Smell and taste sensation/ objectives of the lecture

Transcription

1 Smell and taste sensation/ objectives of the lecture Describe the basic features of the neural elements in the olfactory epithelium and olfactory bulb. Outline the pathway by which impulses generated in the olfactory epithelium reach the olfactory cortex. Describe the location and cellular composition of taste buds. Name the five major taste receptors and signal transduction mechanisms in these receptors. Outline the pathways by which impulses generated in taste receptors reach the insular cortex. Introduction Smell (olfaction) and taste (gustation) are generally classified as visceral senses because of their close association with gastrointestinal function. Physiologically, they are related to each other. The flavors of various foods are in large part a combination of their taste and smell. Consequently, food may taste different if one has a cold that depresses the sense of smell. Both smell and taste receptors are chemoreceptors that are stimulated by molecules dissolved in mucus of the nose and saliva of the mouth. Olfactory epithelium and olfactory bulbs (figure 1: a & b) Olfactory sensory neurons which are specialized cells of the smell, are located in a specialized portion of the nasal mucosa, the yellowish pigmented olfactory epithelium. In humans, it covers an area of 10 cm 2 in the roof of the nasal cavity near the septum. The human olfactory epithelium contains about 50 million bipolar olfactory sensory neurons interspersed with glia-like supporting (sustentacular) cells and basal stem cells. New olfactory sensory neurons are generated by basal stem cells as needed to replace those damaged by exposure to the environment. The olfactory epithelium is covered by a thin layer of mucus secreted by the supporting cells and Bowman glands, which lie beneath the epithelium.

2 Each olfactory sensory neuron has a short, thick dendrite that projects into the nasal cavity where it terminates in a knob containing 6 12 cilia and protrude into the mucus overlying the olfactory epithelium. Odorant molecules (chemicals) dissolve in the mucus and bind to odorant receptors on the cilia of olfactory sensory neurons. The mucus provides the appropriate molecular and ionic environment for odor detection. The axons of the olfactory sensory neurons pass through the cribriform plate of the ethmoid bone and enter the olfactory bulbs. In the olfactory bulbs, the axons of the olfactory sensory neurons contact the primary dendrites of the mitral cells and tufted cells to form anatomically discrete synaptic units called olfactory glomeruli. Free endings of many trigeminal pain fibers are found in the olfactory epithelium. They are stimulated by irritating substances, which leads to the characteristic odor of such substances as peppermint, menthol, and chlorine. Activation of these endings by nasal irritants also initiates sneezing, lacrimation, respiratory inhibition, and other reflexes. Figure 1-a: olfactory epithelium

3 Figure 1-b: showing olfactory bulb Olfactory cortex (figure 2) The axons of the mitral and tufted cells pass posteriorly to terminate on apical dendrites of pyramidal cells in five regions of the olfactory cortex: anterior olfactory nucleus, olfactory tubercle, piriform cortex, amygdala, and entorhinal cortex. From these regions, information travels directly to the frontal cortex or via the thalamus to the orbitofrontal cortex. Conscious discrimination of odors is dependent on the pathway to the orbitofrontal cortex. The orbitofrontal activation is generally greater on the right side than the left; thus, cortical representation of olfaction is asymmetric. The pathway to the amygdala is probably involved with the emotional responses to olfactory stimuli, and the pathway to the entorhinal cortex is concerned with olfactory memories.

4 Figure 2: olfactory cortex Odorant receptors The odorant receptors are G protein coupled receptors. When an odorant molecule binds to its receptor, the G protein subunits (α, β, γ) dissociate. The α- subunit activates adenylate cyclase to catalyze the production of camp, which acts as a second messenger to open cation channels, increasing the permeability to Na +, K -, and Ca 2+. The net effect is an inward-directed Ca 2+ current which produces the graded receptor potential. Odor detection threshold Odor-producing molecules (odorants) are generally small, containing from 3 to 20 carbon atoms; and molecules with the same number of carbon atoms but different structural configurations have different odors. Relatively high water and lipid solubility are characteristic of substances with strong odors. The odor detection thresholds are the lowest concentration of a chemical that can be detected. The wide range of thresholds illustrates the remarkable sensitivity of the odorant receptors.

