Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

Smell and taste sensation/ objectives of the lecture

■ Describe the basic features of the neural elements in the olfactory epitheliumand olfactory bulb.■ Outline the pathway by which impulses generated in the olfactory epitheliumreach 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 theinsular 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 in the roof of the nasal cavity called olfactory epithelium. The human olfactory epithelium contains about 50 million bipolar olfactory sensory neurons. The olfactory epithelium is covered by a thin layer of mucus.

Each olfactory sensory neuron contain 6–12 cilia which 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 axons of the olfactory sensory neurons pass through the cribriform plate of the ethmoid bone and enter the olfactory bulbs.

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

Figure 1-a: olfactory epithelium

Figure 1-b: showing olfactory bulb

Olfactory cortex (figure 2)

The axons from olfactory bulb 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.

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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 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.

Figure 2: olfactory cortex

Odor detection thresholdOdor-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.

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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.

-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.

AdaptationIt 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 50–70 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.

If the sensory nerve is cut, the taste buds it innervates degenerate and eventually disappear.

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

Figure 3: taste bud

Figure 4: papillae of the tongue

Taste pathways (figure 5)

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 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

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

medial lemniscus and project directly to 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 nerves to the nucleus of tractus solitarius contain fibers from all types of taste receptors, without any clear localization of types.

Taste thresholds & intensity discrimination

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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.

■ 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

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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 areA. 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 receptorsA. 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?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.

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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 disturbanceB. HyperosmiaC. Auditory problemsD. Taste and odor abnormalitiesE. No major sensory deficits

5. Which of the following are incorrectly paired?A. ENaC : Sour tasteB. Gustducin : Bitter tasteC. T1R3 family of GPCR : Sweet tasteD. Heschel sulcus : SmellE. 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.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?

Neurophysiology/special senses/smell and taste Lect. Dr. Zahid M. kadhim

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 tasteB. Facial twitchingC. Droopy eyelidD. Ipsilateral facial paralysisE. All of the above