Tongue

TASTE PATHWAYS

The chemosensitive cells found in the taste buds of the tongue, epiglottis, and larynx are innervated by three groups of sensory neurons.

Sensory Neurons. Taste cells synapse with sensory axons that run in the chorda tympani and greater superficial petrosal branches of the facial nerve, which carries taste sensation via the geniculate ganglion. The lingual branch of the glossopharyngeal nerve carries taste sensation via the petrosal (inferior) ganglion of the glossopharyngeal nerve. The superior laryngeal branch of the vagus nerve passes through the nodose (inferior) ganglion. All three groups of cells terminate in the medullary nucleus of the tract of solitarius.

Central Connections. From the nucleus of the tract of solitarius, second-order neurons project mostly ipsilateral (some fibers may cross over in the medial lemniscus) up the solitariothalamic bundle to the ipsilateral ventral posteromedial nucleus of the thalamus. Third-order neurons from the VPM nucleus pass through the posterior limb of the internal capsule to the taste region of the sensory cortex, located just below the face. The hypothalamic and amygdalar taste connections, on the other hand, appear to be primarily involved in reflex and motivational responses to taste stimuli, and thus control food intake.

The reflex-type brainstem connections between the taste nuclei and the autonomic nuclei for salivation (superior and inferior salivatory nuclei) mediate salivation reflexes that accompany taste responses to food stimuli on the tongue. “Gustation sweating,” facial and forehead sweating in response to eating spicy foods, is a normal response, although it can be pathologic if profuse.

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