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Flashcards in Chemosensation - taste and smell Deck (32):

what are the objectives of this lecture?

1. Describe some simple tests you can do in a clinical setting that will enable you to distinguish a deficit of olfaction from a deficit of taste and recognize the clinical terms for losses of these senses (ageusia = loss of taste; anosmia = loss of smell).
2. Describe the differences in morphology and functioning of receptor cells for taste, trigeminal and olfactory modalities.
3. Describe the difference in route of access of odorants to the olfactory epithelium during orthonasal and retronasal stimulation.
4. Describe the way odor information is transmitted from the receptor epithelium to the olfactory bulb and compare how different odors are represented within the receptor sheet and within the bulb.
5. List the output pathways and targets of the olfactory bulb and describe the behaviors or cognitive events that are associated with each of the telencephalic olfactory target areas.
6. List the three types of gustatory papillae and indicate which cranial nerve provides their gustatory and general cutaneous innervation to that area of the tongue.
7. Trace the neural pathways and name the central nuclei conveying taste information from taste buds to primary gustatory cortex.
8. Describe how variations in taste sensitivity to particular chemicals may relate to overall health status of individuals.
9. Describe how variations in taste sensitivity to particular chemicals may relate to overall health status of individuals


Why is "taste" a misnomer?

The colloquial use of the term “taste” is imprecise, and in fact, what is usually called a “taste” sensation involves stimulation of three sensory systems: olfaction (smell), gustation (taste proper) and chemesthesis (detection of irritant chemicals by trigeminal nerve endings).


What do taste buds recognize?

The sense of taste utilizes taste buds present in the oral cavity (tongue and palate) to detect sweet, sour, salty, bitter, and the taste of monosodium glutamate (umami)


How does (general) olfaction work?

Olfaction utilizes olfactory receptor neurons located in the neuroepithelium lining the roof of the nasal cavity to detect and identify thousands of smells or flavors delivered into the nose either through the nostrils, or, in the case of flavors in foods, retronasally


The trigeminal nerve contributes to chemoreception how?

Finally, chemesthesis utilizes chemosensory receptors localized in the sensory processes that trigeminal neurons extend into the nasal and buccal cavities to detect irritant or noxious stimuli (e.g. capsaicin, the main irritant component of chili peppers, CO2 in carbonated beverages or acetic acid).


The sense of smell uses what receptor cell

Ciliated, bipolar neuron
*in CN 1
*olfactory bulb = nucleus


The sense of taste uses what receptor cell?

Modified epithelial cell
(Synapses onto nerve fiber from cranial ganglion cell)
*Nucleus of the Solitary Tract = CNS nucleus


The sense of chemesthesis uses what receptor cell?

Free Nerve Ending of
Cranial Ganglion Cell
*CN V (mostly)
*spinal trigeminal nucleus = main CNS nucleus


What's up with taste papillae?

Lingual taste buds lie on specialized bumps or grooves, called taste papillae
*The three kinds of taste papillae are classified according to their shape and number of taste buds:
*fungiform papillae are located all over the anterior end of the tongue,
*foliate papillae on the sides, and circumvallate papillae at the posterior part of the tongue.
*Stimulation of a single taste papilla can be sufficient to identify the taste quality of the stimulus.
*Filiform papillae are non-taste papillae and serve as tactile organs.


Describe the shared innervation of taste buds along the tongue and soft palate

The chorda tympani branch of the facial nerve innervates fungiform taste papillae in the anterior 2/3 of the tongue,
*while the glossopharyngeal nerve innervates the circumvallate papillae.
*Taste buds located in the soft palate are innervated by the superior petrosal branch of the facial nerve, and
*the taste buds in the extreme posterior tongue, oropharynx and epiglottis are supplied by the vagus.
*Taste buds in these posterior areas are thought to be important in gag reflexes designed to stop intake of spoiled foods or noxious compounds.


How do the taste pores transmit information to the CNS?

Organization of a Taste Bud.
*The gustatory fibers enter the taste bud (intragemmal fibers) whereas the majority of somatosensory fibers innervate the surrounding epithelium (Perigemmal Fibers).
*Tastants dissolved in saliva reach the elongate receptor cells through the taste pore, which is a small opening in the epithelium.
*Taste receptor cells synapse onto the intragemmal fibers.
*Proliferative basal cells generate new taste cells just as they generate new cells in the surrounding epithelium.


