P - Taste and Smell

how are taste buds organized?

- 3,000-10,000 buds/tongue scattered throughout 3 types of papillae - 50 taste cells/bud - each taste cell has its own afferent nerve fiber (1:1 ratio) → taste cells have microvilli that stick out through a taste pore into the oral mucosa - picks up taste - taste cells are replaced every 7-10 days → stresses on taste cells can come from heat, infections, mechanical damage, and toxins → sustentacular (supporting cells) keep cells alive by ensuring that mitosis is happening properly

importance of gustation (taste) and olfaction (smell)

- allows us to differentiate between danger (toxins) and nutrition in foods - both are tied to the CNS (caudal orbital cortex) -- act as a reward center -- induce emotional and behavioral responses (food drive)

Signal transduction beyond receptor cilia (smell)

- each olfactory receptor cell expresses one single odorant receptor gene → all the receptors on all its cilia are the exact same - olfactory receptor cell nerves release glutamate onto olfactory bulb glomeruli Glomeruli = well defined micro-regions that receive info from specific zones; ~2000 - in picture, purple = 1 zone, orange = another zone, green = another zone - 1 glomerulus → 1 mitral cell → mitral cell goes to cortex (which is also organized into micro-regions)

Brain regions for olfaction

- usually other special senses pass through the thalamus before going to cortex but smell doesn't do that??? the olfactory tract (coming from olfactory bulb) can go 2 places: Medial olfactory area: the axons will be projected to hypothalamus or primitive response areas - primitive responses: salivating or licking lips in response to smell of food OR Lateral olfactory area: sends axons to less primitive areas; sends to 2 areas → amygdala/hippocampus: emotional responses and civilized responses - emotional = smell of food your mom used to make = happy girl - civilized response = likes and dislikes that make you sick → thalamus/ orbitofrontal cortex: long-term memories - odor memories and conscious likes and dislikes that don't make you sick

CNS taste transmission

1. sensory afferents within 3 CN innervate taste buds activation of taste buds excite gustatory aff fibers that go to: - anterior 2/3 tongue thru facial n (trigeminal n + chorda tympani) - posterior 1/3 tongue thru CNIX - posterior pts of mouth (epiglottis) thru CNX 2. all signals go to gustatory nucleus (nucleus of solitary tract) in medulla where they synapse 3. ventral posterior medial nucleus (thalamus) - 1/3 of info crosses but 2/3 stays ipsilateral -- synapse here 4. from thalamus to gustatory cortex in ventral pt of postcentral gyrus

How does desensitization to odors occur?

2 ways: 1. Adaptation in the receptors cAMP activates Ca++/calmodulin dependent protein kinase II (CaMK) → after a while, CaMK: - inactivates adenylyl cyclase → reduces cAMP → closes Na+/Ca++ channels - activates phosphodiesterase to cleave cAMP → repress opening of cation channels - we see a 50% reduction after ~1 sec (very fast response) 2. Adaptation in CNS - granule cells (GR) provide CNS negative feedback inhibition (lateral inhibition) of the afferent olfactory bulb via synapses on mitral cells → reduction of odors persistent a few minutes (more long term)

supertasters

25% of population - heightened sensitivity (10-100x taste bud density = more taste cells) - usually only affects one taste - increased sweet sensitivity = decreased BW and CVD - increased bitter sensitivity = increased risk for cancer

Hyperosmia

= increase in sense of smell genetic thing where you have more olfactory epi → more olfactory receptors → stronger sense of smell

Anosmia and hyposmia

= loss or decrease in sense of smell - frequently associated with aging (with age, basal cells don't do as good of job replacing olfactory neurons - Congenital (less olfactory epi for ex) - Head trauma (damage a part of smell conduction pathway) - Upper respiratory tract infections/allergies: ↑ mucus production → ↑thickness → harder for odorants to go across mucus and bind to receptors on cilia - Drugs: inhaled glucocorticoids (cause apoptosis of bipolar cells), radiation/chemo (in areas of olfactory pathway -- face, brain), streptomycin, codeine

which neurotransmitter is used to transmit taste sensations?

