Monkey insular cortex neurons respond to baroreceptive and somatosensory convergent inputs

Forebrain neural patterns associated with sex differences in autonomic and cardiovascular function during baroreceptor unloading

Generally, women demonstrate smaller autonomic and cardiovascular reactions to stress, compared with men. The mechanism of this sex-dependent difference is unknown, although reduced baroreflex sensitivity may be involved. Recently, we identified a cortical network associated with autonomic cardiovascular responses to baroreceptor unloading in men. The current investigation examined whether differences in the neural activity patterns within this network were related to sex-related physiological responses to lower body negative pressure (LBNP, 5, 15, and 35 mmHg). Forebrain activity in healthy men and women ( n = 8 each) was measured using functional magnetic resonance imaging with blood oxygen level-dependent (BOLD) contrast. Stroke volume (SV), heart rate (HR), and muscle sympathetic nerve activity (MSNA) were collected on a separate day. Men had larger decreases in SV than women ( P < 0.01) during 35 mmHg LBNP only. At 35 mmHg LBNP, HR increased more in males then females (9 ± 1 beats/min vs. 4 ± 1 beats/min, P < 0.05). Compared with women, increases in total MSNA were similar at 15 mmHg LBNP but greater during 35 mmHg LBNP in men [1,067 ± 123 vs. 658 ± 103 arbitrary units (au), P < 0.05]. BOLD signal changes ( P < 0.005, uncorrected) were identified within discrete forebrain regions associated with these sex-specific HR and MSNA responses. Men had larger increases in BOLD signal within the right insula and dorsal anterior cingulate cortex than women. Furthermore, men demonstrated greater BOLD signal reductions in the right amygdala, left insula, ventral anterior cingulate, and ventral medial prefrontal cortex vs. women. The greater changes in forebrain activity in men vs. women may have contributed to the elevated HR and sympathetic responses observed in men during 35 mmHg LBNP.

Mécanismes cérébraux impliqués dans l’interaction entre la douleur et les émotions

INTRODUCTION

Pain is an unpleasant and intrusive sensation, warning of actual or potential tissue damage. Over the last fifteen years, functional cerebral imaging research has demonstrated the involvement of many cerebral structures in the experience of pain.

BACKGROUND

Intimately linked to the notion of suffering, the affective dimension of pain relies on neurophysiological systems partly distinct anatomically from those involved more specifically in its sensory dimension. Some pathways convey nociceptive information to the somatosensory cortex and the insula, contributing to the sensory aspects of pain (e.g.: sensory intensity), and secondarily, to its affective dimension. Other pathways project directly to the anterior cingulate cortex, the insula, the amygdala and to the prefrontal cortices, which are structures involved in the affective dimension of pain (unpleasantness of pain and regulation of autonomic and behavioral responses). Interestingly, these latter regions are an integral part of the cerebral emotional networks.

PERSPECTIVES AND CONCLUSION

This close anatomical relationship between pain and emotions circuits could explain the powerful emotional impact of pain as well as the reciprocal modulatory effect of emotions on pain observed in clinical and experimental studies. More specifically, this modulatory effect might reflect interactions between emotional and nociceptive systems in the prefrontal and cingulate cortices, ventral striatum, amygdala and hippocampal regions. Taken together, these observations further attest to the emotional nature of pain experience.

Neuronal responses to a delayed-response delayed-reward go/nogo task in the monkey posterior insular cortex

Anatomical connections of the insular cortex suggest its involvement in cognition, emotion, memory, and behavioral manifestation. However, there have been few neurophysiological studies on the insular cortex in primates, in relation to such higher cognitive functions. In the present study, neural activity was recorded from the monkey insular cortex during performance of a delayed-response delayed-reward go/nogo task. In this task, visual stimuli indicating go or nogo responses associated with reward (reward trials) and with no reward (no-reward trials) were presented after eye fixation. In the reward trials, the monkey was required to release a button during presentation of the 2nd visual stimuli after a delay period (delay 1). Then, a juice reward was delivered after another delay (delay 2). The results indicated that the neurons responding in each epoch of the task were topographically localized within the insular cortex, consistent with the previous anatomical studies indicating topographical distributions of afferent inputs from other subcortical and cortical sensory areas. Furthermore, some insular neurons 1) nonspecifically responded to the visual cues and during fixation; 2) responded to the visual cues predicting reward and during the delay period before reward delivery; 3) responded differentially in go/nogo trials during the delay 2; and 4) responded around button manipulation. The observed patterns of insular-neuron responses and the correspondence of their topographical localization to those in previous anatomical studies suggest that the insular cortex is involved in attention- and reward-related functions and might monitor and integrate activities of other brain regions during cognition and behavioral manifestation.

