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Introductory Neurobiology Week 8 Day 1 Notes

by: lucy allen

Introductory Neurobiology Week 8 Day 1 Notes Biol 3640

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Notes for day one of week 8 (Tuesday, 2/23/2016).
Introductory Neurobiology
Dr. John C Kinnamon
Class Notes
introductory neurobiology, neurobiology
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This 7 page Class Notes was uploaded by lucy allen on Tuesday March 1, 2016. The Class Notes belongs to Biol 3640 at University of Denver taught by Dr. John C Kinnamon in Fall 2016. Since its upload, it has received 12 views. For similar materials see Introductory Neurobiology in Biology at University of Denver.


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Date Created: 03/01/16
Gustation -chemical senses include: -taste (gustation) -smell (olfaction) -vomeronasal chemoreception -the common chemical sense -garlic, wasabi, chili peppers and pain receptors, unmyelinated c fibers in your mouth Taste and Smell -represent the most phylogenetically old sensory systems -can go back to single celled organisms, they respond to chemicals, and are either repelled or attracted to these chemicals -they are the sensory systems that let us detect and discriminate the molecules in our environment -these senses help to link the external environment with internal needs -are vivid emotionally and perceptually strong tugs on your memories Other Types of Chemoreception -muscle sensors, detect lactic acid, burning during heavy exercise -circulatory sensors: oxygen and carbon dioxide receptors for maintenance of proper oxygenation and keeping the CO down2to a proper level -digestive tract sensors: receptors for various ingested substances -many use the same sensory transduction pathways as taste buds -organs of gustation on the tongue and on the palate, less on the epiglottis -holding your nose closed when chewing a jelly belly jelly bean -cannot taste the flavor, just get texture and some sweetness -once nostrils are opened and air is allowed in, the "smell" coming up through your pharynx can be sensed Taste -gustation -ageusia: loss of taste -dysgeusia: inappropriate tastes -people on medications, specifically SSRIs, have a metallic taste -organs of taste: taste buds -taste buds are accumulations of cells in the epithelium, 100-150 cells per taste bud, spindle shaped cells, each taste bud has an opening to the outside world called a taste pore, where the initial events in sensory transduction take place There is no tongue map! -mistranslation of a German scientific article -perpetuated through literature for over 100 years -all taste buds in your mouth respond to all of the primary flavors Cutaneous Receptors in the mouth -also participate in sensing -texture -fat in fancier ice cream vs. less fatty ice cream -temperature -steak, perfect temperature = ideal flavor -taste buds in the mouth are innervated by cranial nerves -rear of the mouth: 9th cranial nerve, glossopharyngeal nerve -epiglottis: innervated by 10th cranial nerve, vagus nerve Taste Buds are Contained Within Gustatory Papillae -fungiform papillae -interior 2/3 of the tongue -foliate papillae -located in the rear sides (lateral surfaces) of the tongue -in rows -several hundred taste buds -vallate papillae -rear surface of the tongue -located in a chevron (v-shaped) structure -maybe a dozen in the chevron organization -in primates taste buds line the inner wall -rodents and bunny rabbits: taste buds line both walls Three Cell Types in Mammalian Taste Buds -type I -spindle shape, terminate in long apical microvilli which project into the taste pore -glial-like function: not believed to function in gustation (do not have gustatory receptors), but envelop other cells and act to deactivate ATP which is released by the type II cells -type II: receptor cells -terminate in short, brush-like microvilli -usually have a very large, round nucleus -easily distinguished from skinny electron dense nucleus of Type I cells -signal bitter, sweet, and umami but not all on the same cell -three subtypes of type II cells -reminder: umami: 5th primary taste, taste for 'savory' -monosodium glutamate is a classic umami chemical -sushi bar: seaweed wrap is a strong umami stimulus, concentrated umami -type III -terminates in a single large microvillus -we can distinguish the three types just on the basis of their termination -respond to acidic (sour) stimuli and maybe* to salty stimuli (still up for debate) -nuclei characterized by deep invaginations (see slide 25) -slide 18 -electron micrograph of a longitudinal section of a bunny rabbit's taste bud -slide 19 -high magnification image of taste pore showing the termination of different cell types in the oral cavity There are also Basal Cells -differentiate into the type I, II and III cells -edge cells are peripheral cells Taste Cells Labeled With Antibodies That Recognize Particular Proteins -green cells are type III, labeled with an antibody to 5-HT (serotonin) -red cells:??? -for years it was not known how taste cells communicated with sensory nerve fibers -there are conventional synapses from some of the taste cells on to the nerve fibers -classic synapses use snare proteins for docking the synaptic vesicle to the presynaptic membrane -presynaptic and postsynaptic snare proteins -they form a complex and the docked vesicle is then ready for stimulation by calcium coming in via voltage gated calcium channels -initiates the fusion and exocytosis of the neurotransmitter Difference Between type II and III communication with nerve fibers -type III -classic -presynaptic capacitance, contributes to the membrane -release GABA and serotonin, maybe ATP -type II -have no vesicles -no SNAP25 -no voltage gated calcium channels -no increased membrane capacitance -no evidence for vesicle fusion with the presynaptic membrane -release of transmitter is blocked by channel blockers, not by i -NT released is ATP -we believe ATP is released through a non-vesicular channel through channels, not via a classic synaptic vesicle mechanism -there are complex interactions between taste buds in mammalian taste cells Mechanisms of Transmitter Release are Different for Type III and Type II Cells -stimulation of sensory afferent nerve fibers and type III cells (presynaptic cell), once stimulated it either via ATP or by sour (acidic) stimuli, releases GABA and serotonin onto the nerve fiber but it also acts on the type II cell and down-regulates