Gustation: the sense of taste

Taste buds:

- Taste buds: onion-shaped organs housing taste receptors - Gustatory cells: receptor cells detect tastants (live 7 to 9 days) - Supporting cells: sustain gustatory cells - Basal cells: neural stem cells that replace gustatory cells

Gustatory cells are

-Gustatory cells: neuroepithelial chemoreceptive cells of taste buds - Gustatory microvillus (taste hair) forms dendritic ending - Microvillus often extends through taste pore, to tongue surface - Tastants (tasty molecules) dissolve in saliva and stimulate microvillus

• Transduction in gustatory cells

- For sweet, bitter, and umami the tastants are molecules o Tastant binds to specific cell membrane receptor o G protein is activated causing formation of 2nd messenger o Results in cell depolarization - For salty and sour the tastants are ions o The tastant depolarizes the cell directly - Depolarized gustatory cell releases neurotransmitter stimulating primary neuron (in CN VII or CN IX)

Gustation:

- Gustatory cells are chemoreceptors within taste buds - Papillae of tongue - Filiform papillae: short and spiked o No taste buds (no role in gustation); help manipulate food o Located on anterior two-thirds of tongue surface - *Fungiform papillae: mushroom-shaped o Each contains a few taste buds o Located on tip and sides of tongue -*Vallate (circumvallate) papillae: largest, least numerous o Contain most of the taste buds o Located in a row of 10-12 along posterior dorsal tongue surface. Raised edges surrounding a depression - *Foliate papillae: leaflike ridges o Not well developed o House a few taste buds in early childhood o Located on posterior lateral tongue

• Gustatory pathway

- Primary neuron in cranial nerve brings signal to Nucleus solitarius within medulla o Medullary activity triggers salivation and stomach secretions o Nauseating stimuli instead trigger gag or vomiting - Signal is relayed to thalamus - Then relayed to primary gustatory cortex in insula for conscious taste o Taste is integrated with temperature, texture, and especially smell o Food has less taste if olfaction is blocked (e.g., having a cold)

• Gustatory pathways

- Sensory (primary) neurons connect to multiple gustatory cells in the tongue and project to the medulla o In anterior parts of tongue, sensory neurons are part of the facial nerve (CN VII) o In posterior one third of tongue, sensory neurons are part of the glossopharyngeal nerve (CN IX) - Secondary medullary neurons project to thalamus - Tertiary thalamic neurons project to primary gustatory cortex in insula

• Five basic taste sensations spread over broad regions of the tongue

- Sweet o Produced by organic compounds, e.g., sugar or artificial sweeteners - Salty o Produced by metal ions, e.g., Na* and K+ - Sour o Associated with acids, e.g., vinegar - Bitter o Produced by alkaloids, e.g., unsweetened chocolate - Umami o Taste related to amino acids producing meaty flavor