Chemosensation - taste and smell Flashcards
what are the objectives of this lecture?
- 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).
- Describe the differences in morphology and functioning of receptor cells for taste, trigeminal and olfactory modalities.
- Describe the difference in route of access of odorants to the olfactory epithelium during orthonasal and retronasal stimulation.
- 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.
- 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.
- 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.
- Trace the neural pathways and name the central nuclei conveying taste information from taste buds to primary gustatory cortex.
- Describe how variations in taste sensitivity to particular chemicals may relate to overall health status of individuals.
- 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)
*CN VII, IX, X
*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.
