenergic) and both dorsal and median raphe (serotonergic) nuclei. Response to eRAVANS was significantly greater than iRAVANS for all nuclei, and greater than GANctrl in LC and raphe nuclei. Furthermore, eRAVANS, but not iRAVANS, enhanced cardiovagal modulation, confirming enhanced eRAVANS response on both central and peripheral neurophysiological levels.

CONCLUSION

7T fMRI localized brainstem response to taVNS, linked such response with autonomic outflow, and demonstrated that taVNS applied during exhalation enhanced NTS targeting.

Electric stimulation of ears accelerates body weight loss mediated by high-fat to low-fat diet switch accompanied by increased white adipose tissue browning in C57BL/6 J mice

Background

Weight reduction frequently occurs in patients receiving vagus nerve stimulation (VNS) therapy. Therefore, we hypothesized that during dietary intervention for weight loss, auricular electric stimulation (AES), an alternative of VNS, accelerates weight loss by increasing white adipose tissue (WAT) browning and increases energy expenditure.

Methods

C57BL/6J male mice were fed a high-fat diet for 5 wk. to induce obesity, then switched to a low-fat diet for 5 wk. and allocated into 3 groups to receive 2 Hz electric stimulation on ears, electrode clamps only, or nothing (AES, Sham and Ctrl, respectively).

Results

Switching to a low-fat diet reduced body weight progressively in all 3 groups, with the greatest reduction in the AES group. In accordance with a mild decrease in feed intake, hypothalamus mRNA levels of Npy, AgRP tended to be reduced, while Pomc tended to be increased by AES. Mice in the AES group had the highest concentrations of norepinephrine in serum and inguinal WAT, and expression levels of uncoupling protein-1 (UCP-1) and tyrosine hydroxylase in inguinal WAT. Furthermore, their subcutaneous adipocytes had multilocular and UCP-1⁺ characteristics, along with a smaller cell size.

Conclusion

AES, by increasing WAT browning, could be used in conjunction with a low-fat diet to augment weight loss in addition to suppressing appetite.

A controlled trial of transcutaneous vagus nerve stimulation for the treatment of pharmacoresistant epilepsy

This study explored the efficacy and safety of transcutaneous vagus nerve stimulation (t-VNS) in patients with pharmacoresistant epilepsy. A total of 60 patients were randomly divided into two groups based on the stimulation zone: the Ramsay-Hunt zone (treatment group) and the earlobe (control group). Before and after the 12-month treatment period, all patients completed the Self-Rating Anxiety Scale (SAS), the Self-Rating Depression Scale (SDS), the Liverpool Seizure Severity Scale (LSSS), and the Quality of Life in Epilepsy Inventory (QOLIE-31). Seizure frequency was determined according to the patient's seizure diary. During our study, the antiepileptic drugs were maintained at a constant level in all subjects. After 12 months, the monthly seizure frequency was lower in the treatment group than in the control group (8.0 to 4.0; P=0.003). This reduction in seizure frequency was correlated with seizure frequency at baseline and duration of epilepsy (both P>0.05). Additionally, all patients showed improved SAS, SDS, LSSS, and QOLIE-31 scores that were not correlated with a reduction in seizure frequency. The side effects in the treatment group were dizziness (1 case) and daytime drowsiness (3 cases), which could be relieved by reducing the stimulation intensity. In the control group, compared with baseline, there were no significant changes in seizure frequency (P=0.397), SAS, SDS, LESS, or QOLIE-31. There were also no complications in this group.

The auriculo-vagal afferent pathway and its role in seizure suppression in rats

BACKGROUND

The afferent projections from the auricular branch of the vagus nerve (ABVN) to the nucleus tractus solitaries (NTS) have been proposed as the anatomical basis for the increased parasympathetic tone seen in auriculo-vagal reflexes. As the afferent center of the vagus nerve, the NTS has been considered to play roles in the anticonvulsant effect of cervical vagus nerve stimulation (VNS). Here we proposed an "auriculo-vagal afferent pathway" (AVAP), by which transcutaneous auricular vagus nerve stimulation (ta-VNS) suppresses pentylenetetrazol (PTZ)-induced epileptic seizures by activating the NTS neurons in rats.

