Vagal input to lateral area 3a in cat cortex

Neural Correlates of Pain-Autonomic Coupling in Patients With Complex Regional Pain Syndrome Treated by Repetitive Transcranial Magnetic Stimulation of the Motor Cortex

OBJECTIVES

Complex regional pain syndrome (CRPS) is a chronic pain condition involving autonomic dysregulation. In this study, we report the results of an ancillary study to a larger clinical trial investigating the treatment of CRPS by neuromodulation. This ancillary study, based on functional magnetic resonance imaging (fMRI), evaluated the neural correlates of pain in patients with CRPS in relation to the sympathetic nervous system and for its potential relief after repetitive transcranial magnetic stimulation of the motor cortex.

MATERIALS AND METHODS

Eleven patients with CRPS at one limb (six women, five men, aged 52.0 ± 9.6 years) were assessed before and one month after the end of a five-month repetitive transcranial magnetic stimulation (rTMS) therapy targeting the motor cortex contralateral to the painful limb, by means of electrochemical skin conductance (ESC) measurement, daily pain intensity scores on a visual numerical scale (VNS), and fMRI with motor tasks (alternation of finger movements and rest). The fMRI scans were analyzed voxelwise using ESC and VNS pain score as regressors to derive their neural correlates. The criterion of response to rTMS therapy was defined as ≥30% reduction in VNS pain score one month after treatment compared with baseline.

RESULTS

At baseline, ESC values were reduced in the affected limb vs the nonaffected limb. There was a covariance of VNS with brain activation in a small region of the primary somatosensory cortex (S1) contralateral to the painful side on fMRI investigation. After rTMS therapy on motor cortex related to the painful limb, the VNS pain scores significantly decreased by 22% on average. The criterion of response was met in six of 11 patients (55%). In these responders, at one month after treatment, ESC value increased and returned to normal in the CRPS-affected limb, and overall, the increase in ESC correlated with the decrease in VNS after motor cortex rTMS therapy. At one month after treatment, there also was a covariance of both variables (ESC and VNS) with fMRI activation of the S1 region previously mentioned. The fMRI activation of other brain regions (middle frontal gyrus and temporo-parietal junction) showed correlation with ESC values before and after treatment. Finally, we found a positive correlation at one month after treatment (not at baseline) between VNS pain score and fMRI activation in the temporo-parietal junction contralateral to painful side.

CONCLUSIONS

This study first shows a functional pain-autonomic coupling in patients with CRPS, which could involve a specific S1 region. However, the modulation of sympathetic sudomotor activities expressed by ESC changes was rather correlated with functional changes in other brain regions. Finally, the pain relief observed at one month after rTMS treatment was associated with a reduced activation of the temporo-parietal junction on the side in which rTMS was performed. These findings open perspectives to define new targets or biomarkers for using rTMS to treat CRPS-associated pain.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT02817880.

Convergence of forepaw somatosensory and motor cortical projections in the striatum, claustrum, thalamus, and pontine nuclei of cats

The basal ganglia and pontocerebellar systems regulate somesthetic-guided motor behaviors and receive prominent inputs from sensorimotor cortex. In addition, the claustrum and thalamus are forebrain subcortical structures that have connections with somatosensory and motor cortices. Our previous studies in rats have shown that primary and secondary somatosensory cortex (S1 and S2) send overlapping projections to the neostriatum and pontine nuclei, whereas, overlap of primary motor cortex (M1) and S1 was much weaker. In addition, we have shown that M1, but not S1, projects to the claustrum in rats. The goal of the current study was to compare these rodent projection patterns with connections in cats, a mammalian species that evolved in a separate phylogenetic superorder. Three different anterograde tracers were injected into the physiologically identified forepaw representations of M1, S1, and S2 in cats. Labeled fibers terminated throughout the ipsilateral striatum (caudate and putamen), claustrum, thalamus, and pontine nuclei. Digital reconstructions of tracer labeling allowed us to quantify both the normalized distribution of labeling in each subcortical area from each tracer injection, as well as the amount of tracer overlap. Surprisingly, in contrast to our previous findings in rodents, we observed M1 and S1 projections converging prominently in striatum and pons, whereas, S1 and S2 overlap was much weaker. Furthermore, whereas, rat S1 does not project to claustrum, we confirmed dense claustral inputs from S1 in cats. These findings suggest that the basal ganglia, claustrum, and pontocerebellar systems in rat and cat have evolved distinct patterns of sensorimotor cortical convergence.

