بسم الله الرحمن الرحيم

MECHANISMS OF TASTE & OLFACTION By Dr. Khaled Khalil

By the end of this lecture, you should be able to: Describe the primary taste sensations. Explain the structure and mechanism of stimulation of taste buds. Discuss the taste pathway. Describe the mechanism of stimulation of olfactory cells. Discuss the olfactory pathway. Explain the role of central nervous system in olfaction. Explain the clinical correlation of taste sensations and olfaction.

Taste sensation Primary taste modalities All tastes are varying combinations of four primary tastes: salty, sour , sweet, and bitter and umami, a meaty or savory. These primary taste sensations are elicited by the following stimuli: Salt taste is stimulated by chemical salts, especially NaCl (table salt). Sour taste is caused by acids. The citric acid content of lemons.

Sweet taste is evoked by glucose and other organic molecules with similar structures but no calories, such as saccharin, aspartame, and other artifcial sweeteners, can also interact with “sweet” receptor binding sites. Bitter taste is elicited by caffeine, nicotine, strychnine and morphine . Umami taste is triggered by amino acids, especially glutamate (in monosodium glutamate) . The presence of amino acids, as found in meat for example, serves as a marker for a desirable, nutritionally protein-rich food.

Taste receptors Taste receptor cells are present in in taste buds located in taste papillae on the dorsum of the tongue, pharynx and upper esophagus. A taste bud consists of a group of 50 to 150 receptor cells, as well as supporting cells and basal cells. The chemoreceptor cells synapse at their bases with primary afferent nerve fibers. The apices of taste receptor cells have microvilli to increase surface area exposed to oral contents.

Tongue Map

Each receptor cell responds in varying degrees to all five primary tastes but is generally preferentially responsive to one of the taste modalities. The richness of fine taste discrimination beyond the primary tastes depends on subtle differences in the stimulation patterns of all the taste buds in response to various substances, Taste perception is also influenced by information derived from other receptors, especially odor as well as psychological factors associated with past experiences with the food.

Taste bud

Mechanism of stimulation of taste buds: For a chemical substance (tastant) to bind to the taste receptor, it must be in a solution or dissolved in saliva. Taste binding protein (TBP) (produced by Ebner's glands that secrete mucus into the cleft around circumvallate papillae) transports the tastant and concentrates it at taste buds. Then the tastant binds to its specific receptors in the microvilli of gutstatory cells. The binding of the tastant to a taste receptor opens or closes ion channels, causing potential changes in the taste receptor cell and release of the chemical transmitter e.g. glutamate which excite the nerve fibers to generate nerve impulses.

For each tastant, there is a different mechanism :

Neural pathway of taste sensation

Central Gustatory Pathway First order neurons: It carries taste sensation from the taste buds to the gustatory nucleus (nucleus solitarius) as follow; a) From anterior 2/3 of the tongue are carried by the chorda tympani nerve (branch of the facial nerve). b) From the posterior 1/3 of the tongue are carried by the glossopharyneal nerve. c) From the pharynx, hard and soft palate and epiglottis are carried by the vagus nerve.

Second order neurons: From the gustatory nucleus (nucleus solitarius) axon of 2nd order neuron ascend in the ipsilateral (same side) medial lemniscus and pass directly to the ventral posteromedial nucleus of the thalamus (VPMNT). Third order neurons: From the thalamus (VPMNT), the axons of the 3rd order neurons pass in the sensory radiation to the face area of the somatosensory cortex in the ipsilateral postcentral gyrus. They also pass to the anterior part of the insula which is anterior to the face area of the postcentral gyrus. This area that mediates conscious perception of taste and taste discrimination.

Disorders of taste sensation Hypogeusia : Decreased ability to taste due to nerve lesion, aging, cigarette smoking. Ageusia : Inability to taste due to Vit. B12 or zinc deficiency. Dysgeusia : unpleasant perception of taste giving metalic or rancid taste.

Smell (Olfactory) Sensation The olfactory (smell) sensation is a chemical sense involving receptors which are sensitive to volatile chemical substances. Olfactory Mucosa: It is located on the superior nasal concha and the upper third of the nasal septum. The total area of the olfactory mucosa in both sides is about 5 cm2.

Stimulation of the olfactory receptor: 1) The odorant substance must be; Volatile in order to be sniffed into the nose to reach the olfactory receptors. Sufficiently water soluble in order to pass through mucous layer coating the mucosa to reach the olfactory cells Lipid soluble in order to penetrate the cells membrane of the olfactory cells.

2) The olfactory cells are stimulated when air blasts upwards into the superior region of the nose. The amount of air reaching this region is greatly increased by sniffing, an action that includes contraction of the lower part of the nares on the septum, deflecting the airstream upward. 3) The odorant substance, on coming in contact with the olfactory mucosa, first diffuses into the mucus that covers the cilia, then binds with odorant-binding proteins (OBP) that trap and concentrate the substances and transfer them to the olfactory receptors.

3) The odorant receptors are G-protein coupled receptors 3) The odorant receptors are G-protein coupled receptors. The specific G-protein in olfactory receptors is called Golf (G-­protein in olfactory receptor). Binding the odour to the receptor activates Golf, which stimulates adenylate cyclase to produce cAMP. cAMP opens Na, Ca channels resulting in depolarization and generation of receptor potential which at threshold levels will fire an action potential

The Olfactory Pathways 1st order neuron: Olfactory receptors are the first order neurons. Each has a single nonmyelinated axon which must pass through the cribriform plate to end in the glomeruli in the olfactory bulb.

Olfactory bulb and 2nd order neurons: The second order neurons are the mitral and tufted cells present in the olfactory bulbs. Their dendrites synapse with the axons of the olfactory receptors in globular structures called the olfactory glomeruli. Each glomerulus contains the synapses of about 25,000 olfactory receptors and 100 of second order neurons. There are about 60 such glomeruli in each bulb. Axons of the mitral and tufted cells form the olfactory tract that leaves the bulb to the olfactory cortex.

A) The medial olfactory area: Olfactory Cortex: The axons of the mitral and tufted cells form the olfactory tract which enters the brain between the cerebellum and mesencephalon, where they divide into 2 tracts that terminate on medial and lateral olfactory areas A) The medial olfactory area: The medial olfactory area consists of a group of nuclei located immediately anterior to the hypothalamus e.g. septal nuclei, and primitive portions of limbic system. This olfactory area is concerned with: Primitive responses to olfaction, such as licking the lips, salivation caused by the smell of food. Primitive emotional drives associated with smell.

B) The Lateral Olfactory Area: In the lateral olfactory area, fibers may take one of 2 pathways: 1) To Prepyriform and pyriform cortex plus the cortical portion of the amygdaloid nuclei ----> then pass into almost all portions of the limbic system, especially the hippocampus. It is concerned with: Learning to like or dislike certain foods. Behavioral responses to food e.g. development of absolute aversion to foods that have caused nausea and vomiting Olfactory conditioned reflexes. 2) To the dorsomedial thalamic nucleus and then project to the orbitofrontal cortex.This system is responsible for: The conscious perception of olfaction i.e. analysis, interpretation and significance of the odour.

Disturbance of olfaction: Anosmia: means complete absence of the sense of smell Hyposmia: means diminished olfactory sensitivity. More than 75% of humans over the age of 80 have an impaired ability to identify smells. Dysosmia: means distorted sense of smell e.g. Olfactory hallucinations due to lesion in the prepyriform cortex. Hypersensitivity for both smell and taste as in Addison disease

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