RESULTS

The afferent projections from the ABVN to the NTS were firstly observed in rats. ta-VNS increased the first grand mal latency of the epileptic seizure and decreased the seizure scores in awake rats. Furthermore, when the firing rates of the NTS neurons decreased, epileptiform activity manifested as electroencephalogram (EEG) synchronization increased with 0.37±0.12 s delay in anaesthetized rats. The change of instantaneous frequency, mean frequency of the NTS neurons was negative correlated with the amplitude of the epileptic activity in EEG traces. ta-VNS significantly suppressed epileptiform activity in EEG traces via increasing the firing rates of the neurons of the NTS. In comparison with tan-VNS, the anticonvulsant durations of VNS and ta-VNS were significantly longer (P<0.01). There was no significant difference between the anticonvulsant durations of VNS and ta-VNS (P>0.05). The anticonvulsant effect of ta-VNS was weakened by reversible cold block of the NTS.

CONCLUSIONS

There existed an anatomical relationship between the ABVN and the NTS, which strongly supports the concept that ta-VNS has the potential for suppressing epileptiform activity via the AVAP in rats. ta-VNS will provide alternative treatments for neurological disorders, which can avoid the disadvantage of VNS.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis and upper cervical cord following capsaicin injection into oral and craniofacial regions in rats

To define the somatotopic arrangement of neurons in the trigeminal spinal subnucleus caudalis and upper cervical cord activated by acute noxious stimulation of various orofacial sites, pERK expression was analyzed in these neurons. After capsaicin injection into the tongue, lower gum, upper and lower lips, or mental region, pERK-like immunoreactive (pERK-LI) cells were distributed mainly in the dorsal half of the trigeminal spinal nucleus interporalis (Vi) and caudalis (Vc) transition zone (Vi/Vc zone), middle Vc, and Vc and upper cervical cord transition zone (Vc/C2 zone). pERK-LI cells were distributed throughout the dorsal to ventral portion of the Vi/Vc zone, middle Vc, and Vc/C2 zone following capsaicin injection into the anterior hard palate, upper gum, buccal mucosa, or vibrissal pad and in the ventral portion of the Vi/Vc zone, middle Vc, and Vc/C2 zone following snout, ophthalmic, or ocular injection of capsaicin. The rostrocaudal distribution area of pERK-LI cells was more extensive from the Vi/Vc zone to the Vc/C2 zone after intraoral injection than that after facial injection, and the rostrocaudal distribution of pERK-LI cells from the Vi/Vc zone to the Vc/C2 zone had a somatotopic arrangement, with the snout being represented most rostrally and ophthalmic, ocular, or mental regions represented most caudally. These findings suggest that the pERK-LI cells expressed from the Vi/Vc zone to the Vc/C2 zone following injection of capsaicin in facial and intraoral structures may be differentially involved in pain perception in facial and intraoral sites.

The Role of the Craniospinal Nerves in Mediating the Antinociceptive Effect of Transcranial Electrostimulation in the Rat

Transcranial electrostimulation (TES) has been reported to elicit significant analgesia, but its mechanism of action has not been elucidated. In a recently introduced clinically relevant rat model of TES we have validated and characterized the TES antinociceptive effect, suggesting involvement of the sensory nerves of the rat's scalp in mediating that effect. In this study, we have further investigated the role of the craniospinal nerves by attempting to block the TES antinociceptive effect with local anesthetic injected under the TES electrodes. We also applied different transcutaneous electrical nerve stimulation modalities through the TES electrodes and compared the elicited antinociceptive effect to that of TES. The antinociceptive effect was assessed by measuring nociceptive thresholds in the tail-flick latency test in awake, unrestrained male rats. Data were analyzed by one-way analysis of variance followed by the Bonferroni t-test. The TES antinociceptive effect was significantly reduced after local anesthetic injection, and administration of 100 Hz transcutaneous electrical nerve stimulation was, over time, capable of eliciting the same degree of antinociceptive effect as TES. We conclude that sensory craniospinal nerves play a critical role in mediating the TES antinociceptive action and offer a hypothesis on the underlying mechanism(s) responsible for this action.