The Phantom Satiation Hypothesis of Bariatric Surgery

The excitation of vagal mechanoreceptors located in the stomach wall directly contributes to satiation. Thus, a loss of gastric innervation would normally be expected to result in abrogated satiation, hyperphagia, and unwanted weight gain. While Roux-en-Y-gastric bypass (RYGB) inevitably results in gastric denervation, paradoxically, bypassed subjects continue to experience satiation. Inspired by the literature in neurology on phantom limbs, I propose a new hypothesis in which damage to the stomach innervation during RYGB, including its vagal supply, leads to large-scale maladaptive changes in viscerosensory nerves and connected brain circuits. As a result, satiation may continue to arise, sometimes at exaggerated levels, even in subjects with a denervated or truncated stomach. The same maladaptive changes may also contribute to dysautonomia, unexplained pain, and new emotional responses to eating. I further revisit the metabolic benefits of bariatric surgery, with an emphasis on RYGB, in the light of this phantom satiation hypothesis.

Mechanisms underlying the sensation of dyspnea

Dyspnea is defined as a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity. It is a common symptom among patients with respiratory diseases that reduces daily activities, induces deconditioning, and is self-perpetuating. Although clinical interventions are needed to reduce dyspnea, its underlying mechanism is poorly understood depending on the intertwined peripheral and central neural mechanisms as well as emotional factors. Nonetheless, experimental and clinical observations suggest that dyspnea results from dissociation or a mismatch between the intended respiratory motor output set caused by the respiratory neuronal network in the lower brainstem and the ventilatory output accomplished. The brain regions responsible for detecting the mismatch between the two are not established. The mechanism underlying the transmission of neural signals for dyspnea to higher sensory brain centers is not known. Further, information from central and peripheral chemoreceptors that control the milieu of body fluids is summated at higher brain centers, which modify dyspneic sensations. The mental status also affects the sensitivity to and the threshold of dyspnea perception. The currently used methods for relieving dyspnea are not necessarily fully effective. The search for more effective therapy requires further insights into the pathophysiology of dyspnea.

Somatosensory evoked fields predict response to vagus nerve stimulation

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There is a need to preoperatively identify children who will respond to VNS.

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There is similarity between the afferent vagus and median nerve projections to S1.

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Median nerve somatosensory evoked fields identify those who will respond to VNS.

There is an unmet need to develop robust predictive algorithms to preoperatively identify pediatric epilepsy patients who will respond to vagus nerve stimulation (VNS). Given the similarity in the neural circuitry between vagus and median nerve afferent projections to the primary somatosensory cortex, the current study hypothesized that median nerve somatosensory evoked field(s) (SEFs) could be used to predict seizure response to VNS. Retrospective data from forty-eight pediatric patients who underwent VNS at two different institutions were used in this study. Thirty-six patients (“Discovery Cohort”) underwent preoperative electrical median nerve stimulation during magnetoencephalography (MEG) recordings and 12 patients (“Validation Cohort”) underwent preoperative pneumatic stimulation during MEG. SEFs and their spatial deviation, waveform amplitude and latency, and event-related connectivity were calculated for all patients. A support vector machine (SVM) classifier was trained on the Discovery Cohort to differentiate responders from non-responders based on these input features and tested on the Validation Cohort by comparing the model-predicted response to VNS to the known response. We found that responders to VNS had significantly more widespread SEF localization and greater functional connectivity within limbic and sensorimotor networks in response to median nerve stimulation. No difference in SEF amplitude or latencies was observed between the two cohorts. The SVM classifier demonstrated 88.9% accuracy (0.93 area under the receiver operator characteristics curve) on cross-validation, which decreased to 67% in the Validation cohort. By leveraging overlapping neural circuitry, we found that median nerve SEF characteristics and functional connectivity could identify responders to VNS.