ATP blocking ATP → no taste - there are other modulators to go from bud to brain

ageusia and hypogeusia

ageusia = absence of taste hypogeusia = diminished taste causes: - lingual/glossopharyngeal nerve injury or olfactory dysfunction - neurological disorders (Bell's Palsy, MS) - drugs (chemotherapeutics, ACE inhibitors, anti-histamines, anti-depressants, anti-cholinergic) → cisplatin (chemo), captopril (ACE inhib) → anti-cholinergics = decreased saliva production - poor oral hygiene (increased bacteria), tobacco/alcohol, aging (death of taste cells) - Vit B3, zinc deficiencies - anti-histamines, anti-depressants, and anti-cholinergics tell us that those NT are involved in transducing taste in CNS

dysgeusia

aka parageusia (unpleasant perception of taste) - metallic, salty, foul, or rancid taste (chem/radiation), or alterations in taste sensitivity - 5-HT (serotonin) and NE reuptake inhibitors usually temporary; taste can normalize after a while

taste cell receptors

all heterodimeric umami - T1R1/T1R3 sweet - T1R2/T1R3 bitter - T2Rs sour - PKD1L3/PKD2L1 - Otop1 salt - ENac/VR-1

Peanut butter and alzheimers

decreased ability to smell peanut butter smell is associated with alzheimers because the area in the brain where peanut butter is perceived is close to there damage occurs in early stages of alzheimers?

taste cell degeneration

due to afferent nerve fiber damage - nerve fibers send trophic factors to the taste cells to keep them alive and ensure proper mitosis with age ( >45+), you lose trophic factors → degeneration of taste cells (change in taste sensations) taste cells + innervation = survival

labelled-line model

each taste cell expresses receptor for one 1' taste there is specificity btw each cell type, receptor, and info that afferent sensory neuron transmits convergence hypothesis - some cells have shared afferent connection so at high concentrations, both will be stimulated

taste transmission - leading theory

each taste fiber responds to one stimulus from each taste cell in a single taste bud CNS receives input from a population of taste receptors w distinctive pattern of response response pattern across many fibers then encode for particular taste sensation binding of taste receptor to taste substance either directly opens ion channels or uses 2nd msger to open ion channel

3 types of papillae

foliate - margins of tongue, taste buds on sides, associated w/ von Ebner's glands circumvallate - posterior 1/3 of tongue, taste buds on lateral sides, associated with von Ebner's glands fungiform - anterior 2/3 tongue, taste buds on dorsal surface filiform papillae lack taste buds!! (tactile) von Ebner's glands = serous glands that secrete saliva

which chemical substances do we have taste receptors for?

found on tips of taste cells 13 probable chemical substances: - sodium (2) - potassium (2) - chloride (1) - adenosine (1) - hydrogen ion (1) - inosine (1) - sweet (2) - bitter (2) - glutamate (1) these receptors used in combination all code for the 5 primary tastes

where is taste perceived on the tongue?

fun fact, actually really really sad fact: the tongue map doesn't exist all five primary tastes can be perceived everywhere on the tongue --> this is bc taste is created by action potentials in the brain, not the tongue itself - there are more bitter receptors on back of tongue but still located throughout tongue

PTC in herbivores vs omni/carnivores

herbivores are not sensitive to PTC (bitter taste) - i think so they can actually eat the plants?? - have larger livers to get rid of all the toxins in some naturally bitter foods i have that herbivores have less bitter receptors so they have less food limitations omni/carnivores are more sensitive to bitter tastes

odorants

humans can distinguish - 100 smell sensation - organic odorants (3-20C) → 400k - 1 trillion odor molecules odorants must be - slightly water soluble (watery mucus) - slightly lipid soluble (membrane - helps get to cell wall) we have odor thresholds for different substances (methanol, we need a very high concentration to smell; T-butyl mecaptan, we need very low concentrations to smell so added to natural gases so humans can smell a leak)

Transduction mechanism of olfactory receptors

intensity of olfactory stimulation is proportional to the log of stimulus strength → increase stimulus strength = increase intensity of olfactory stimulation 1. Odorant binds to GPCR (Golf) 2. Alpha subunit of GPCR activates adenylyl cyclase → ↑cAMP 3. cAMP opens cyclic-nucleotide gated cation channel 4. ↑ cell permeability to Na+ (and Ca++) → depolarization 5. bc of ↑ Ca++, Cl- channels open → Cl- exits cell 6. Graded potential in receptor cilia - if depolarization reaches threshold, AP generated from soma to axon 7. Axon releases glutamate to stimulate olfactory bulb glomeruli Graded potential = more R stimulation → ↑ Na+/Ca++ channels open → more depolarization → bigger graded potential

fat taste perception

it is debated whether fat should be considered a taste - we have fat receptors in our mouth and GI system - fats are broken down by fat lipases - fats are generally energy rich - may play a role in obesity and ghrelin signaling

7 primary smell sensations

made up of 100 indiv smells - camphoraceous "essential oils from leaves" - musky "skunk or deer scent" - floral "flower" - pepperminty "menthol notes" - ethereal "ether / cleaning fluid" - pungent "acetone, mustard seed" - putrid "decaying"

How is taste perceived?