Neurophysiological and anatomical variability of the greater auricular nerve

OBJECTIVE

To study the normal variation in greater auricular nerve (GAN) conduction and anatomical course.

MATERIALS AND METHODS

GAN nerve conduction was studied in 77 healthy adults while 17 GANs were dissected in 10 cadavers.

RESULTS

The largest sensory nerve action potentials (SNAP) were recorded when the stimulator was placed 6 or 7 cm from the external acoustic meatus (EAM). Amplitude decreased significantly with age (P = 0.001). The dissected GANs entered the subcutaneous layer between 55 and 105 mm (median 68 mm) from EAM. The postauricular branch was found in 13 of 17 cases.

CONCLUSION

A large variation was observed both anatomically and by way of electrical stimulation. SNAP variation does probably reflect the anatomical variation in GAN course, branching, and terminal distribution. The best sites for stimulation were found 6 and 7 cm from EAM at the posterior sternocleiodomastoid border. The maximal amplitude, right:left amplitude ratio, and distal latency may be useful variables in GAN conduction studies.

Distribution of Fos-like immunoreactivity in the caudal brainstem of the rat following noxious chemical stimulation of the temporomandibular joint

Central expression of the protooncogene c-fos was used to examine areas receiving noxious sensory input from the rat temporomandibular joint (TMJ). Fos-like immunoreactivity (Fos-LI) in the caudal brainstem was visualized 2 hours after unilateral injection of the small-fiber-specific excitant/inflammatory irritant mustard oil into the TMJ region. Control animals received injection of either mustard oil into the subcutaneous fascia overlying the masseter muscle or mineral oil vehicle into the TMJ region. In all groups, Fos-LI was consistently observed ipsilaterally in the spinal trigeminal nucleus and cervical dorsal horn and, bilaterally, in the nucleus of the solitary tract and the ventrolateral medulla. The expression of Fos-LI ipsilaterally in the paratrigeminal nucleus was variable. Within the trigeminal sensory complex, Fos-LI was restricted to subnucleus caudalis and the caudal portions of subnucleus interpolaris near the level of the obex. Approximately 12% of Fos-LI cells in subnucleus caudalis and in the cervical dorsal horn were found in laminae III-VI. Compared to TMJ mustard oil injection, mineral oil injection produced less Fos-LI at all rostrocaudal levels, whereas subcutaneous mustard oil injection produced less Fos-LI in caudal subnucleus caudalis but similar amounts in the cervical dorsal horn. Neither of these injections yielded significant ipsilateral responses in subnucleus caudalis, indicating that Fos-LI in this region following TMJ mustard oil injection could be ascribed solely to small-fiber stimulation in the deep TMJ region. The wide rostrocaudal distribution of Fos-LI within the caudal brainstem reflects the distribution of TMJ-responsive nociceptive neurons that may underlie the spread and referral of pain from the TMJ region.