Somatotopic organization of the processing of muscle and cutaneous pain in the left and right insula cortex: a single-trial fMRI study

The insula is involved in processing noxious information. It is consistently activated by acute noxious stimuli, can elicit pain on stimulation, and lesions encompassing the insula can alter pain perception. Anatomical tracing, electrophysiological and functional brain imaging investigations have suggested that the insula is somatotopically organized with respect to noxious cutaneous inputs. It has also recently been revealed that the anterior insula displays differential activation during cutaneous compared with muscle pain. Given this difference, it is important to determine if an insula somatotopy also exists for muscle pain. Using high-resolution functional magnetic resonance imaging (fMRI) we compared insula activation patterns in 23 subjects during muscle and cutaneous pain induced in the right leg and forearm. Group and frequency analyses revealed somatotopically organized signal increases in the posterior contralateral (left) and ipsilateral (right) anterior insula. Within the posterior contralateral insula, signal increases during both cutaneous and muscle forearm pain were located lateral and anterior to those evoked by leg pain, whereas in the ipsilateral anterior insula the pattern was reversed. Furthermore, within the ipsilateral anterior insula, muscle pain activated a region anterior to that activated by cutaneous pain. This somatotopic organization may be crucial for pain localization or other aspects of the pain experience that differ depending on both stimulation site and type of tissue activated. This study reveals that the insula is organized somatopically with respect to muscle and cutaneous pain and that this organization is further separated according to the tissue in which the pain originates.

Power spectrum analysis and cardiovascular morbidity in anxiety disorders

Spectral analysis of heart rate variability (HRV) and related measures has been shown to be a reliable noninvasive technique enabling quantitative assessment of cardiovascular autonomic regulatory responses to autonomic regulatory mechanisms; it provides a dynamic probe of sympathetic and parasympathetic tone, reflecting the interactions between the two. Over 20 studies reported abnormalities of HRV in anxiety, and patients with heart disease and anxiety are at increased risk for morbidity and mortality. Psychiatric drugs partly correct abnormalities of HRV and, recently, autonomic drugs (beta-blockers) have been studied in anxiety disorders. The authors call for further studies, especially in patients with co-existing anxiety disorders and heart disease, incorporating assessment of HRV.

A PET activation study of brush-evoked allodynia in patientswith nerve injury pain

Acute experimental brush-evoked allodynia induces a cortical activation pattern that differs from that typically seen during experimental nociceptive pain. In this study, we used positron emission tomography to measure changes in regional cerebral blood flow (rCBF) in patients with clinical allodynia. Nine patients with peripheral nerve injury were scanned during rest, brush-evoked allodynia, and brushing of normal contralateral skin. PET data were analyzed for the whole group and for single subjects. Allodynic stimulation activated the contralateral orbitofrontal cortex (BA 11) in every patient. Whereas normal brushing activated most strongly the contralateral insular cortex, allodynic brushing produced an ipsilateral activation in this area. Another important difference between normal and allodynic brushing was the absence of a contralateral primary somatosensory cortex (SI) activation during allodynic brushing. No thalamic activation was observed during allodynic or control brushing. Although no anterior cingulate cortex (ACC) activation could be demonstrated in the group analysis, single subject analysis revealed that four patients activated this region during brush-evoked allodynia. A direct post hoc comparison of brush -and allodynia-induced rCBF changes showed that allodynia was associated with significantly stronger activations in orbitofrontal cortex and ipsilateral insula whereas non-painful brushing more strongly activated SI and BA 5/7. These findings indicate that activity in the cortical network involved in the sensory-discriminative processing of nociceptive pain is downregulated in neuropathic pain. Instead, there is an upregulation of activity in the orbitofrontal and insular cortices, which is probably due to the stronger emotional load of neuropathic pain and higher computational demands of processing a mixed sensation of brush and pain.

