The gustatory pathway

1. The gustatory pathway

2. Taste Receptor Cells • Arise from epithelial cells and are located on taste buds in the papillae of the tongue • Detect different taste qualities each with a unique chemosensory mechanism • Salty • Sour • Bitter • Sweet • Umami • Innervated by the primary afferent fibers of cranial nerves VII, IX, and X

3. Taste buds on the anterior tongue and palate innervated by the intermediate nerve (VII) • Posterior tongue and pharynx innervated by glossopharyngeal nerve (IX) • Epiglottis and larynx innervated by vagus nerve (X) • The afferent fibers of these cranial nerves synapse with many taste cells between single or multiple taste buds • Intermediate nerve afferents enter the brain stem at the pontomedullary junction • Glossopharyngeal and vagus nerve afferents enter the brain stem in the rostral medulla Cranial Nerves

4. Ascending Gustatory Pathway • The afferent fibers of the cranial nerves collect in the rostral solitary nucleus • Axons ascend ipsilaterally in the central tegmental tract of the brain stem and terminate in the parvocellular division of the ventral posterior medial nucleus of the thalamus • From the thalamus, neurons project to the insular cortex, the posterior limb of the internal capsule, and the operculum (primary gustatory areas)

5. The insular cortex projects to the orbitofrontal cortex • Both cortices are part of the limbic system • The limbic system is responsible for the behavioral and emotional significance of taste

6. The gustatory cortex is located completely beneath the cortical surface. PET scan – Cortical activation after presentation of 5% sucrose solution

7. The Gustatory System is Complex • Much about the system is still unknown • Labeled Line Model – suggests that different tastes have segregated pathways to the brain • Across Fiber Theory – different tastes are represented by different activity across a neural population