Taste and Other Visceral Sensory Inputs to the Hypothalamus

Ascending axons from the nucleus of the solitary tract travel through the brainstem, and a large proportion of them synapse in the parabrachial nucleus. From there, axons continue on to the thalamus (for conscious appreciation of taste), amygdala (for taste associations), and hypothalamus (presumably for regulation of feeding). The inputs to the hypothalamus and amygdala are augmented by a smaller number of axons that reach these sites directly from the nucleus of the solitary tract. In primates, there is evidence that some axons from the taste portion of the nucleus of the solitary tract may reach the thalamus directly, without requiring a relay in the parabrachial nucleus. Taste neurons in the thalamus are located adjacent to the tongue somatosensory area, and they innervate the insular cortex, which is the primary taste cortex.

The posterior two thirds of the nucleus of the solitary tract receives inputs from other internal organs via the glossopharyngeal and vagus nerves. These terminate in a roughly topographic order, with gastrointestinal inputs in the middle part of the nucleus and cardiorespiratory in the caudal part. The nucleus of the solitary tract provides local inputs to cell groups in the medulla that control gastrointestinal functions, including gastric acid secretion and gut motility as well as cardiovascular and respiratory reflexes (e.g., the baroreceptor reflex that stabilizes blood pressure when moving from a lying to a standing position, and the increase in both respiratory rate and blood pressure when there is a high level of carbon dioxide in the blood).

Other axons from the posterior two thirds of the nucleus of the solitary tract terminate in the parabrachial nucleus. Parabrachial neurons then contact the visceral sensory thalamus, which, in turn, projects to the insular cortex, where sensations such as gastric fullness or air hunger reach conscious appreciation. Other parabrachial outputs are joined by smaller numbers of axons from the nucleus of the solitary tract itself in projecting to the amygdala, where they may be involved in visceral conditioned reflexes. Parabrachial inputs to the hypothalamus may play a role in a wide range of functions, from regulation of behaviors such as feeding and drinking to control of secretion of hormones such as vasopressin (during hypovolemia) and oxytocin (during emesis).

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