Chemosensory Systems Flashcards
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. Know the clinical terms for losses of these senses (ageusia = loss of taste; anosmia = loss of smell).
Smell—Scientists have developed an easily administered “scratch-and-sniff” test to evaluate the sense of smell.
Taste—Patients react to different chemical concentrations in taste testing; this may involve a simple “sip, spit, and rinse” test, or chemicals may be applied directly to specific areas of the tongue.
Simple test:
• In clinic, 2/3 of people will complain of loss of taste. However, once you actually check for loss of taste, instead, 90% have actually lost or reduced their sense of smell.
Describe the differences in morphology and functioning of receptor cells for taste, trigeminal and olfactory modalities.
- Smell: Receptor cell = ciliated bipolar neuron innervated by CN I w/ olfactory bulb as primary sensory nucleus.
- Taste: Receptor cell = modified epithelial cell that synapses onto nerve fiber from cranial ganaglion cell to CN VII, IX, X. The primary CNS nucleus is the NST.
- Chemisthesis: Receptor cell: free nerve ending of cranial ganglion cell innervated by CNV. Detection of irritant chemicals by trigeminal nerve endings. Primary sensory nucleus is Spinal Trigeminal Nucleus.
Describe the difference in route of access of odorants to the olfactory epithelium during orthonasal and retronasal stimulation.
- Ortho-nasal = actually through your nostrils.
* Retro-nasal = through your mouth (ew) traveling through the concha to get to the olfactory epithelium.
Describe the way odor information is transmitted from the receptor epithelium to the olfactory bulb. Compare how different odors are represented within the receptor sheet
and within the bulb.
- Axons of olfactory neurons cross the ethmoid bone and converge on glomeruli.
- Olfactory receptor neurons expressing the same olfactory receptor protein project their axons into the SAME glomerulus. Convergence of axons that stem from receptor neurons w/ same chemical specificity is basis for ability of olfactory system to recognize structurally dissimilar odorants.
•Primary principle of encoding odor quality = odor-specific map of glomeruli in olfactory bulb.
Each odorant stimulates a number of receptors. Info on odor identity is NOT coded by labeled line or localized to a single glomerulus. Odor identity is distributed in wide areas of the glomerular layer so it entails PATTERN RECOGNITION!!!
List the output pathways and targets of the olfactory bulb. What behaviors or cognitive events are associated with each of the telencephalic olfactory target areas?
Output of olfactory bulb projects directly to the cortex (NOT THROUGH THE THALAMUS!!!):
•Axons of output cells from the bulb collect into the lateral olfactory tract.
•Axons project directly onto olfactory cortex + portion of amygdyla
•Olfactory cortex: consists of lateral olfactory gyrus and part of the uncus.
•Other major connections with limbic system: amygdal, olfactory tubercle, and entorhinal cortex.
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.
1) . Fungiform papillae: located in the anterior end of the tongue innervated by the facial nerve.
2) . Foliate papillae: on the side of the tongue innervated by the facial nerve.
3) . Circumvallate: located at the posterior end of the tongue and innervated by the glossopharyngeal nerve.
All taste buds in the pharynx, extreme posterior tongue, esophagus, and epiglottis are supplied by the vagus nerve.
Trace the neural pathways and name the central nuclei conveying taste information from taste buds to primary gustatory cortex.
Central taste pathway: primary afferents from tongue run in the facial, glossopharyngeal, and vagus nerves. When axons enter the CNS they synapse on second order neurons in the rostral area of the ipsilateral Nucleus of the solitary tract (NST). The 2nd order cells in rostral portions of the NT are the Tase Lemniscus, and they send axons bilaterally to the medial part of the ventrobasal thalamus
Secondary gustatory cortex: located in the orbitofrtonal surface, receives projections from the primary gustatory area located on the interior insula and from olfactory insula areas.
Orbitofrontal cortex: where integration of taste and smell leads to perception of FLAVOR!!!
Describe how variations in taste sensitivity to particular chemicals may relate to overall health status of individuals.
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Contrast the neural pathways for taste and smell.
Taste has to go through the thalamus while smell does not!
What is the presumed neural correlate for the peculiar ability of odors to evoke emotional memories?
Amygdala + olfactory tubercle are interconnected with hypothalamus. Gives subconscious regulation of homeostasis, circadian, reproductive and other biological activity patterns.
Entorhinal cortex: feeds into hippocampus, major player in storage and retrieval of MEMORIES!
How are odors represented in the olfactory bulb?
Axons of the olfactory neurons converge on the glomeruli present at the outer layer of the olfactory bulb. Olfactory receptor neurons expressing the same olfactory receptor protein project their axons to the same glomerulus. In this way, the odor is “mapped” onto the olfactory bulb according to odor quality
What are the mechanisms of taste and olfactory transduction?
Secondary gustatory cortex: located in the orbitofrtonal surface, receives projections from the primary gustatory area located on the interior insula and from olfactory insula areas.
Orbitofrontal cortex: where integration of taste and smell leads to perception of FLAVOR!!!
How can you tell the difference between taste, smell and trigeminal chemoreception in a clinical setting? How can you determine which modality is affected by a dysfunction or illness?
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