The thalamus plays a major role as a site of convergence of somatic and visceral inputs. Visceral inputs into the thalamus were examined using electrical stimulation of visceral nerves (82,104-106), or natural stimulation of visceral organs (107-110). In monkeys, medial thalamus receives viscerosomatic input via thoracic STT neurons (111), whereas neurons in lateral thalamus are activated by input through the STT and the dorsal column (14). Lateral thalamic neurons can also be excited by colorectal distension or urinary bladder distension and by convergent input elicited by noxious stimulation of somatic receptive fields in proximal lower body regions (107). In fact, the majority of lateral thalamic somatosensory neurons in squirrel monkeys receive somatovisceral and viscerovisceral inputs from naturally stimulated visceral organs (112). In the rat, neurons in and near the thalamic ventrobasal complex respond to stimulation of different visceral organs, including the uterus, the cervix, the vagina, and the colon (12,14,113). Colorectal distension or colon inflammation excites neurons in the ventral posterolateral nucleus of thalamus (12,14,112,113) and in the medial thalamus at the level of the nucleus submedius (114).
On the other hand, the sensation of angina can be evoked by microstimulation in the region of the thalamic principal sensory nucleus (the ventrocaudal nucleus) in humans (115)—a nucleus that corresponds to the ventral posterior nucleus in the cat and the monkey (116,117). Microstimulation in the area ventral and posterior to the ventrocaudal thalamus in the human brain evoked visceral pain sensations and triggered in some cases pain "memories" (118). Electrical stimulation of the thalamic ventrobasal complex in animals inhibits viscerosensory processing in normal rats but facilitates visceral hypersensitivity in rats with neonatal colon pain (119). These observations coupled with an extensive repertoire of experimental data suggest that the thalamus, particularly the posterolateral nucleus, is involved in the processing of visceral information, including both noxious and innocuous visceral inputs.
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