Upon a elongated taste cell binding a tastant, what happens?

Depolarization of taste cells leads to release of transmitter from the basal portion of the cell.
*In the taste system ATP (acting on neural P2X receptors) is crucial in transmission of taste information. Drugs which block P2X receptors (to relieve pain) can cause loss of taste which can limit patient compliance with these treatments.


Distinguishing taste can be thought of as analogous to the higher order determination of color from just three different types of color cones. Why is this?

*Apparently, the central nervous system extracts information from the population of afferent fibers activated by a particular chemical stimulus on the tongue to determine its sensory characteristics.
*Individual afferent neurons contact a number of taste buds, and within each taste bud each fiber innervates a few receptor cells.
*The individual taste receptor cells are sensitive mostly to one class of taste stimuli.
*Electrical recordings from taste nerves show that most single fibers predominantly signal the presence of a particular taste quality, sweet or bitter, while other fibers are more broadly tuned e.g. to ionic stimuli.


How do different tastants depolarize a taste receptor cell?

the taste system detects a diverse array of molecules and to do so must utilize a host of different mechanisms.
*Some stimuli, e.g. salts or protons, can permeate ion channels directly to depolarize the receptor cell.
* Other taste substances, e.g. sugars, bitter and glutamate rely on metabotropic (G- protein-linked) receptors.


What's the bitter-receptor to sweet-receptor ratio?

*bitter is way more represented than any other taste receptor cell
**Of the 30 or so human taste receptor genes identified to date, 27 encode bitter receptors. Individual differences in primary structure of the receptor proteins (polymorphisms) can lead to different degrees of sensitivity to particular taste substances. The most common example is differential sensitivity to PROP (Propylthiouracil). Such differences in sensitivity may affect both diet and tolerance for bitter-tasting medicines.


people who can't taste PROP have worse respiratory infections why?

The cells in airways express the PROP (special class of bitter) taste receptor.
*CAN BE USED to detect and respond to bacterial signaling molecules.
*When the airway epithelial cells detect the presence of large populations of bacteria, the epithelial cells mount a local defense and also alert elements of the innate immune system.


Describe the central taste pathway

*slightly different pathways for either reflex or conscious sensation (like any other sense)
*cell bodies of taste afferents (which synapse to taste receptor cells) are in the respective CN VII, IX, or X nuclei
*synapse on 2nd order neurons in nucleus of the solitary tract (ipsilateral)
*project bilaterally to VPM (thalamus) for conscious appreciation of taste
*project bilaterally to hypothalamus and amygdala for sub-conscious reactions to taste and control of appetite


what is the taste lemniscus?

*pathway to the cortex and integration of taste and olfaction
*Second order cells (in rostral portions of NST) send axons bilaterally to the medial part of the ventrobasal thalamus (VPMpc)
*The thalamic neurons then send their axons into the insular cortex (the insula and the operculum).
* The pathway to insular cortex presumably serves the conscious appreciation of taste.
*Secondary gustatory cortex, located in the orbitofrontal surface, receives projections from the primary gustatory area located on the anterior insula and from olfactory areas of the insula.
*The orbitofrontal cortex is presumably where integration taste and smell leads to the perception of flavor.


The nucleus of the solitary tract participates in reflex arcs how?

Also the NST has reflex connections to nuclei in the brainstem involved in gagging (nuc. ambiguus), swallowing (e.g. nuc. ambiguus, hypoglossal nuc.) and salivation (sup. & inf. Salivatory nuc.).


What is unique/special about olfactory receptor cells?

*they are themselves neurons
*the olfactory neurons are bipolar neurons (i.e. a dendrite at one end [apically] and an axon emitted from the other end [basally]) and that they send a single, very thin (0.2 m in diameter) unmyelinated axon towards the olfactory bulb as part of the olfactory nerve (CN I).


olfactory neurons are exposed to the environment and are therefore unique in what way from other neurons?

Since the olfactory neurons are exposed to the external environment, they are subject to attack by bacteria, viruses and environmental toxins.
*Perhaps because of this, olfactory neurons are the only neurons that are continuously undergoing neurogenesis and replacement. Thus the olfactory receptor cells you are using today are entirely different than the ones you used a month ago.


Odarant receptor proteins look like what other proteins?

The olfactory receptor proteins possess seven transmembrane spanning regions and resemble other G-protein coupled receptors such as those that bind some neurotransmitters and hormones (such as serotonin and epinephrine).