microvilli from taste cells are exposed to mouth contents - taste substances cause depolarization of taste cell and generate a receptor potential → activates gustatory afferent nerve fibers - binding of a tastant induces a strong immediate signal → weaker, continuous signal is transmitted as long as taste bud is exposed to taste stimulus each taste bud responds to one of five primary taste stimuli in low concentration - at high concentrations, most buds can be excited by two or more primary taste stimuli

olfaction - smell

oldest, least understood special sense poorly developed in humans - SA of olfactory membrane smaller in humans 350 olfactory receptors are known (not very specific) - largest family of genes known

olfactory epithelium

olfactory receptor neurons - 100 Million Olfactory cells (30-60 d lifespan) - 2.4 cm2 - support cells (keep olfactory receptor cells alive) and basal Cells (grow and divide to become olfactory receptor cells) cells have Olfactory Cilia (dendrites) = mucus layer - 4-25 per cell - cover epithelial layer - have odorant receptors - 1 receptor type / olfactory cell - 350 different receptors for odorants (low specificity) - mucus from Bowman's Glands acts like a solvent for odorants (enhances time they have to interact w olfactory receptor cells) Olfactory receptor cells (bipolar) --> Axons of Olfactory nerves --> olfactory bulb Unmyelinated axons bc they are so short and speed is not critical to its function

major innervation of papillae

post 1/3 (circumvallate/foliate) - glossopharyngeal nerve ant 2/3 (fungiform) - chorda tympani (CN VII)

What are the different types of potentials that occur in olfactory receptor cell

receptor cilia: graded potential (proportional to receptors activated) soma/cell body: spikes with graded potential?? Based on transmembrane difference potential produced by sensory receptors - idk what this means axon: spikes (no graded potentials anymore)

importance of saliva

secreted by salivary glands and von Ebner's glands - cleanses taste buds to allow for new taste sensations - has lingual lipase to break down fats (so fats can bind to their receptors better) -- salivary amylase in saliva likely has a similar role for sugars

sour taste perception

sources: - H+ ions - helps us detect spoiled and rotten foods

umami taste perception

sources: - L-glutamate - usually associated with meats, cheese, MSG - these foods are high in easily digested proteins w/ probiotics (fermented foods)

salty taste perception

sources: - NaCl - important in regulating ECF volume, blood pressure, perfusion i missed things ahahaha :)

bitter taste perception

sources: - long-chain nitrogen - alkaloids - usually found in quinine, caffeine, strychnine (poison-- seizures), nicotine - sensation of bitter taste is important in allowing us to identify toxins (ex: poisonous plants are rich in toxic alkaloids) -- may cause people to reject bitter foods - however, sulforaphane found in bitter foods (brussel sprouts, broccoli) has been proven to reduce risk of cancer -- those who reject bitter foods will eat less of these foods, thus increasing their risk of cancer

sweet taste perception

sources: - sugar - glycols - alcohols - aldehydes - ketones - some AAs - energy rich - naturally occurring sweets (ex: honey, fruits) are also rich in micronutrients

what helps us detect taste?

taste buds - have taste cells that respond to stimuli smell and texture (tactile receptors) also influence how we taste things

taste complexity and adaptation

taste: 5^5 = 3,125 possible combinations scientists think we are limited to 120 diff tastes (5x4x3x2x1) - able to distinguish btw 10000 chemicals - less discriminating than smell taste + other input = flavor flavor depends on - taste receptors (genetic diff) - odor receptors (80% of taste) - nocieptors - pain - transduce info abt cool/spicy stuff (mint/capsaicin) - thermoreceptors (cold/sweet- cold makes something seem sweet, spicy/hot- hoT makes something seem spicier) - texture/pressure - fat (fats solubilizes some spices) preference/aversion likely learned hence CNS mediated taste sensations adapt rapidly... buds v CNS?

thresholds for taste

threshold = [minimum] in order to perceive taste intensity of taste = log [x] usually need a 30% change in concentration in order to perceive a change in the intensity of the taste - humans are bad at detecting changes in taste sensitivity: bitter > sour > salt > sweet - lower thresholds for bitter and sour bc this allows us to detect toxins and rotten foods - higher thresholds for salt and sweet bc we need these for energy and ECF V maintenance - with sweet, most things aren't too sweet so that we develop an aversion, instead it is pleasant so we have emotional and behavioral changes that make us seek it out -- applies to salt too

taste cell transduction via multiple ion channels

umami/sweet/bitter - GPCR → PLC → IP3 → Ca release → open TRPM5 - Na/K entry → depolarization of indiv taste cells sour - direct ion entry - H+ ions enters w some K maybe → positive charges entering → depolarization salt- direct ion entry - Na, K, NH4, Ca can enter that channel → depolarization bc all positive charged

phenylthiocarbamide (PTC)

used to test ability to detect bitter tastes - done by licking PTC paper - those who have increased bitter sensitivity will be able to taste the PTC dominant trait