Chapter 9: Smell & Taste

The section on olfaction details the anatomy of the olfactory epithelium, which includes bipolar olfactory sensory neurons, supporting cells, and regenerative basal stem cells. It explains the molecular physiology of signal transduction, where odorants bind to specific G-protein-coupled receptors (GPCRs) on the cilia of receptor cells. This binding triggers the dissociation of the G-protein alpha subunit, activating adenylyl cyclase to produce cyclic AMP (cAMP), which subsequently opens cation channels to allow the influx of sodium and calcium ions, leading to depolarization and action potentials. The text traces the neural pathway from the olfactory nerves through the cribriform plate to the olfactory bulb, where axons synapse with mitral and tufted cells within glomeruli. It highlights the role of lateral inhibition by periglomerular and granule cells in refining odor discrimination. The summary further maps the projections to the olfactory cortex structures—including the anterior olfactory nucleus, piriform cortex, amygdala, and entorhinal cortex—and finally to the orbitofrontal cortex for conscious perception. The discussion then shifts to gustation, describing the structure of taste buds located on fungiform, circumvallate, and foliate papillae, and their innervation by the facial, glossopharyngeal, and vagus nerves. Detailed attention is given to the transduction mechanisms for the five primary taste modalities: salt and sour utilize ion channels (such as epithelial sodium channels and proton-sensitive channels), while sweet, bitter, and umami activate GPCRs (T1R and T2R families) and second messenger systems involving inositol triphosphate and intracellular calcium release. The central pathway is outlined from the nucleus of the tractus solitarius (NTS) to the thalamus and the gustatory cortex in the insula. Finally, the chapter addresses clinical abnormalities such as anosmia, hyposmia, ageusia, and dysgeusia, examining their causes, ranging from trauma and infection to drug side effects and nutritional deficiencies.