Cortical regions associated with autonomic cardiovascular regulation during lower body negative pressure in humans

The purpose of the present study was to determine the cortical structures involved with integrated baroreceptor-mediated modulation of autonomic cardiovascular function in conscious humans independent of changes in arterial blood pressure. We assessed the brain regions associated with lower body negative pressure (LBNP)-induced baroreflex control using functional magnetic resonance imaging with blood oxygen level-dependent (BOLD) contrast in eight healthy male volunteer subjects. The levels of LBNP administered were 5, 15 and 35 mmHg. Heart rate (HR; representing the cardiovascular response) and LBNP (representing the baroreceptor activation level) were simultaneously monitored during the scanning period. In addition, estimated central venous pressure (CVP), arterial blood pressure (ABP) and muscle sympathetic nerve activity were recorded on a separate session. Random effects analyses (SPM2) were used to evaluate significant (P < 0.05) BOLD signal changes that correlated separately with both LBNP and HR (15- and 35-mmHg versus 5-mmHg LBNP). Compared to baseline, steady-state LBNP at 15 and 35 mmHg decreased CVP (from 7 +/- 1 to 5 +/- 1 and 4 +/- 1 mmHg, respectively) and increased MSNA (from 12 +/- 1 to 23 +/- 3 and 36 +/- 4 bursts min(-1), respectively, both P < 0.05 versus baseline). Furthermore, steady-state LBNP elevated HR from 54 +/- 2 beats min(-1) at baseline to 64 +/- 2 beats min(-1) at 35-mmHg suction. Both mean arterial and pulse pressure were not different between rest and any level of LBNP. Cortical regions demonstrating increased activity that correlated with higher HR and greater LBNP included the right superior posterior insula, frontoparietal cortex and the left cerebellum. Conversely, using the identical statistical paradigm, bilateral anterior insular cortices, the right anterior cingulate, orbitofrontal cortex, amygdala, midbrain and mediodorsal nucleus of the thalamus showed decreased neural activation. These data corroborate previous investigations highlighting the involved roles of the insula, anterior cingulate cortex and amygdala in central autonomic cardiovascular control. In addition, we have provided the first evidence for the identification of the cortical network involved specifically with baroreflex-mediated autonomic cardiovascular function in conscious humans.

Neural-cardiac coupling in threat-evoked anxiety

Anxiety is a debilitating symptom of many psychiatric disorders including generalized anxiety disorder, mood disorders, schizophrenia, and autism. Anxiety involves changes in both central and peripheral biology, yet extant functional imaging studies have focused exclusively on the brain. Here we show, using functional brain and cardiac imaging in sequential brain and cardiac magnetic resonance imaging (MRI) sessions in response to cues that predict either threat (a possible shock) or safety (no possibility of shock), that MR signal change in the amygdala and the prefrontal and insula cortices predicts cardiac contractility to the threat of shock. Participants with greater MR signal change in these regions show increased cardiac contractility to the threat versus safety condition, a measure of the sympathetic nervous system contribution to the myocardium. These findings demonstrate robust neural-cardiac coupling during induced anxiety and indicate that individuals with greater activation in brain regions identified with aversive emotion show larger magnitude cardiac contractility increases to threat.

Neuroanatomic deficits in congenital central hypoventilation syndrome

Congenital Central Hypoventilation Syndrome (CCHS) patients exhibit compromised autonomic regulation, reduced breathing drive during sleep, diminished ventilatory responses to chemoreceptor stimulation, and diminished air hunger perception. The syndrome provides an opportunity to partition neural processes regulating breathing and cardiovascular action. No obvious lesions appear with conventional magnetic resonance imaging; however, T2 relaxometry procedures can detect reduced cell or fiber density or diminished myelination not found with routine evaluation. High-resolution T1, proton density, and T2-weighted brain images were collected from 12 patients and 28 age- and gender-matched controls. Voxel-by-voxel T2 maps were generated from the proton density and T2-weighted images and evaluated by voxel-based-relaxometry procedures. Normalized and smoothed T2 maps were compared between groups using analysis of covariance at each voxel, with age and ventricle size included as covariates. Patients showed damaged or maldeveloped tissue, principally right-sided, including white matter from the level of the anterior cingulate cortex caudally to the level of the posterior cingulate and laterally to the posterior superior temporal cortex. Portions of the posterior, mid, and anterior cingulate, as well as the internal capsule, putamen, and globus pallidus and basal forebrain extending to the anterior and medial thalamus were affected. Deficits in the cingulum bundle and mid-hippocampus and ventral prefrontal cortex appeared, as well as the right cerebellar cortex and deep nuclei. Neuroanatomic deficiencies in limbic structures suggest a structural basis for reduced air hunger perception, thermoregulatory and autonomic deficiencies in the syndrome, while cerebellar deficits may also contribute to breathing and cardiovascular dysregulation.