Selective deafferentation of convergent inputs to trigeminal subnucleus caudalis: effects on calcitonin gene-related peptide distribution

Electrophysiological studies (Sessle, 1987, 1991) suggest that trigeminal deafferenting injuries can cause an "unmasking" of existing but normally suppressed convergent inputs to the spinal trigeminal nucleus, including many that arise from the cervical spinal cord. However, the spatial arrangement of this projection has not been examined, particularly with reference to nociceptive components that might become involved in pathological changes leading to chronic pain. Therefore, the purpose of this study was to apply selective interruptions of the trigeminal and/or cervical primary afferent inputs to the spinal trigeminal subnucleus caudalis (Vc) in the cat, followed by (1) demonstration and quantification of axonal degeneration in the spinal trigeminal tract to determine the extent of trigeminal-cervical primary afferent overlap; and (2) an analysis of lesion-induced alterations in the distribution of calcitonin gene-related peptide immunoreactivity (CGRP-IR) in laminae I and II of Vc, since recent evidence strongly suggests that CGRP is involved in pathophysiological elevations of central nervous system neuronal excitability. Degenerating fibers were found throughout the spinal tract following a trigeminal rhizotomy or tractotomy, with the largest numbers adjacent to the rostral two-thirds of Vc, but with a significant number extending caudally to at least the level of C2. CGRP-IR was reduced or eliminated from the rostral one-third and periobex region of Vc, except for a dorsomedial zone that was minimally affected. Retention of CGRP-IR was greater at more caudal levels. Following a combined trigmeninal and cervical tractotomy, fiber degeneration was massive throughout the spinal tract, yet a population of small myelinated fibers persisted at 60 days after surgery. Concomitantly, CGRP-IR was profoundly reduced throughout Vc, except for a small dorsomedial zone of retention, which became more extensive caudally. A cervical tractotomy resulted in moderate numbers of degenerating fibers adjacent to the caudal one-third of Vc, and this number declined rostrally; however, degenerating fibers could be seen at the level of the obex. CGRP-IR was reduced in the dorsomedial and ventrolateral zones of Vc, particularly in its caudal one-third. Electron-microscopic analysis revealed a population of CGRP-IR boutons, most of which were of the simple axodendritic type with asymmetrical contacts. A few examples of axoaxonic contacts were observed. Loss of labeled boutons observed with the electron microscope was consistent with light-microscopic quantitative results. Those boutons that were retained were variable in size and displayed simple axodendritic contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Central projections of C4-C8 dorsal root ganglia in the rat studied by anterograde transport of WGA-HRP

Injections of WGA-HRP were made in the rat C4-C8 dorsal root ganglia (DRGs) individually to study the central projections and their relations to each other. The main dorsal horn projections from these DRGs to the dorsal horn lamina II extended for about two segments rostrally and caudally to the injected DRG, whereas the projections to laminae I, III, and IV were less restricted rostrocaudally. Comparisons of the dorsal horn projections from the DRGs investigated indicated a tendency for a somatotopic organization, which was most prominent in lamina II. Labeled central branches from the C4-8 DRGs could be traced in the dorsal column as far caudally as 12-17 segments caudal to the level of entrance. Most of these fibers appeared to end in the medial dorsal horn base, including the column of Clarke. Labeling of primary afferents in the ventral horn generally extended for at least 3-4 segments rostral and caudal to the level of the injected DRG. Projections to the central cervical nucleus were most prominent from the C4 DRG and gradually became less prominent from the more caudal DRGs. Heavy projections to the cuneate nucleus (Cun) originated from the C7 and C8 DRG, whereas those from the C4-C6 DRGs were less extensive. The Cun projections from the different DRGs appeared to overlap, and the same was true for the projections to the external cuneate nucleus. Projections to the gracile nucleus, the vestibular nuclear complex, including nucleus X, and to trigeminal sensory nuclei were seen from all DRGs investigated.