its function -activity of the type III cell can inhibit that of the type II cell -atypical mitochondria: tubular cristae -only found close to -we believe that the large atypical mitochondria are very metabolically active and they are the source of the ATP that is released by the type II cells onto the nerve fibers and onto the other type III cells -depolarization comes down the membrane of the type II cell, reaches the junction where there are the specialized channels (CALHM1), which when depolarized allow passage of ATP from the type II cell onto P2X receptors on the afferent nerve terminal -stimulates the afferent nerve terminal, sodium comes in and depolarizes the afferent nerve terminal -then the ATP is taken back up into the presynaptic cell (type II) where it is recycled Conclusions -specialized contacts between type II cells in nerve fibers where atypical mitochondria, only at closed apposition of atypical mitochondria -individual nerve fibers form contacts with either type II or III cells but not both -several type II cells can converge onto a single nerve fiber Taste Qualities Appetitive vs. Aversive Gustatory Stimuli -appetitive: you enjoy it, you want to have more -aversive: sour milk, something very bitter, an alkaloid you don't want more of -only one transduction mechanism for hearing, seeing, and feeling but not taste -sourness and saltiness act directly on channels -bitterness and sweetness are signaled by second messengers -although taste cells act like neurons they are not neurons, they are specialized epithelial cells -type III cells: receive information, conduct action potentials (membrane depolarization), calcium influx, release of neurotransmitter via vesicles, activation of gustatory afferent neurons (can form synapses) Taste Stimuli Have Diverse Chemical Structures -bitter are complex (most bitter: denatonium) -salty are more simple (sodium) Salty Taste -uses amiloride-sensitive mechanisms and amiloride-insensitive mechanisms for sensory transductions -given for high blood pressure, blocks sodium channels in the kidneys -sodium passes right through the ion channel, depolarizing the salty sense Sour Taste -originally through there were a variety of ion channels signaling sour taste -theory: acid sensing -theory: hyperpolarizing -direct passage of hydrogen ions through ion channel -blocking of potassium channel -TRP channel (PKDL1) -turns out all sour sensing cells are immunopositive to antibodies for the PKDL marker -you can knock-out PKD and animals will still sense sour taste -even though PKD is a marker for sour cells, it is not involved in the signaling transduction pathway -recent finding: hydrogen ions pass straight through an ion channel Sour + Salt = Sweeter -put salt on grapefruit or watermelon, it will taste sweeter -dilute salt solutions are sweet when drank -miracle berry: comes from miraculin -does not work at neutral pH but something acidic (lemon) makes a strong sweet sensation -does not effect the acid pathway, overpowers it -only humans and primates are sensitive to miraculin -can get them online! -effects can last up to an hour -used in some African countries to cover up sour taste of bad milk, so it will taste sweet Bitter, Sweet, Umami Taste -G-protein coupled second messenger pathways -bitter taste is signaled by T2R (T=taste, 2=second taste discovered, R = receptor) -about 30 T receptors -often associated with harmful stimuli, poisons, aversive stimuli -not known all of the chemical structures elicit the taste (alkaloids) -genetic variation (prop) -N6 -sweet taste is signaled by type II cells' heterodimer of T1R2 and T1R3 -studying tip: "23" vs. "13" for umami -each subunit has 7 transmembrane alpha-helices -cats are missing T1R2, do not taste sweet stimuli -gymnemic acid: taste modifier that blocks sweet receptors -umami is signaled by type II cells' heterodimer of T1R1 and T1R3 -savory -fish, limes, tomatoes, sun-dried tomatoes, cheese, wine, shrimp, meat -foods rich in glutamate -fish sauce, parmesan cheese -pizza has more umami stimulation than food from a Chinese restaurant, due to tomatoes AND cheese -pathway of transduction is all the same, difference is a receptor -beta-gamma subunit is important for taste here, when activated it stimulates phospholipase C beta 2, producing IP3 and diacyl glycerol -IP3 binds IP3 receptor in smooth ER, causing calcium to be released from the intracellular stores -opens up the TRPM5 channel, and sodium enters -depolarizes the cell, opening up more voltage-gated sodium channels generating an action potential -causes the COM1*(CHECK) channels to release ATP, which acts on P2 receptors in the afferent nerve fiber -white strip of paper -super tasters: very bitter taste, hard to stand, aversive -tasters: bitter but tolerable -no tasters: do not taste the bitter flavor -taster status and number of fungiform papillae are correlated -super tasters: many fungiform papillae, small in size -non tasters: a few fungiform papillae, large in size -salt can be a modifier for bitter stimuli when cooking -tonic water + salt -margaritas? Doritos vs. Fritos -Dorito -cheddar cheese, monosodium glutamate, Romano cheese -feast of umami -Frito -corn and salt -bbq potato chips vs. regular potato chips -bbq potato chips have more umami, you want more of the bbq ones Is Chinese Restaurant Syndrome Real? -from the amount of MSG consumed in the restaurant -more MSG in a pizza How is Taste Information Encoded? -different receptors respond preferentially to 1 of the 5 basic tastes -taste receptors are distributed across the tongue -labeled line coding -one taste cell responds to one primary, and all of the taste cells responding to a single primary would synapse onto the same nerve fiber -controversies -major unresolved question 1: recent research in kinnamon's lab supports the LLC hypothesis -T1Rs and T2Rs found in different cells, also supporting LLC hypothesis -some taste cells respond to more than one stimulus, against LLC hypothesis -salty and acidic stimuli -bitter and sweet stimuli -across fiber pattern coding/ population coding -relying on higher up in the brain to take this information and to sort out the different stimuli by comparing the input from 100s to 1,000s of nerve fibers -current belief: taste coding is a combination of both LLC and AFPC Taste Preferences- Learned and Innate -we learn to like things we did not when young (beer, vegetables) -other preferences are innate (lemons)


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