5 Determination of differences in the intensity of any given odor is poor. The concentration of an odor-producing substance must be changed by about 30% before a difference can be detected. Adaptation It is common knowledge that when one is continuously exposed to even the most disagreeable odor, perception of the odor decreases and eventually ceases. This sometimes beneficent phenomenon is due to the fairly rapid adaptation, or desensitization. TASTE SENSE Taste buds (figure 3 &4) The specialized sense organ for taste (gustation) consists of approximately 10,000 taste buds, which are ovoid bodies measuring μm. There are 2 morphologically distinct types of cells within each taste bud: basal cells and taste cells which found at various stages of development. Taste cells are the sensory neurons that respond to taste stimuli or tastants. Each taste bud has between 50 and 100 taste cells. The apical ends of taste cells have microvilli that project into the taste pore, a small opening on the dorsal surface of the tongue where tastes cells are exposed to the oral contents. Each taste bud is innervated by about 50 nerve fibers, and conversely, each nerve fiber receives input from an average of five taste buds. The basal cells arise from the epithelial cells surrounding the taste bud. They differentiate into new taste cells, and the old cells are continuously replaced with a half-time of about 10 days. If the sensory nerve is cut, the taste buds it innervates degenerate and eventually disappear. In humans, the taste buds are located in the mucosa of the epiglottis, palate, and pharynx in the walls of papillae of the tongue. The fungiform papillae are rounded structures most numerous near the tip of the tongue; the circumvallate papillae are prominent structures arranged in a V on the back of the tongue; the foliate papillae are on the posterior edge of the tongue.

6 The von Ebner s glands (also called gustatory glands or serous glands) secrete saliva into the cleft around the circumvallate and foliate papillae. Secretions from these glands may function to cleanse the mouth to prepare the taste receptors for a new stimulant. Figure 3: taste bud Taste pathways (figure 5) Figure 4: papillae of the tongue The sensory nerve fibers from the taste buds on the anterior two-thirds of the tongue travel in the chorda tympani branch of the facial nerve, and those from the posterior third of the tongue reach the brain stem via the glossopharyngeal

7 nerve. The fibers from areas other than the tongue (eg, pharynx) reach the brain stem via the vagus nerve. On each side, taste fibers in these three nerves unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata. From there, axons of second-order neurons ascend in the ipsilateral medial lemniscus and project directly to the ventral posteromedial nucleus of the thalamus. From the thalamus, the axons of the third-order neurons pass to neurons in the anterior insula and the frontal operculum in the ipsilateral cerebral cortex. This region is rostral to the face area of the postcentral gyrus, which is probably the area that mediates conscious perception of taste. Figure 5: taste pathway Taste modalities, receptors & transduction Humans have five established basic tastes: sweet, sour, bitter, salt, and umami. The umami taste was added to the four classic tastes relatively recently. Its receptor triggered particularly by the monosodium glutamate used so extensively in Asian cooking. The taste is pleasant and sweet but differs from the standard sweet taste. Although for many years it was thought that the surface of the tongue had special areas for each of the first four of these sensations, it is now known that all tastants are sensed from all parts of the tongue and adjacent structures. Afferent

8 nerves to the nucleus of tractus solitarius contain fibers from all types of taste receptors, without any clear localization of types. Salt and sour tastes are triggered by activation of ionotropic receptors; sour, bitter, and umami tastes are triggered by activation of G-protein coupled receptor. Taste thresholds & intensity discrimination The ability of humans to discriminate differences in the intensity of tastes, like intensity discrimination in olfaction, is relatively crude. A 30% change in the concentration of the substance being tasted is necessary before an intensity difference can be detected. Taste threshold refers to the minimum concentration at which a substance can be perceived. The threshold concentrations of substances to which the taste buds respond vary with the particular substance. Bitter substances tend to have the lowest threshold. Some toxic substances such as strychnine have a bitter taste at very low concentrations, preventing accidental ingestion of this chemical, which causes fatal convulsions. CHAPTER SUMMARY Olfactory sensory neurons, supporting (sustentacular) cells, and basal stem cells are located in the olfactory epithelium within the upper portion of the nasal cavity. The cilia located on the dendritic knob of the olfactory sensory neuron contain odorant receptors that are coupled to G proteins. Axons of olfactory sensory neurons contact the dendrites of mitral and tufted cells in the olfactory bulbs to form olfactory glomeruli. Information from the olfactory bulb travels via the lateral olfactory stria directly to the olfactory cortex, including the anterior olfactory nucleus, olfactory tubercle, piriform cortex, amygdala, and entorhinal cortex. Taste buds are the specialized sense organs for taste and are comprised of basal stem cells and three types of taste cells. Taste buds are located in the mucosa of the epiglottis, palate, and pharynx and in the walls of papillae of the tongue.