Electrophysiological studies on human pain perception

OBJECTIVE

We reviewed the recent progress in electrophysiological studies using electroencephalography (EEG), magnetoencephalography (MEG) and repetitive transcranial magnetic stimulation (rTMS) on human pain perception.

METHODS

For recording activities following A delta fiber stimulation relating to first pain, several kinds of lasers such as CO2, Tm:YAG and argon lasers are now widely used. The activity is frequently termed laser evoked potential (LEP), and we reviewed previous basic and clinical reports on LEP. We also introduced our new method, epidermal stimulation (ES), which is useful for recording brain activities by the signals ascending through A delta fibers. For recording activities following C fiber stimulation relating to second pain, several methods have been used but weak CO2 laser stimuli applied to tiny areas of the skin were recently used.

RESULTS

EEG and MEG findings following C fiber stimulation were similar to those following A delta fiber stimulation except for a longer latency. Finally, we reviewed the effect of rTMS on acute pain perception. rTMS alleviated acute pain induced by intracutaneous injection of capsaicin, which activated C fibers, but it enhanced acute pain induced by laser stimulation, which activated A delta fibers.

CONCLUSIONS

One promising approach in the near future is to analyze the change of a frequency band. This method will probably be used for evaluation of continuous tonic pain such as cancer pain, which evoked response studies cannot evaluate.

Kortikale Repr�sentation von Schmerz

Contrary to the traditional view that the cerebral cortex is not involved in pain perception an extensive cortical network associated with pain processing has been revealed during the past decades. This network consistently includes the primary (S1) and secondary somatosensory cortices (S2), the insular cortex, and the anterior cingulate cortex (ACC). These cortical areas are organized in parallel and contribute to different dimensions of pain experience. The S1 cortex is mainly involved in discriminative aspects of pain, while the S2 cortex seems to have an important role in cognitive aspects of pain perception. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in pain-related learning and memory. The ACC is closely related to pain affect and may subserve the integration of general affect, cognition, and response selection. Furthermore, first pain appears to be particularly related to activation of S1 whereas second pain is closely related to ACC activation.

Neurons in and near insular cortex are responsive to muscular contraction and have sympathetic and/or cardiac-related discharge

Insular cortex (IC) is recognized as a potential site for "central command" of cardiorespiratory responses during exercise. Muscular contraction (MC) decreased the discharge rate of most IC neurons. Activity of most contraction sensitive neurons was either not altered by elevating blood pressure or showed a response converse to that of MC. IC may thus have a role in central command but the area is clearly modulated by MC.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

A comparative magnetoencephalographic study of cortical activations evoked by noxious and innocuous somatosensory stimulations

We recorded somatosensory-evoked magnetic fields and potentials produced by painful intra-epidermal stimulation (ES) and non-painful transcutaneous electrical stimulation (TS) applied to the left hand in 12 healthy volunteers to compare cortical responses to noxious and innocuous somatosensory stimulations. Our results revealed that cortical processing following noxious and innocuous stimulations was strikingly similar except that the former was delayed approximately 60 ms relative to the latter, which was well explained by a difference in peripheral conduction velocity mediating noxious (Adelta fiber) and innocuous (Abeta fiber) inputs. The first cortical activity evoked by both ES and TS was in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side. The following activities were in the bilateral secondary somatosensory cortex (SII), insular cortex, cingulate cortex, anterior medial temporal area and ipsilateral SI. The source locations did not differ between the two stimulus modalities except that the dipole for insular activity following ES was located more anterior to that following TS. Both ES and TS evoked vertex potentials consisting of a negativity followed by a positivity at a latency of 202 and 304 ms, and 134 and 243 ms, respectively. The time course of the vertex potential corresponded to that of the activity of the medial temporal area. Our results suggested that cortical processing was similar between noxious and innocuous stimulation in SI and SII, but different in insular cortex. Our data also implied that activities in the amygdala/hippocampal formation represented common effects of noxious and tactile stimulations.

