154. Physiology of Taste – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Gustation is the visceral chemical sense responsible for taste. It helps assess food quality, regulate food intake, stimulate digestion, and protect against harmful substances. The five primary tastes are sweet, salty, sour, bitter, and umami.
• Taste receptors are present mainly in taste buds on the tongue. Fungiform papillae are found near the tip and anterior margins, circumvallate papillae form a V-shaped row before the sulcus terminalis, and foliate papillae lie on the posterolateral tongue. Filiform papillae are numerous but do not contain taste buds. Taste buds are also present in the palate, pharynx, epiglottis and larynx.
• A taste bud contains receptor, supporting and basal cells. Tastants enter through the taste pore and interact with microvilli of receptor cells. Gustatory receptor cells are modified epithelial cells rather than primary sensory neurons and are continuously replaced from basal stem cells.
• Taste innervation is important for examinations: the anterior two-thirds of the tongue is carried mainly by the chorda tympani of the facial nerve, the posterior one-third by the glossopharyngeal nerve, and taste from the epiglottis, pharynx and larynx by the vagus nerve.
• The taste pathway is: taste receptors → cranial nerves VII, IX and X → nucleus of the solitary tract → ventral posteromedial nucleus of thalamus → primary gustatory cortex in the anterior insula and frontal operculum. Higher centres, especially the orbitofrontal cortex, integrate taste with smell, texture and temperature to produce flavor.
• The traditional tongue map is incorrect. All regions containing taste buds can detect all five primary tastes, although sensitivity may vary slightly between regions.
• Gustatory transduction begins when dissolved tastants bind to receptors on taste-cell microvilli. This produces a graded receptor potential. Depolarization opens calcium channels, and increased intracellular calcium causes neurotransmitter release onto primary sensory nerve endings. Adenosine triphosphate is an important neurotransmitter released by many taste receptor cells.
• Salty taste is produced mainly by sodium ions entering receptor cells through epithelial sodium channels, causing depolarization. Sour taste is mainly produced by hydrogen ions, which reduce potassium permeability and depolarize the receptor cell.
• Umami is the savory taste produced mainly by glutamate and is mediated largely by T1R1–T1R3 receptors. Bitter substances activate T2R receptors and are particularly important for detecting potentially toxic substances. Sweet taste is mediated mainly by T1R2–T1R3 receptors and commonly signals energy-rich nutrients.
• Taste quality is encoded mainly by population coding. Individual receptor cells and sensory fibres may respond to several tastes but usually show greater sensitivity to one modality. The brain identifies the taste from the overall pattern of activity across many sensory neurons.
• Bitter substances generally have the lowest detection thresholds, providing early warning against harmful chemicals. Taste intensity is influenced by tastant concentration, area stimulated, temperature, duration of stimulation and interactions between different tastants. Continuous stimulation produces taste adaptation.
• Age, sex, saliva production, nutritional status, medications, tobacco use, culture and previous dietary experience can modify taste. Sodium deficiency may increase the preference for salty foods. Conditioned taste aversion can cause long-lasting avoidance of a food associated with previous illness.
• Ageusia is complete loss of taste, hypogeusia is reduced taste sensitivity, and dysgeusia is abnormal or distorted taste perception. Xerostomia, aging, infections, medications, nutritional deficiencies and disorders of the gustatory pathway may cause taste disturbances. Selective taste blindness, such as inability to taste phenylthiocarbamide, may be genetically determined.
• Taste is tested by applying different taste solutions separately to each side of the tongue, with rinsing between tests. Taste stimulation produces taste–salivary and cephalic digestive reflexes, increasing salivary, gastric and pancreatic secretions.