Alteration of coupling between brain and heart induced by sedation with propofol and midazolam

For a comprehensive understanding of the nervous system, several previous studies have examined the network connections between the brain and the heart in diverse conditions. In this study, we identified coupling between the brain and the heart along the continuum of sedation levels, but not in discrete sedation levels (e. g., wakefulness, conscious sedation, and deep sedation). To identify coupling between the brain and the heart during sedation, we induced several depths of sedation using patient-controlled sedation with propofol and midazolam. We performed electroencephalogram (EEG) spectral analysis and extracted the instantaneous heart rate (HR) from the electrocardiogram (ECG). EEG spectral power dynamics and mean HR were compared along the continuum of sedation levels. We found that EEG sigma power was the parameter most sensitive to changes in the sedation level and was correlated with the mean HR under the effect of sedative agents. Moreover, we calculated the Granger causality (GC) value to quantify brain-heart coupling at each sedation level. Additionally, the GC analysis revealed noticeably different strengths and directions of causality among different sedation levels. In all the sedation levels, GC values from the brain to the heart (GCb→h) were higher than GC values from the heart to the brain (GCh→b). Moreover, the mean GCb→h increased as the sedation became deeper, resulting in higher GCb→h values in deep sedation (1.97 ± 0.18 in propofol, 2.02 ± 0.15 in midazolam) than in pre-sedation (1.71 ± 0.13 in propofol, 1.75 ± 0.11 in midazolam; p < 0.001). These results show that coupling between brain and heart activities becomes stronger as sedation becomes deeper, and that this coupling is more attributable to the brain-heart direction than to the heart-brain direction. These findings provide a better understanding of the relationship between the brain and the heart under specific conditions, namely, different sedation states.

Contributions of Nociresponsive Area 3a to Normal and Abnormal Somatosensory Perception

Traditionally, cytoarchitectonic area 3a of primary somatosensory cortex (SI) has been regarded as a proprioceptive relay to motor cortex. However, neuronal spike-train recordings and optical intrinsic signal imaging, obtained from nonhuman sensorimotor cortex, show that neuronal activity in some of the cortical columns in area 3a can be readily triggered by a C-nociceptor afferent drive. These findings indicate that area 3a is a critical link in cerebral cortical encoding of secondary/slow pain. Also, area 3a contributes to abnormal pain processing in the presence of activity-dependent reversal of gamma-aminobutyric acid A receptor-mediated inhibition. Accordingly, abnormal processing within area 3a may contribute mechanistically to generation of clinical pain conditions. PERSPECTIVE: Optical imaging and neurophysiological mapping of area 3a of SI has revealed substantial driving from unmyelinated cutaneous nociceptors, complementing input to areas 3b and 1 of SI from myelinated nociceptors and non-nociceptors. These and related findings force a reconsideration of mechanisms for SI processing of pain.

Stomach-brain synchrony reveals a novel, delayed-connectivity resting-state network in humans

Resting-state networks offer a unique window into the brain’s functional architecture, but their characterization remains limited to instantaneous connectivity thus far. Here, we describe a novel resting-state network based on the delayed connectivity between the brain and the slow electrical rhythm (0.05 Hz) generated in the stomach. The gastric network cuts across classical resting-state networks with partial overlap with autonomic regulation areas. This network is composed of regions with convergent functional properties involved in mapping bodily space through touch, action or vision, as well as mapping external space in bodily coordinates. The network is characterized by a precise temporal sequence of activations within a gastric cycle, beginning with somato-motor cortices and ending with the extrastriate body area and dorsal precuneus. Our results demonstrate that canonical resting-state networks based on instantaneous connectivity represent only one of the possible partitions of the brain into coherent networks based on temporal dynamics.

The brain is always active. Even when it is not receiving sensory input, it generates its own spontaneous activity. This activity shapes how we interpret future sensory signals and creates our inner mental world. Moreover, this spontaneous activity is not random. When a healthy volunteer lies inside a brain scanner without performing any task, his or her brain shows predictable patterns of activity. Specific groups of brain regions – often with related roles – become active at the same time as one another. Each set of regions is referred to as a resting state network.

Of course, the brain does not operate in isolation from the rest of the body. Our internal organs continuously send signals to the brain via the spinal cord and cranial nerves. Specialized cells in the stomach wall in particular produce a slow rhythmic pattern of electrical activity. Known as the gastric rhythm, this activity helps ensure that the stomach muscles contract at the correct speed for digestion. But the stomach also produces this rhythm even when empty, suggesting that it has other roles too.

