Chapter 030. Disorders of Smell, Taste, and Hearing (Part 1)

Harrisons Internal Medicine Chapter 30. Disorders of Smell, Taste, and HearingSmellThe sense of smell determines the flavor and palatability of food and drink and serves, along with the trigeminal system, as a monitor of inhaled chemicals, including dangerous substances such as natural gas, smoke, and air pollutants. Olfactory dysfunction affects ~1% of people under age 60 and more than half of the population beyond this age.DefinitionsSmell is the perception of odor by the nose. Taste is the perception of salty, sweet, sour, or bitter by the tongue. Related sensations during eating such as somatic sensations of coolness, warmth,...
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Chapter 030. Disorders of Smell, Taste, and Hearing (Part 1) Chapter 030. Disorders of Smell, Taste, and Hearing (Part 1) Harrisons Internal Medicine > Chapter 30. Disorders of Smell, Taste,and Hearing Smell The sense of smell determines the flavor and palatability of food and drinkand serves, along with the trigeminal system, as a monitor of inhaled chemicals,including dangerous substances such as natural gas, smoke, and air pollutants.Olfactory dysfunction affects ~1% of people under age 60 and more than half ofthe population beyond this age. Definitions Smell is the perception of odor by the nose. Taste is the perception of salty,sweet, sour, or bitter by the tongue. Related sensations during eating such assomatic sensations of coolness, warmth, and irritation are mediated through thetrigeminal, glossopharyngeal, and vagal afferents in the nose, oral cavity, tongue,pharynx, and larynx. Flavor is the complex interaction of taste, smell, and somaticsensation. Terms relating to disorders of smell include anosmia, an absence of theability to smell; hyposmia, a decreased ability to smell; hyperosmia, an increasedsensitivity to an odorant; dysosmia, distortion in the perception of an odor;phantosmia, perception of an odorant where none is present; and agnosia, inabilityto classify, contrast, or identify odor sensations verbally, even though the ability todistinguish between odorants or to recognize them may be normal. An odorstimulus is referred to as an odorant. Each category of smell dysfunction can befurther subclassified as total (applying to all odorants) or partial (dysfunction ofonly select odorants). Physiology of Smell The olfactory epithelium is located in the superior part of the nasal cavitiesand is highly variable in its distribution between individuals. Over time theolfactory epithelium loses its homogeneity, as small areas undergo metaplasiaproducing islands of respiratory-like epithelium. This process is thought to besecondary to insults from environmental toxins, bacteria, and viruses. The primarysensory neuron in the olfactory epithelium is the bipolar cell. The dendriticprocess of the bipolar cell has a bulb-shaped vesicle that projects into the mucouslayer and bears six to eight cilia containing odorant receptors. On average, eachbipolar cell elaborates 56 cm2 (9 in.2) of surface area to receive olfactory stimuli.These primary sensory neurons are unique among sensory systems in that they areshort-lived, regularly replaced, and regenerate and establish new centralconnections after injury. Basal stem cells, located on the basal surface of theolfactory epithelium, are the progenitors that differentiate into new bipolar cells(Fig. 30-1). Figure 30-1 Olfaction. Olfactory sensory neurons (bipolar cells) are embedded in asmall area of specialized epithelium in the dorsal posterior recess of the nasalcavity. These neurons project axons to the olfactory bulb of the brain, a smallovoid structure that rests on the cribriform plate of the ethmoid bone. Odorantsbind to specific receptors on olfactory cilia and initiate a cascade of actionpotential events that lead to the production of action potentials in the sensoryaxons. Between 50 and 200 unmyelinated axons of receptor cells form the fila ofthe olfactory nerve; they pass through the cribriform plate to terminate withinspherical masses of neuropil, termed glomeruli, in the olfactory bulb. Olfactoryensheathing cells, which have features resembling glia of both the central andperipheral nervous systems, surround the axons along their course. The glomeruliare the focus of a high degree of convergence of information, since many morefibers enter than leave them. The main second-order neurons are mitral cells. Theprimary dendrite of each mitral cell extends into a single glomerulus. Axons of themitral cells project along with the axons of adjacent tufted cells to the limbicsystem, including the anterior olfactory nucleus and the amygdala. Cognitiveawareness of smell requires stimulation of the prepiriform cortex or amygdaloidnuclei. A secondary site of olfactory chemosensation is located in the epitheliumof the vomeronasal organ, a tubular structure that opens on the ventral aspect ofthe nasal septum. In humans, this structure is rudimentary and nonfunctional,without central projections. Sensory neurons located in the vomeronasal organdetect pheromones, nonvolatile chemical signals that in lower mammals triggerinnate and stereotyped reproductive and social behaviors, as well asneuroendocrine changes. The sensation of smell begins with introduction of an odorant to the cilia ofthe bipolar neuron. Most odorants are hydrophobic; ...