Cortical processing of human somatic and visceral sensation

Gastric distention correlates with activation of multiple cortical and subcortical regions

BACKGROUND & AIMS

The pathophysiology of functional dyspepsia may involve abnormal processing of visceral stimuli at the level of the central nervous system. There is accumulating evidence that visceral and somatic pain processing in the brain share common neuronal substrates. However, the cerebral loci that process sensory information from the stomach are unknown. The aim of this study was to localize the human brain regions that are activated by gastric distention.

METHODS

Brain (15)O-water positron emission tomography was performed in 15 right-handed healthy volunteers during baseline and distal gastric distentions to 10 mm Hg, 20 mm Hg, threshold pain, and moderate pain. Pain, nausea, and bloating were rated during baseline and distentions (0-5 scale). Statistical subtraction analysis of brain images was performed between distentions and baseline.

RESULTS

Symptoms increased with distending stimulus intensity (maximum pain, 2.1 +/- 0.4; nausea, 2.2 +/- 0.4; bloating, 3.7 +/- 0.2). Paralleling increases in distention stimulus and symptoms, progressive increases in activation (P < or = 0.05), were observed in the thalami, insula bilaterally, anterior cingulate cortex, caudate nuclei, brain stem periaqueductal gray matter, cerebellum, and occipital cortex.

CONCLUSIONS

Symptomatic gastric distention activates structures implicated in somatic pain processing, supporting the notion of a common cerebral pain network.

A study of the cortical processing of ano-rectal sensation using functional MRI

Investigation of human ano-rectal physiology has concentrated largely on understanding the motor control of defecation and continence mechanisms. However, little is known of the physiology of ano-rectal sensation. There are important differences in the afferent innervation and sensory perception between the rectum and anal canal. This suggests that there could also be differences in the brain's processing of sensation from these two areas; however, this possibility remains unexplored. The aim of our study was to identify the cerebral areas processing anal (somatic) and rectal (visceral) sensation in healthy adults, using functional MRI. Eight male subjects with an age range of 21-39 years were studied on two separate occasions, one for rectal and the other for anal stimulation studies. Single shot gradient echo planar imaging was performed using a 1.5 tesla Phillips MRI scanner. For each subject, a series of 40 image sets containing 24 slices of the brain was obtained during periods of rapid phasic non-painful distension of the rectum or anal canal, alternating with rest periods, without stimulation. After motion correction, the images were analysed using cross correlation to identify the cerebral areas activated by the stimulus. Rectal stimulation resulted in bilateral activation of the inferior primary somatosensory, secondary somatosensory, sensory association, insular, peri-orbital, anterior cingulate and prefrontal cortices. Anal canal stimulation resulted in activation of areas similar to rectal stimulation, but the primary somatosensory cortex was activated at a more superior level, and there was no anterior cingulate activation. In conclusion, anal and rectal sensation resulted in a similar pattern of cortical activation, including areas involved with spatial discrimination, attention and affect. The differences in sensory perception from these two regions can be explained by their different representation in the primary somatosensory cortex. The anterior cingulate cortex was only activated by rectal stimulation, suggesting that the viscera have a greater representation on the limbic cortex than somatic structures, and this explains the greater autonomic responses evoked by visceral sensation in comparison with somatic sensation.

Brain responses to changes in bladder volume and urge to void in healthy men

Knowledge of how changes in bladder volume and the urge to void affect brain activity is important for understanding brain mechanisms that control urinary continence and micturition. This study used PET to evaluate brain activity associated with different levels of passive bladder filling and the urge to void. Eleven healthy male subjects (three left- and eight right-handed) aged 19-54 years were catheterized and the bladder filled retrogradely per urethra. Twelve PET scans were obtained during two repetitions of each of six bladder volumes, with the subjects rating their perception of urge to void prior to and after each scan. Increased brain activity related to increasing bladder volume was seen in the periaqueductal grey matter (PAG), in the midline pons, in the mid-cingulate cortex and bilaterally in the frontal lobe area. Increased brain activity relating to decreased urge to void was seen in a different portion of the cingulate cortex, in premotor cortex and in the hypothalamus. Both activation patterns were predominantly bilaterally symmetric and none of the effects could be attributed to the presence of the catheter. However, in some subjects, mostly those reporting intrusive sensations from the urethral catheter, there was a discrepancy between filling volume and urge so that they reported high urge with low volumes. As this 'mismatch' decreased, activation increased bilaterally in the somatosensory cortex. Our findings support the hypothesis that the PAG receives information about bladder fullness and relays this information to areas involved in the control of bladder storage. Our results also show that the network of brain regions involved in modulating the perception of the urge to void is distinct from that associated with the appreciation of bladder fullness.

Functional neuroimaging of visceral sensation

The use of functional brain imaging techniques has led to considerable advances in our understanding of brain processing of human visceral sensation. The use of complementary techniques such as functional MRI, positron emission tomography, magnetoencephalography, and EEG has led to the identification of a network of brain areas that process visceral sensation. These studies suggest that unlike somatic sensation, which has an intense homuncular representation in the primary somatosensory cortex (SI), visceral sensation is primarily represented in the secondary somatosensory cortex, whereas representation in SI is vague. This difference could account for the poor localization of visceral sensation in comparison with somatic sensation. However, in a manner similar to that of somatic sensation, visceral sensation is represented in the paralimbic and limbic structures such as the insular, anterior cingulate, and prefrontal cortices. These areas are likely to mediate the affective and cognitive components of visceral sensation. Recent studies suggest that negative emotional factors such as fear, and cognitive factors such as attention can modulate the brain processing of visceral sensation in the insular and anterior cingulate cortices. In addition, alterations in the pattern of cortical processing of visceral sensation have been described in patients with functional gastrointestinal pain. It is likely that future research into the factors that modulate the brain processing of visceral sensation in health and disease are likely to improve further our understanding of the pathophysiology of functional visceral pain disorders.