gustatory receptors and the neural pathway for gustation

Vallate

12, very large, circular; form inverted V shape at back of tongue, houses 100-300 taste buds.

Gustatory Receptor Cells

50 in each taste bud. Gustatory Hair (projects from gustatory receptor cell to external surface through taste pore). Taste pore (opening in taste bud). At base they synapse with dendrites of first-order neurons that form first part of gustatory pathway; dendrites of first-order neurons branch profusely and contact many gustatory receptor cells called taste buds.

Receptor Potential

Arises differently in different tastants; Salty: Na+ in salty food enter gustatory receptor cells via Na+ channels in plasma membrane; Na+ triggers depolarization of cells causing release of neurotransmitter. Sour: Hydrogen (H+) flow into gustatory receptor cells via H+ channels, deplarizes cell, triggering release of neurotransmitter; also influence opening and closing of other channels. Sweet, Umami, Bitter: Do not enter gustatory receptor cells; they bind to receptors on plasma membrane linked to G proteins; G proteins activate several second messengers inside gustatory receptor cells; this causes depolarization and release of neurotransmitter. Different tastes arise from activation of different groups of taste neurons; that is how the body can differentiate from one taste to the next.

Taste Thresholds and Adaptation

Bitter and sour substances have a low threshold (higher sensitivity); sweet and salty are a high threshold (less sensitivity) Adaptation to a specific taste can occur in 1-5 minutes of continuous stimulation; due to changes that occur in taste receptors, in olfactory receptors, in neurons of gustatory pathway of CNS.

Gustation

Chemical sense; five primary tastes (sour, sweet, bitter, salty, umami) Umami is savory taste; odors can pass upward into nasal cavity, can stimulate olfactory receptors; taste can stimulate the olfactory receptors 1000 times more strongly than gustatory receptors.

Tastants

Chemicals that stimulate gustatory receptor cells; is dissolved in saliva, then makes contact with plasma membrane of gustatory hairs (sites of taste transduction), then receptor potential is created that stimulates exocytosis of synaptic vesicles from gustatory receptor cells; liberated neurotransmitter triggers nerve impulse in first-order sensory neurons that synapse with gustatory receptor cells.

Papillae

Elevations on the tongue that have taste buds; increase surface area, rough texture for tongue (easier for food to be moved around mouth). Three types: Vallate (circumvallate) Papillae, Fungiform Papillae, Foliate Papillae.

Filiform Papillae

Entire surface of tongue; pointed, threadlike protrusions; make it easier for food to move around mouth; no tastebuds, only tactile receptors.

Foliate Papillae

Located in small trenches on lateral side of tongue, usually disappear in childhood.

Fugiform Papillae

Mushroom shaped elevations, scattered over entire area of tongue, contain five taste buds each.

Taste Buds

Receptors for sensation of taste located in taste buds, most in tongue, some in soft palate (posterior portion of mouth), pharynx (throat), epiglottis (cartilage lid over voice box); taste buds decline with age. Taste Bud: Oval body; three kinds of epithelial cells: Supporting cells, Gustatory receptor cells, Basal cells

Basal Cells

Stem cells found at periphery of taste bud near connective tissue layer, produce supporting cells that develop into gustatory receptor cells.

Supporting Cells

Surround the Gustatory receptor cells.

Gustatory Pathway

Three cranial nerves contain axons for first-order gustatory neurons that innervate taste buds; Facial Nerve (VII) innervates taste buds of anterior 2/3 of tongue; Glossopharyngeal Nerve innervates taste buds of posterior 1/3 of tongue; Vagus Nerve innervates taste buds of throat and epiglottis. 1. From taste buds nerve impulses propagate along cranial nerves to gustatory nucleus in medulla oblongata 2. some axons then carry taste signals to limbic system and hypothalamus, others to the thalamus 3. taste signals from thalamus travel to primary gustatory area in parietal lobe of cerebral cortex (conscious perception of taste).