To find out what these might be, Rebollo et al. placed electrodes on the abdomen of healthy volunteers lying inside brain scanners. By examining the volunteers’ spontaneous brain activity, Rebollo et al. identified a new resting state network that is active in synchrony with the gastric rhythm. The regions within this so-called gastric network are not active at the same time as each other, but instead become active in a specific sequence that is repeated at each gastric cycle. Many of the regions within the gastric network belong to other resting state networks too. Some of the regions help regulate automatic bodily functions such as heart rate, while others process information about the body’s position in space.

The existence of the gastric network suggests a link between the automatic regulation of processes such as digestion, and spontaneous brain activity. Future studies could examine whether this link impacts perception and cognition, and whether this link plays a role in disorders where the connection between the digestive system and the brain appears to be altered.

The state of sleep and the current brain paradigm

Up to the present time cerebral cortex was considered as substrate for realization of the highest psychical functions including consciousness. Cortical sensory areas were regarded as structures specialized for processing of information coming from one particular modality (visual, auditory, somatosensory, and so on). However, studies of cortical activity in sleep-wake cycle demonstrated that during sleep the same neurons in the same cortical areas switch to processing of signals coming from the various visceral systems. After awakening these visceral responses disappear and the neurons return to processing of the information coming from the exteroreceptors. These observations indicate that most likely cortical areas are universal processors, which perform particular operations with incoming information independent of its origin. During wakefulness, results of the information processing on the cortical level should be directed to structures connected with organization of behavior and consciousness, while during sleep cortical outputs should be redirected to structures performing integration of the visceral information. Thus, results of sleep studies indicate that current brain paradigm should be changed.

Activity of somatosensory-responsive neurons in high subdivisions of SI cortex during locomotion

Responses of neurons in the primary somatosensory cortex during movements are poorly understood, even during such simple tasks as walking on a flat surface. In this study, we analyzed spike discharges of neurons in the rostral bank of the ansate sulcus (areas 1-2) in 2 cats while the cats walked on a flat surface or on a horizontal ladder, a complex task requiring accurate stepping. All neurons (n = 82) that had receptive fields (RFs) on the contralateral forelimb exhibited frequency modulation of their activity that was phase locked to the stride cycle during simple locomotion. Neurons with proximal RFs (upper arm/shoulder) and pyramidal tract-projecting neurons (PTNs) with fast-conducting axons tended to fire at peak rates in the middle of the swing phase, whereas neurons with RFs on the distal limb (wrist/paw) and slow-conducting PTNs typically showed peak firing at the transition between swing and stance phases. Eleven of 12 neurons with tactile RFs on the volar forepaw began firing toward the end of swing, with peak activity occurring at the moment of foot contact with floor, thereby preceding the evoked sensory volley from touch receptors. Requirement to step accurately on the ladder affected 91% of the neurons, suggesting their involvement in control of accuracy of stepping. During both tasks, neurons exhibited a wide variety of spike distributions within the stride cycle, suggesting that, during either simple or ladder locomotion, they represent the cycling somatosensory events in their activity both predictively before and reflectively after these events take place.

Pain without nociception?

We describe a young woman with complete cervical spinal cord transsection, who developed significant abdominal pain, triggered by gastric distension and deep abdominal palpation. On the basis of the nature of her spinal cord injury, her brain-gut axis was limited to vagal pathways. Studies in mammalian models of human visceral sensation consistently showed that the subdiaphragmatic vagus contains a homogeneous population of afferents that are activated by low-intensity stimuli, which are generally believed to be important in regulating autonomic function and perhaps contributing to visceral sensory experiences triggered by such low-intensity stimuli (e.g. fullness, nausea), but not pain, although many fibers encode stimuli well into the noxious range. In contrast, spinal afferent pathways include fibers with high-activation thresholds that are thought to represent specialized nociceptors. This illustrative case argues against an exclusive role of specialized nociceptive pathways in visceral pain, but supports a concept of intensity coding with the composite of vagal and spinal input contributing to conscious perception and pain.