Increased cellular activity in rat insular cortex after water and salt ingestion induced by fluid depletion

Insular cortex (IC) receives inputs from multiple sensory systems, including taste, and from receptors that monitor body electrolyte and fluid balance and blood pressure. This work analyzed metabolic activity of IC cells after water and sodium ingestion induced by sodium depletion. Rats were injected with the diuretic furosemide (10 mg/kg body wt), followed 5 min later by injections of the angiotensin-converting enzyme inhibitor captopril (5 mg/kg body wt). After 90 min, some rats received water and 0.3 M NaCl to drink for 2 h while others did not. A third group had access to water and saline but was not depleted of fluids. All rats were killed for processing of brain tissue for Fos-immunoreactivity (Fos-ir). Nondepleted animals had weak-to-moderate levels of Fos-ir within subregions of IC. Fluid-depleted rats without fluid access had significantly increased Fos-ir in all areas of IC. Levels of Fos-ir were highest in fluid-depleted rats that drank water and sodium. Fos-ir levels were highest in anterior regions of IC and lowest in posterior regions of IC. These results implicate visceral, taste, and/or postingestional factors in the increased metabolic activity of cells in IC.

Impaired heart rate variability in euthymic bipolar patients

BACKGROUND

Power spectral analysis of heart rate variability (HRV) has recently been shown to be a reliable non-invasive test for quantitative assessment of the central sympathovagal interaction that modulates cardiovascular autonomic function.

METHODS

We studied 39 euthymic bipolar patients and 39 controls, matched for age and sex. A high-resolution electrocardiogram was obtained during complete rest. Spectral analysis of R-R intervals was performed by the fast Fourier transform algorithm.

RESULTS

Euthymic bipolar patients at rest are characterized by markedly low HRV, independent of specific drug treatments.

CONCLUSION

While these results of autonomic stability in euthymic bipolar patients are counter-intuitive, the findings may be important in understanding pathophysiology of bipolar affective illness.

Brain mechanisms of pain affect and pain modulation

Recent animal studies reveal ascending nociceptive and descending modulatory pathways that may contribute to the affective-motivational aspects of pain and play a critical role in the modulation of pain. In humans, a reliable pattern of cerebral activity occurs during the subjective experience of pain. Activity within the anterior cingulate cortex and possibly in other classical limbic structures, appears to be closely related to the subjective experience of pain unpleasantness and may reflect the regulation of endogenous mechanisms of pain modulation.

Neurophysiology and functional neuroanatomy of pain perception

The traditional view that the cerebral cortex is not involved in pain processing has been abandoned during the past decades based on anatomic and physiologic investigations in animals, and lesion, functional neuroimaging, and neurophysiologic studies in humans. These studies have revealed an extensive central network associated with nociception that consistently includes the thalamus, the primary (SI) and secondary (SII) somatosensory cortices, the insula, and the anterior cingulate cortex (ACC). Anatomic and electrophysiologic data show that these cortical regions receive direct nociceptive thalamic input. From the results of human studies there is growing evidence that these different cortical structures contribute to different dimensions of pain experience. The SI cortex appears to be mainly involved in sensory-discriminative aspects of pain. The SII cortex seems to have an important role in recognition, learning, and memory of painful events. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in affective aspects of pain-related learning and memory. The ACC is closely related to pain unpleasantness and may subserve the integration of general affect, cognition, and response selection. The authors review the evidence on which the proposed relationship between cortical areas, pain-related neural activations, and components of pain perception is based.

Functional imaging of brain responses to pain. A review and meta-analysis (2000)