Does one olfactory neuron recognize a bunch of different odorants?

No, Each olfactory receptor neuron predominantly expresses one olfactory receptor protein.
*however, if the chemical fit is close enough, there may be cross over


How does depolarization of an olfactory neuron occur?

*starts in the cilia, and is amplified across the cell by EFFLUX OF CL
*When an odorant binds to the receptor protein, the associated G-protein (Golf) activates adenylate cyclase which locally generates cAMP. The cAMP opens a nearby cAMP-gated ion channel which permits influx of Na+ and Ca2+. The local increase in Ca2+ opens adjacent Ca2+-gated Chloride channels. Since Cl- levels in the receptor cell are quite high, opening a Cl- channel results in outflow of Cl- thereby further depolarizing the cell. This amplified depolarization is sufficient to drive the cell to threshold thereby triggering an action potential.


Is it true you kidney and lung can smell?

Examples of tissues exhibiting functional olfactory receptors include kidney and lung
*probably to detect noxious stimuli? dissolved chemicals that mediate a certain elimanaion response perhaps?


After an olfactory neuron senses an odorant, where does it project that signal to?

Axons of the olfactory neurons penetrate the ethmoid bone (cribriform plate) and converge on glomeruli, which are spherical neuropil structures (a tangle of axons and dendrites) present at the outer layer of the olfactory bulb. In each of these glomeruli, approximately 1000 axons (each from a single olfactory neuron) make excitatory synaptic connections with the apical dendrite of 2-25 mitral (or tufted) cells


the primary principle of encoding odor quality is through a odor-related map of glomeruli in the olfactory bulb. Why?

*a given chemical will induce a given population of olfactory neurons which project to a given glomerulus (olfactory neurons that express receptor a will project to glomerulus b and no other)
*Olfactory receptor neurons expressing the same olfactory receptor protein project their axons to the same glomerulus. This convergence of axons that stem from receptor neurons with the same chemical specificity has been postulated to be the basis for the ability of the olfactory system to recognize among structurally dissimilar odorants.


Can one odorant only depolorize one type of olfactory neuron?

When a subject is stimulated with a pure olfactory compound and the response of the olfactory bulb is recorded, many glomeruli respond to each individual odorant. As a result, it appears that each odorant can stimulate a number of receptors. The response to single odorants is usually not localized to a single glomerulus but is distributed in wide areas of the glomerular layer. Therefore, identification of an odor entails recognition of the pattern of activity across all glomeruli of the olfactory bulb.


The olfactory neurons are unique in that they don't relay through a 2nd order neuron right away. Instead they project straight to the cortex. But what areas of the cortex do they project to?

*These axons (olfactory tract, which is a collection of olfactory neuron axons) project directly into olfactory cortex and a portion of the amygdala.
*olfactory tubercle
*piriform cortex
*entorhinal cortex


How is the conscious sense of smell transduced (CNS pathway)

The axons of output cells from the bulb collect into the lateral olfactory tract.
*These axons project directly into olfactory cortex and a portion of the amygdala.
*The olfactory cortex consists of the lateral olfactory gyrus and part of the uncus.
*Olfactory cortex is divided into several areas including the piriform cortex, the accessory olfactory nucleus and the olfactory tubercle. The piriform cortex projects to the orbitofrontal cortex, both directly and via MD nucleus of the thalamus. Orbitofrontal cortex is an association area for olfactory and taste information, and it is thought that this pathway results in our conscious appreciation of smell


Describe the sub-architecture of the olfactory cortex

*Olfactory cortex is divided into several areas including the piriform cortex, the accessory olfactory nucleus and the olfactory tubercle. The piriform cortex projects to the orbitofrontal cortex, both directly and via MD nucleus of the thalamus. Orbitofrontal cortex is an association area for olfactory and taste information, and it is thought that this pathway results in our conscious appreciation of smell


Besides the conscious perception of smell, what are the other CNS systems the olfactory cortex projects to?

The other major connections are associated with the limbic system, the amygdala, olfactory tubercle and entorhinal cortex.
*Amygdala and olfactory tubercle are interconnected with the hypothalamus which is intimately involved in the subconscious regulation of homeostasis, circadian, reproductive and other biological activity patterns.
*The entorhinal cortex feeds into the hippocampus which is a major player in the storage and retrieval of memories.

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