9 There are taste receptors for sweet, sour, bitter, salt, and umami. Signal transduction mechanisms include passage through ion channels, binding to and blocking ion channels, and GPCR requiring second messenger systems. The afferents from taste buds in the tongue travel via the seventh, ninth, and tenth cranial nerves to synapse in the nucleus of the tractus solitarius. From there, axons ascend via the ipsilateral medial lemniscus to the ventral posteromedial nucleus of the thalamus, and on to the anterior insula and frontal operculum in the ipsilateral cerebral cortex. MULTIPLE CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. A young boy was diagnosed with congenital anosmia, a rare disorder in which an individual is born without the ability to smell. Odorant receptors are A. located in the olfactory bulb. B. located on dendrites of mitral and tufted cells. C. located on neurons that project directly to the olfactory cortex. D. located on neurons in the olfactory epithelium that project to mitral cells and from there directly to the olfactory cortex. E. located on sustentacular cells that project to the olfactory bulb. 2. A 37-year-old female was diagnosed with multiple sclerosis. One of the potential consequences of this disorder is diminished taste sensitivity. Taste receptors A. for sweet, sour, bitter, salt, and umami are spatially separated on the surface of the tongue. B. are synonymous with taste buds. C. are a type of chemoreceptor. D. are innervated by afferents in the facial, trigeminal, and glossopharyngeal nerves. E. all of the above 3. Which of the following does not increase the ability to discriminate many different odors?

10 A. Many different receptors. B. Pattern of olfactory receptors activated by a given odorant. C. Projection of different mitral cell axons to different parts of the brain. D. High β-arrestin content in olfactory neurons. E. Sniffing. 4. As a result of an automobile accident, a 10-year-old boy suffered damage to the brain including the periamygdaloid, piriform, and entorhinal cortices. Which of the following sensory deficits is he most likely to experience? A. Visual disturbance B. Hyperosmia C. Auditory problems D. Taste and odor abnormalities E. No major sensory deficits 5. Which of the following are incorrectly paired? A. ENaC : Sour taste B. Gustducin : Bitter taste C. T1R3 family of GPCR : Sweet taste D. Heschel sulcus : Smell E. Ebner glands : Taste acuity 6. A 9-year-old boy had frequent episodes of uncontrollable nose bleeds. At the advice of his physician, he underwent surgery to correct a problem in his nasal septum. A few days after the surgery, he told his mother he could not smell the cinnamon rolls she was baking in the oven. Which of the following is true about olfactory transmission? A. An olfactory sensory neuron expresses a wide range of odorant receptors. B. Lateral inhibition within the olfactory glomeruli reduces the ability to distinguish between different types of odorant receptors. C. Conscious discrimination of odors is dependent on the pathway to the orbitofrontal cortex. D. Olfaction is closely related to gustation because odorant and gustatory receptors use the same central pathways.

11 E. All of the above. 7. A 31-year-old female is a smoker who has had poor oral hygiene for most of her life. In the past few years she has noticed a reduced sensitivity to the flavors in various foods which she used to enjoy eating. Which of the following is not true about gustatory sensation? A. The sensory nerve fibers from the taste buds on the anterior two-thirds of the tongue travel in the chorda tympani branch of the facial nerve. B. The sensory nerve fibers from the taste buds on the posterior third of the tongue travel in the petrosal branch of the glossopharyngeal nerve. C. The pathway from taste buds on the left side of the tongue is transmitted ipsilaterally to the cerebral cortex. D. Sustentacular cells in the taste buds serve as stem cells to permit growth of new taste buds. E. The pathway from taste receptors includes synapses in the nucleus of the tractus solitarius in the brain stem and ventral posterior medial nucleus in the thalamus. 8. A 20-year-old woman was diagnosed with Bell palsy (damage to facial nerve). Which of the following symptoms is she likely to exhibit? A. Loss of sense of taste B. Facial twitching C. Droopy eyelid D. Ipsilateral facial paralysis E. All of the above