Smell and Taste Physiology 2025 PDF

SMELL & TASTE Ng Sook Luan, PhD [email protected]  Learning Objectives 1. Describe the receptors and pathways for the sensation of smell and taste. 2. Describe some characteristics of the sensation of smell and taste. 3. Explain with an example, the role of smell in the...

SMELL & TASTE Ng Sook Luan, PhD [email protected]  Learning Objectives 1. Describe the receptors and pathways for the sensation of smell and taste. 2. Describe some characteristics of the sensation of smell and taste. 3. Explain with an example, the role of smell in the overall sensation produced by food. Olfaction (Smell) 7 primary classes of olfactory stimulants: camphoraceous (mothballs), musky (perfumes), floral (roses), peppermint (mint gum), ethereal (dry cleaning fluid), pungent (vinegar) & putrid (rotten egg) Sensitivity to odours declines steadily with ageing. Women generally have better olfactory abilities than men, the pattern of decline is similar. Degeneration of smell – loss of olfactory neurons and loss of cells from the olfactory bulbs → loss of olfactory sensitivity & odour identification → diminish appetite & food selection malnutrition in the elderly  Smell desensitisation (phenomenon of adaptation) due to continuous exposure to a particular odour → decreases the perception of odour Olfactory receptors adapt quite slow. Yet from our smell sensation adapt within a minute. Postulated mechanism: after the onset of olfactory stimulus → CNS gradually develops a strong feedback inhibition to suppress relaying smell signals through olfactory bulb  Olfactory cells 10 to 20 million olfactory receptors located in the olfactory epithelium (5cm2 in total area) Olfactory neurons are constantly replaced after a few weeks (2 months) by basal cells The mucus is produced by Bowman’s glands which are spaced among the olfactory cells. Each receptor's axon pierces the ethmoid bone's cribriform plate, enters the olfactory bulb, and terminates in the olfactory glomeruli. Cribriform plate Stimulation of Olfactory Cells Odorant molecules must first penetrate the olfactory mucus layer Bind to a receptor protein on the ciliary membrane Physical factors affect the degree of stimulation: 1. Only volatile substances can be sniffed into the nostrils 2. The odorant must be at least slightly water-soluble, so that able to dissolve in mucus to reach the receptors 3. The odorant must be at least slightly lipid-soluble to help bind to receptor proteins Olfactory Pathway 1. Odorant molecules bind to the ciliated olfactory receptors leading to the activation of second messenger systems, which involves a Golf Protein and cyclic AMP that triggers the opening of sodium channels. The inward current of sodium ions then depolarizes the receptors and the signals then travel to the brain. 2. The olfactory nerve (first-order afferent) passes through the sieve-like cribriform plate into the cranium. The first synaptic relay of the olfactory nerve is the ipsilateral olfactory bulb. 3. The second-order neurons (mitral cells) from the olfactory bulbs enter the olfactory tract and project to several sites at the base of the brain. The olfactory tract enters the brain at the anterior junction between the mesencephalon and cerebrum. 4. The nerves relay medially into the medial olfactory area. The medial olfactory pathways include projection to the olfactory tubercle, amygdala, orbitofrontal cortex, and hypothalamus. 5. The other passes laterally into the lateral olfactory area. The axons of the lateral olfactory pathway synapse in the primary olfactory cortex (piriform cortex). 6. The olfactory pathway is the only sensory system that does not have the obligatory synaptic relay in the thalamus before reaching the cortex. But, olfactory information does reach the mediodorsal thalamus and is then transmitted to the prefrontal and orbitofrontal cortex for conscious perception of odour.  Olfactory Discrimination Olfactory receptors are susceptible to methyl mercaptan (garlic smell) at a < 500 pg/L. Olfactory discrimination is specific Human can recognise ~ 10,000 different odours although olfactory cells only produce about 300 receptor proteins The brain uses hundreds of receptor in different combinations to create the perception of many different smells The ability to distinguish differences in smell intensity is poor. Odour molecules (3-20 carbons) with similar carbon atoms but different structural configurations have different odours. Substances with strong odours have high water and lipid solubility.  