Mechanisms of dyspnea

The mechanisms and pathways of the sensation of dyspnea are incompletely understood, but recent studies have provided some clarification. Studies of patients with cord transection or polio, induced spinal anesthesia, or induced respiratory muscle paralysis indicate that activation of the respiratory muscles is not essential for the perception of dyspnea. Similarly, reflex chemostimulation by CO₂ causes dyspnea, even in the presence of respiratory muscle paralysis or cord transection, indicating that reflex chemoreceptor stimulation per se is dyspnogenic. Sensory afferents in the vagus nerves have been considered to be closely associated with dyspnea, but the data were conflicting. However, recent studies have provided evidence of pulmonary vagal C-fiber involvement in the genesis of dyspnea, and recent animal data provide a basis to reconcile differences in responses to various C-fiber stimuli, based on the ganglionic origin of the C fibers. Brain imaging studies have provided information on central pathways subserving dyspnea: Dyspnea is associated with activation of the limbic system, especially the insular area. These findings permit a clearer understanding of the mechanisms of dyspnea: Afferent information from reflex stimulation of the peripheral sensors (chemoreceptors and/or vagal C fibers) is processed centrally in the limbic system and sensorimotor cortex and results in increased neural output to the respiratory muscles. A perturbation in the ventilatory response due to weakness, paralysis, or increased mechanical load generates afferent information from vagal receptors in the lungs (and possibly mechanoreceptors in the respiratory muscles) to the sensorimotor cortex and results in the sensation of dyspnea.

Phrenic nerve afferent activation of neurons in the cat SI cerebral cortex

Stimulation of respiratory afferents elicits neural activity in the somatosensory region of the cerebral cortex in humans and animals. Respiratory afferents have been stimulated with mechanical loads applied to breathing and electrical stimulation of respiratory nerves and muscles. It was hypothesized that stimulation of the phrenic nerve myelinated afferents will activate neurons in the 3a and 3b region of the somatosensory cortex. This was investigated in cats with electrical stimulation of the intrathoracic phrenic nerve and C(5) root of the phrenic nerve. The somatosensory cortical response to phrenic afferent stimulation was recorded from the cortical surface, contralateral to the phrenic nerve, ispilateral to the phrenic nerve and with microelectrodes inserted into the cortical site of the surface dipole. Short-latency, primary cortical evoked potentials (1 degrees CEP) were recorded with stimulation of myelinated afferents of the intrathoracic phrenic nerve in the contralateral post-cruciate gyrus of all animals (n = 42). The mean onset and peak latencies were 8.5 +/- 5.7 ms and 21.8 +/- 9.8 ms, respectively. The rostro-caudal surface location of the 1 degrees CEP was found between the rostral edge of the post-cruciate dimple (PCD) and the rostral edge of the ansate sulcus, medio-lateral location was between 2 mm lateral to the sagittal sulcus and the lateral end of the cruciate sulcus. Histological examination revealed that the 1 degrees CEP sites were recorded over areas 3a and 3b of the SI somatosensory cortex. Intracortical activation of 16 neurons with two patterns of neural activity was recorded: (1) short-latency, short-duration activation of neurons and (2) long-latency, long-duration activation of neurons. Short-latency neurons had a mean onset latency of 10.4 +/- 3.1 ms and mean burst duration of 10.1 +/- 3.2 ms. The short-latency units were recorded at an average depth of 1.7 +/- 0.5 mm below the cortical surface. The long-latency neurons had a mean onset latency of 36.0 +/- 4.2 ms and mean burst duration of 32.2 +/- 8.4 ms. The long-latency units were recorded at an average depth of 2.4 +/- 0.2 mm below the cortical surface. The results of the study demonstrated that phrenic nerve afferents have a short-latency central projection to the SI somatosensory cortex. The phrenic afferents activated neurons in lamina III and IV of areas 3a and 3b. The cortical representation of phrenic nerve afferents is medial to the forelimb, lateral to the hindlimb, similar to thoracic loci, hence the phrenic afferent SI site in the cat homunculus is consistent with body position (thoracic region) rather than spinal segment (C(5)-C(7)). The phrenic afferent activation of the somatosensory cortex is bilateral, with the ipsilateral cortical activation occurring subsequent to the contralateral. These results support the hypothesis that phrenic afferents provide somatosensory information to the cerebral cortex which can be used for diaphragmatic proprioception and somatosensation.