Brain responses to pain, assessed through positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are reviewed. Functional activation of brain regions are thought to be reflected by increases in the regional cerebral blood flow (rCBF) in PET studies, and in the blood oxygen level dependent (BOLD) signal in fMRI. rCBF increases to noxious stimuli are almost constantly observed in second somatic (SII) and insular regions, and in the anterior cingulate cortex (ACC), and with slightly less consistency in the contralateral thalamus and the primary somatic area (SI). Activation of the lateral thalamus, SI, SII and insula are thought to be related to the sensory-discriminative aspects of pain processing. SI is activated in roughly half of the studies, and the probability of obtaining SI activation appears related to the total amount of body surface stimulated (spatial summation) and probably also by temporal summation and attention to the stimulus. In a number of studies, the thalamic response was bilateral, probably reflecting generalised arousal in reaction to pain. ACC does not seem to be involved in coding stimulus intensity or location but appears to participate in both the affective and attentional concomitants of pain sensation, as well as in response selection. ACC subdivisions activated by painful stimuli partially overlap those activated in orienting and target detection tasks, but are distinct from those activated in tests involving sustained attention (Stroop, etc.). In addition to ACC, increased blood flow in the posterior parietal and prefrontal cortices is thought to reflect attentional and memory networks activated by noxious stimulation. Less noted but frequent activation concerns motor-related areas such as the striatum, cerebellum and supplementary motor area, as well as regions involved in pain control such as the periaqueductal grey. In patients, chronic spontaneous pain is associated with decreased resting rCBF in contralateral thalamus, which may be reverted by analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. It is argued that imaging studies of allodynia should be encouraged in order to understand central reorganisations leading to abnormal cortical pain processing. A number of brain areas activated by acute pain, particularly the thalamus and anterior cingulate, also show increases in rCBF during analgesic procedures. Taken together, these data suggest that hemodynamic responses to pain reflect simultaneously the sensory, cognitive and affective dimensions of pain, and that the same structure may both respond to pain and participate in pain control. The precise biochemical nature of these mechanisms remains to be investigated.

Electrophysiological evidence for reciprocal insulo-insular connectivity of baroreceptor-related neurons

The recent indications of specialized lateralization of cardiovascular regulation within the right and left posterior insular cortex of the rat, suggest the possibility of transcallosal connectivity between these regions. This has not been previously demonstrated using physiological techniques. Extracellular neural recordings in 34 urethane anesthetized male Sprague-Dawley rats demonstrated reciprocal interinsular antidromic and orthodromic activation, elicited with similar median onset latencies (18 ms). The corresponding conduction velocity of these fibers (0.6 m/s) suggests that they may be unmyelinated. Many of the cells showing interhemispheric connectivity also responded to baroreceptor activation, further emphasizing the connectivity pattern in baroreceptor-related units. Both 1 and 25 Hz microstimulation of the contralateral insula indicated that the most frequent orthodromic response was inhibitory, either alone or as part of a biphasic pattern including activation. Chemical stimulation of the insula using L-glutamate was associated with both excitatory and inhibitory orthodromic activation of the contralateral posterior insula, confirming that the orthodromic electrical stimulation was not solely due to activation of fibers of passage. These data suggest that the two insulae may communicate with each other to integrate and balance cardiovascular function between hemispheres.

Baroreceptive and somatosensory convergent thalamic neurons project to the posterior insular cortex in the rat

Connectivity between the rat posterior insula and the ventrobasal thalamus has been demonstrated anatomically. Neurons convergent for baroreceptor and nociceptive input have also been identified in the homologous anterior insula of the primate. Whether similar convergent cells exist in the ventrobasal thalamus was investigated in 30 urethane anesthetized male Sprague--Dawley rats. Six classes of cells were identified in the right ventrobasal thalamus: (a) 83/159 (52%) baroreceptive and nociceptive convergent units; (b) 2/159 (1%) convergent cells responding to baroreceptor activation and light touch; (c) 44/159 (28%) purely nociceptive units; (d)10/159 (6%) purely baroreceptive units; (e) 1/159 (0.6%) cells responding to brush alone and (f) 19/159 (12%) unresponsive units. Of the viscerosomatic convergent cells, 66/85 (78%) were situated in the ventroposterolateral nucleus (VPL), 6/85 (7%) in the ventroposterolateral parvicellular nucleus (VPLpc), and 13/85 (15%) in the ventroposteromedial nucleus (VPM). Fifteen right ventrobasal thalamic units were antidromically activated and 34 units orthodromically activated by right posterior insular microstimulation. Cobalt injection into the right ventrobasal thalamus blocked the right insular response to baroreceptor activation by >70%. These data indicate: (a) baroreceptive and somatosensory nociceptive convergent units exist in the ventrobasal thalamus; (b) thalamic convergent neurons project directly to the ipsilateral posterior insula and receive reciprocal insulothalamic projections; and (c) a significant proportion of baroreceptor input relays to the posterior insula through the ipsilateral ventrobasal thalamus.