Olfactory Discrimination Olfactory discrimination varies in the following: 1. Sense of smell decreases when the mucosa is congested as in cold. 2. Smoking decreases olfactory sensitivity. 3. The ability to identify odours decreases with age. 4. Olfactory sensitivity is greater in hungry Irritant molecules (peppermint, subjects. menthol, chlorine, and ammonia) 5. Women have greater olfactory sensitivities stimulate naked nerve endings of than men. trigeminal nerve (CN V) pain fibres in the olfactory mucous membrane Nerve endings initiate sneezing, lacrimation, and other reflex response to nasal irritants. Gustation (Taste) Taste is closely linked to olfaction. ‘The taste of food is the aroma’ Taste is a major element in food selection and has an important protective role. Tongue’s taste map theory is no longer believed to be true.  Taste Receptors Taste buds - the sense organ for taste Taste buds are grouped in papillae 3 types of papillae: i. Fungiform papillae are mushroom-like and are located on the anterior two- thirds of the tongue (5 taste buds/fungiform papillae are abundant near the tip of tongue). ii. Foliate papillae are folded to form leaf- like structures on the posterior edge of the tongue. iii. Circumvallate papillae are large, round structures encircled by a groove, located on the posterior part of the tongue (~100 taste buds).  Taste Receptors Taste bud – 50 to 150 receptor cells, supporting cells, and regenerative basal cells in each taste bud The basal cells differentiate into new receptor cells, and the older receptor cells are continuously replaced about every 10 days Taste receptors are polarised epithelial cells tucked down into the epithelium so that only a tiny tip protrudes. The cell membrane that extends into the oral cavity is modified into microvilli to increase the surface area in contact with fluid in the mouth. The basal side of the cells forms a synapse with the primary sensory neuron. Each taste bud is innervated by about 50 nerve fibres, and each nerve fibre receives input from an average of 5 taste buds. Primary Taste Sensations Organic substances such as those containing nitrogen and alkaloids cause a bitter taste. By ↑ intracellular Ca2+ levels, leading to ↑ release of synaptic transmitter and activation of taste nerve fibre Acids cause a sour taste, ↑ intracellular H+ ion concentration, which blocks K+ channels Newly discovered category of taste, termed umami, a Japanese term for meaty flavour. The sweet taste is usually caused by organic chemicals (e.g. sugars), through the opening of Na+ channels and closing of K+ channels Salty taste is produced through the opening of Na+ channels Mechanism of Taste Sensations Central Gustatory Pathway) 1. Sensory nerve fibres (first order neurons) from the taste buds on anterior 2/3 of the tongue travels in the chorda tympani branch of the facial nerve (CN VII) posterior 1/3 of the tongue reaches the brain stem via the glossopharyngeal nerve (CN IX) other tongue areas reach the brain stem via the vagus nerve (CN X) 2. Taste fibres enter the nucleus of tractus solitarius → synapse with second-order neurons → cross the midline and join the medial lemniscus → synapse with third-order neurons at the thalamus. 3. Impulses are relayed to the taste projection area in the cerebral cortex at the foot of the postcentral gyrus and in the insula. 4. The taste is represented in the portion of the sensory homunculus that subserves facial sensation. 5. The central gustatory pathway is predominantly uncrossed. Central Gustatory Pathway) 6. Taste reflexes are integrated into the brainstem. 7. From the tractus solitarus, a large number of impulses are transmitted within the brain stem → into the superior and inferior salivatory nuclei → submandibular, sublingual, and parotid glands to help control salivary secretion during a meal Taste Threshold & Intensity Discrimination Taste threshold concentration of different substances varies in the following: 1. Hydrochloric acid – 100 µmol/L 2. Glucose – 80,000 µmol/L 3. Sucrose – 10,000 µmol/L 4. Saccharin (artificial sweetener) – 23 µmol/L The overall sensation produced by food (flavour) is contributed by some pain stimulation (e.g. spicy food), smell consistency or texture, and food temperature.