Biological evolution based on nonrandom variability regulated by the organism

A hypothetical mechanism for rapid and nonrandom emergence of evolutionary adaptations is proposed. It is supposed that some transcription factors and transcription regulators that are able to cross membranes can leave the cells of their origin and move within the organism using a specialized transport system when individual development occurs under conditions extreme for the given species. This system, in particular, connects soma with germline. The supply of germline cells with unusual transcription regulators changes the balance of their nuclear regulatory RNAs, thus initiating RNA-dependent epigenetic modifications of the germline genome and therefore changes in phenotypes of the progeny. It is highly probable that some of these phenotypes are adaptive and lay the basis for the origin of the next biological species. The proposed mechanism can serve as a basis for a new theory of the origin of species.

Gastric fundic distension activates fronto-limbic structures but not primary somatosensory cortex: a functional magnetic resonance imaging study

INTRODUCTION

The brain representation of visceral stimulation bears important similarities to that of somatic stimulation. However, the role of the primary (S1) and secondary (S2) somatosensory cortices in mediating gastric sensation is uncertain.

MATERIALS AND METHODS

Eighteen healthy, right-handed volunteers (age 32 years+/-6.5 years; 14 men) underwent dynamic assessment of the relationship between sensation and fundic barostat distending pressure and volume, and then brain functional magnetic resonance imaging (fMRI) during noxious fundic distension. Cytoarchitectonic probability maps were used to examine in detail the null hypothesis that fundic distension did not produce significant activation of S1 or S2.

RESULTS

Distending volume explained 74% of the variance in gastric sensation, compared to 64% with distending pressure. Incorporating distending volume into the regressor function for our fMRI analyses, we found that noxious fundic distension activated a widespread network of brain regions, including the pontine brainstem, thalami, cerebellum, insular cortex bilaterally, anterior and posterior cingulate cortex, right frontal lobe, and inferior parietal lobules. In detailed analyses, we found no evidence of activation of S1, but did find activation in one region of S2.

DISCUSSION

Our findings suggest that an extensive, predominantly fronto-limbic network of brain regions, including the insular cortex, mediates perception of noxious gastric fundic distension in healthy humans, without significant participation by the primary somatosensory cortex. This and other recent studies lay the groundwork for investigations comparing brain processing of visceral stimuli between healthy volunteers and patients with functional dyspepsia.

"Pusher syndrome" following cortical lesions that spare the thalamus

Stroke patients with "pusher syndrome" show severe misperception of their own upright body orientation although visual-vestibular processing is almost intact. This dissociation argues for a second graviceptive system in humans for the perception of body orientation. Recent studies revealed that the posterior thalamus is an important part of this system. The present investigation aimed to study the cortical representation of this system beyond the thalamus. We evaluated 45 acute patients with and without contraversive pushing following left-or right-sided cortical lesions sparing the thalamus. In both hemispheres, the simple lesion overlap associated with contraversive pushing typically centered on the insular cortex and parts of the postcentral gyrus. The comparison between pusher patients and controls who were matched with respect to age, lesion size, and the frequency of spatial neglect, aphasia and visual field defects revealed only very small regions that were specific for the pusher patients with cortical damage sparing the thalamus. Obviously, the cortical structures representing our control of upright body orientation are in close anatomical proximity to those areas that induce aphasia in the left hemisphere and spatial neglect in the right hemisphere when lesioned. We conclude that in addition to the subcortical area previously identified in the posterior thalamus, parts of the insula and postcentral gyrus appear to contribute at cortical level to the processing of the afferent signals mediating the graviceptive information about upright body orientation.

Vagal-Evoked Activity in the Parafascicular Nucleus of the Primate Thalamus

The physiological effects of ascending vagal afferent activity in the primate forebrain have not been established, and because vagus nerve stimulation (VNS) is useful clinically for treatment of epilepsy and depression, these actions need to be identified. We used a roving microelectrode to record vagal-evoked potentials in the thalamus of the macaque monkey. In addition to the anticipated activation in the gustatory/visceral thalamic relay nucleus, we found an unexpectedly larger and earlier response focus with multi-unit discharges in the adjacent parafascicular nucleus. These data reveal a potent vagal input to this intralaminar nucleus, which is normally considered to be involved in motor control. This finding indicates that a role for this vagal activation site in the anti-epileptic effects of VNS needs to be considered.

