Neural Coding in the rNST

Excerpt

The gustatory system in mammals provides sensory input that is critical for the regulation of ingestive behavior and the avoidance of toxic substances. Taste serves a unique role among sensory systems in the extent to which it interfaces with neural substrates of reward and motivation [1]. For example, sweet- and bitter-tasting stimuli produce inherent preference and avoidance, respectively, and sweet-tasting stimuli can serve as effective reinforcers. The anatomy of the taste system reflects its dual role as both a discriminative system, designed to determine subtle differences in taste quality and intensity, and a motivational one, which underlies the acceptance and rejection of potential foods. Anatomically, the taste system is situated between the external environment and the internal milieu, making taste a rostral extension of the visceral afferent system [2].

The rostral nucleus of the solitary tract (rNST) in the medulla lies at the interface between taste receptors and the brain. Gustatory neurons in the rNST receive input from the chorda tympani and greater superficial petrosal nerves, which are branches of cranial nerve VII, the glossopharyngeal nerve (IX), and the vagus nerve (X). The chorda tympani nerve innervates taste bud cells of the fungiform papillae on the anterior tongue, whereas the greater superficial petrosal nerve innervates those in palatal epithelia. The glossopharyngeal nerve relays taste information from the posterior tongue to the rNST. The vagus nerve innervates receptor cells found near the epiglottis. Gustatory neurons in the rNST serve the critical function of integrating and encoding information received from taste receptors and routing this information to higher centers involved with motivated behavior and perceptual processing. The NST also projects information to nearby structures in the brainstem that are involved in mediating oromotor responses. Many factors influence how rNST neurons respond to taste stimuli and also how information about tastants could be encoded in their spike outputs.