Chapter 54: The Chemical Senses—Taste and Smell Unit 10 Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Taste and Smell • Allows one to separate undesirable or lethal foods from those that are nutritious. • Recognize the proximity of other individuals or animals. • Tied to primitive emotional and behavioral functions of the nervous system. Because the tongue can only indicate texture and differentiate between sweet, sour, bitter, salty, and umami, most of what is perceived as the sense of taste is actually derived from smell. Temp Taste Is a Function of the Taste Bud • 13 possible chemical receptors in taste buds. – 2 for sodium – 2 for potassium – Chloride – Adenosine – Inosine – 2 sweet – 2 bitter – Glutamate – Hydrogen ion metal ions salty sweet NIH savory UMAMI sour Taste Perception • Sour – Caused by H + from acids (citric acid, acetic acid) – Threshold for citric acid: 2 mM • Salty – Caused by ionized salts, mainly sodium – Threshold for NaCl: 10 mM • Sweet – Many chemicals mostly organic compounds – Threshold for sucrose: 20 mM • Umami (Japanese, meaning delicious) – Glutamate – Threshold for MSG: < 10 mM • Bitter – Long chain organic substances containing nitrogen – Alkaloids (quinine, strychnine, nicotine, etc.) – Threshold for quinine: 0.008 mM – Threshold for strychnine: 0.0001 mM – ( survival mechanism since many poisons are bitter) Babies and young children are exceptionally sensitive to the bitter taste sensation. I teague very sensitive Location of Taste Buds • Found on three types of papillae of the tongue. – Circumvallate papillae form a V on posterior surface of tongue. ( 50% of taste buds) – Foliate papillae are located along lateral surfaces of tongue. ( 25% of taste buds) – Fungiform papillae located over flat surface of tongue. ( 25% of taste buds) • Extraglossal taste buds – On the tonsillar pillars, palate, epiglottis, and proximal esophagus. Filiform papillae are the most numerous of the lingual papillae . They are fine, small, cone-shaped papillae covering most of the dorsum of the tongue. They are responsible for giving the tongue its texture and are responsible for the sensation of TOUCH STIFF.ae adapt rapidly some central adaptation I Taste buds • Taste buds contain the receptors for taste. • Human tongue has 3,000–10,000 taste buds. • Average life of a taste bud is 10 days. • Salts, sweet, sour, and umami tastes causes depolarization of the respective taste cells (via different mechanisms). • Bitter causes an internal release of Ca2+, no external Ca2+ is required. • Bitter taste sensation is far more sensitive on molar basis compared to other taste sensations. Figure 54-1 Transmission of Taste Sensations • Activation of taste buds excite taste fibers. • Anterior 2/3 of tongue through facial nerve (VII) • Posterior 1/3 of tongue through glossopharyngeal nerve (IX) • Posterior aspects of the mouth through vagus nerve (X). • All signals transmitted to solitary nucleus • From solitary nucleus to thalamus. • From thalamus to cortex. Figure 54-2 Adaptation of Taste • Taste sensations adapt rapidly. • Adaptation of taste buds themselves accounts for only about 50% of the adaptation. • Central adaptation must occur but the mechanism for this is not known. Ageusia—The Loss of Taste Sensation Often confused with anosmia which is a loss of the sense of smell True ageusia is relatively rare compared to hypogeusia —a partial loss of taste—and dysgeusia —a distortion or alteration of taste. Causes • Neurological damage: Neurological disorders such as Bell’s palsy, Familial dysautonomia, and Multiple sclerosis • Problems with the endocrine system: Cushing's syndrome, hypothyroidism and diabetes mellitus • Medicinal side-effects : antirheumatic drugs, ACE inhibitors, and others • Other causes: Local damage and inflammation, radiation therapy, tobacco use, and denture use. Other known causes include loss of taste sensitivity from aging, anxiety disorder, cancer, renal failure and liver failure. Smell • Least understood of all senses. • Poorly developed in humans. • Olfactory membrane located on the superior part of each nostril. • Contains olfactory cells which contain cilia. • On the cilia are odorant-binding protein receptors. • Binding of chemical odorant to receptor induces the G-protein transduced formation of cAMP which opens sodium channels. The Olfactory Membrane and Bulb Figure 54-3 Odorant-binding receptors are located on cilia Binding induces G-protein activation of adenyl cyclase, which causes formation of cAMP. The cAMP opens sodium channels causing depolarization. Figure 54-4 From Odorant to Action Potential What Substances Can Be Smelled? • Substance must be volatile so that it can be sniffed into nostrils. • Substance must be at least slightly water soluble to penetrate the mucus to reach the olfactory cells. • Substance must be at least slightly lipid soluble to interact with the membrane. • Olfactory receptors adapt very slowly. But olfactory sensation itself adapts rather rapidly. Hence, must involve a central mechanism. Primary Sensations of Smell • Camphoraceous • Musky • Floral • Pepperminty • Ethereal • Pungent • Putrid These are based on psychological testing. It is certain that the list does not represent true primary sensations of smell. Transmission of Smell Sensation to CNS Olfactory nerve is the first cranial nerve Medial (old) and lateral (new) olfactory area Figure 54-5