Basic and clinical aspects of visceral sensation: transmission in the CNS

Pain and discomfort are the leading cause for consultative visits to gastroenterologists. Acute pain should be considered a symptom of an underlying disease, thereby serving a physiologically important function. However, many patients experience chronic pain in the absence of potentially harmful stimuli or disorders, turning pain into the primary problem rather than a symptom. Vagal and spinal afferents both contribute to the sensory component of the gut-brain axis. Current evidence suggests that they convey different elements of the complex sensory experience. Spinal afferents play a key role in the discriminatory dimension, while vagal input primarily affects the strong emotional and autonomic reactions to noxious visceral stimuli. Drugs, surgical and non-pharmacological treatments can target these pathways and provide therapeutic options for patients with chronic visceral pain syndromes.

Visceral and cutaneous pain representation in parasylvian cortex

The ability to localize both touch and pain has been attributed mainly to the primary somatosensory cortex (S1), based on its fine somatotopic mapping of tactile inputs. Recently, S1 has also been implicated in the differentiation of noxious stimulation, such as distinguishing between pain arising from viscera and skin. Recent MEG and fMRI studies show that there is at least a rudimentary tactile topographic representation in the supra-sylvian cortex [encompassing secondary somatosensory area (S2)], suggesting that this area may contribute to touch localization. Nevertheless, the role of this region in pain localization or its role in the differentiation of various types of pain has not been clearly established. Healthy subjects (four males, three females) underwent fMRI-scanning (1.5 T, standard head coil, BOLD analysis) during painful balloon distention of the distal esophagus and painful heat on the midline chest in the zone of referred pain for the esophageal stimulation. Five of the seven subjects exhibited significant activation of the parasylvian region in both experimental conditions, and in each of these five subjects activation related to esophageal pain was represented more laterally within the parasylvian cortex than that associated with cutaneous trunk pain (paired t-test, p's < 0.01). Our results suggest segregation of visceral esophageal and cutaneous chest afferents within parasylvian cortex, possibly implicating this region in the perceptual differentiation of visceral and cutaneous pain.

Lamina I, but not Lamina V, Spinothalamic Neurons Exhibit Responses That Correspond With Burning Pain

Single-unit recordings from monkey spinothalamic tract (STT) neurons reveal that the responses of polymodal nociceptive lamina I STT neurons correspond with the profile of burning pain elicited in human subjects by repeated brief-contact heat. In contrast, lamina V wide-dynamic-range (WDR) neurons show a significantly different response pattern. This finding indicates that burning pain is signaled by modality-selective lamina I neurons and not convergent lamina V WDR neurons.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the macaque monkey

Thalamic terminations from trigeminal, cervical, and lumbosacral lamina I neurons were investigated with Phaseolus vulgaris leucoagglutinin (PHA-L) and labeled dextrans. Iontophoretic injections guided by physiological recordings were restricted to lamina I or laminae I-II. PHA-L-labeled trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. TRITC- and FITC-labeled dextrans were injected at different levels to confirm topography. Terminations consistently occurred in two main locations: a distinguishable portion of posterolateral thalamus identified cytoarchitectonically as the posterior part of the ventral medial nucleus (VMpo) and a portion of posteromedial thalamus designated as the ventral caudal part of the medial dorsal nucleus (MDvc). In addition, isolated fibers bearing boutons of passage were observed in the ventral posterior medial and lateral (VPM and VPL) nuclei, and spinal terminations occurred in the ventral posterior inferior nucleus (VPI). Isolated terminations occasionally occurred in other sites (e.g., suprageniculate, zona incerta, hypothalamic paraventricular n.). Terminations in MDvc occurred in concise foci that were weakly organized topographically (posteroanterior = rostrocaudal). Terminations in VMpo consisted of dense clusters of ramified terminal arbors bearing multiple large boutons that were well organized topographically (anteroposterior = rostrocaudal). Terminations in VMpo colocalized with a field of calbindin-immunoreactive terminal fibers; double-labeled terminals were documented at high magnification. This propitious marker was especially useful at anterior levels, where VMpo can easily be misidentified as VPM. These findings demonstrate phylogenetically novel primate lamina I TSTT projections important for sensory and motivational aspects of pain, temperature, itch, muscle ache, sensual touch, and other interoceptive feelings from the body.