Somatotopic organization of inputs from the hand to the spinal gray and cuneate nucleus of monkeys with observations on the cuneate nucleus of humans

Central termination patterns of primary afferents from the hand and forelimb were studied following subdermal injections of HRP conjugates in macaque monkeys. In the middle layers of the dorsal horn of the spinal cord, afferents from digits 1-5 terminated in a rostrocaudal sequence in separate, elongated columns at cervical levels 5-7. Afferents from the glabrous digits extended to the medial margin of the dorsal gray, while afferents from the dorsal skin of the digits terminated more laterally. Afferents from the dorsal hand and palm terminated lateral to those from the digits, while inputs from the forearm occupied tissue rostral and caudal to the representation of the hand. In the cuneate nucleus, terminations from each digit formed an elongated column that was densely labelled in the central pars rotunda and sparsely labelled in both the rostral and caudal reticular poles. Within the pars rotunda, digits 1-5 were represented in order from lateral to medial. Inputs from the digit tips terminated ventral to inputs from the proximal digits. Afferents from the dorsal skin of the digits terminated in an even more dorsal position, while the most dorsal portion of the pars rotunda related to the glabrous and dorsal hand. Within the pars rotunda, terminations from specific parts of the hand overlapped parcellated clusters of neurons. These clusters were densely reactive for cytochrome oxidase (CO) and were surrounded by myelinated fibers. Much sparser label in the reticular poles was found consistently only after injections in the glabrous digits. Inputs to the poles appeared diffuse and overlapping while preserving some somatotopic order. When treated for CO or stained for Nissl substance or myelin, the pars rotunda of humans showed parcellation patterns that closely resembled the patterns seen in monkeys. From the relationship of inputs to the CO dense cell clusters in monkeys, it was possible to postulate in detail the somatotopic organization of inputs to pars rotunda of humans. The present results provide a comprehensive description of the somatotopic patterns of termination of afferents from the skin of the hand and forearm in the spinal cord and cuneate nucleus of macaque monkeys. A direct relationship of afferent somatotopy and identifiable cell clusters in the pars rotunda of the cuneate nucleus is further demonstrated. Finally, the patterns of cell clusters in the pars rotunda of macaque monkeys and humans suggest that the somatotopic organization of the cuneate nucleus may be very similar in human and nonhuman primates.

Central distribution of cervical primary afferents in the rat, with emphasis on proprioceptive projections to vestibular, perihypoglossal, and upper thoracic spinal nuclei

The projections of primary afferents from rostral cervical segments to the brainstem and the spinal cord of the rat were investigated by using anterograde and transganglionic transport techniques. Projections from whole spinal ganglia were compared with those from single nerves carrying only exteroceptive or proprioceptive fibers. Injections of horseradish peroxidase (HRP) or wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) were performed into dorsal root ganglia C2, C3, and C4. Free HRP was applied to the cut dorsal rami C2 and C3, greater occipital nerve, sternomastoid nerve, and to the C1/2 anastomosis, which contains afferents from suboccipital muscles and the atlanto-occipital joint. WGA-HRP injections into ganglia C7 and L5 were performed for comparative purposes. Injections of WGA-HRP or free HRP into rostral cervical dorsal root ganglia and HRP application to C2 and C3 dorsal rami produced labeling in dorsal and ventral horns at the level of entrance, the central cervical nucleus, and in external and main cuneate nuclei. From axons ascending to pontine and descending to upper thoracic spinal levels, medial collaterals were distributed to medial and descending vestibular, perihypoglossal and solitary nuclei, and the intermediate zone and Clarke's nucleus dorsalis in the spinal cord. Lateral collaterals projected mainly to the trigeminal subnucleus interpolaris and to lateral spinal laminae IV and V. Results from HRP application to single peripheral nerves indicated that medial collaterals were almost exclusively proprioceptive, whereas lateral collaterals were largely exteroceptive with a contribution from suboccipital proprioceptive fibers. WGA-HRP injections into dorsal root ganglia C7 and L5 failed to produce significant labeling within vestibular and periphypoglossal nuclei, although they demonstrated classical projection sites within the brainstem and spinal cord. The consistent collateralisation pattern of rostral cervical afferents along their whole rostrocaudal course enables them to contact a variety of precerebellar, vestibulospinal, and preoculomotor neurons. These connections reflect the well-known significance of proprioceptive neck afferents for the control of posture, head position, and eye movements.