19 Taste & Olfactory System

5 basic taste modalities

sweet, bitter, umami, sour, salty

sweet taste due to what?

sugars such as fructose and sucrose

bitter taste due to what?

poisonous (at some level) substances, ie quinine or caffeine

trggered by by K+, Mg2+

umami taste due to what?

taste of amino acid L-glutamate (monosodium glu, MSG)

sour taste due to what?

triggered by H+ ions (acids, vinegars, citrics)

salty taste due to what?

triggered by Na+ ions

how do we taste a wide variety of flavors even though there’s only a few taste types?

1) foods activate different combination of tastes

2) distinctive flavor comes from taste and smell occurring simultaneously

3) other sensory modalities contribute to unique food-tasting experience (temperature, texture, nociception)

where are taste receptors

in taste buds (which are receptor organs) on tongue papillae

also some in palate, pharynx, epiglottis, nasal cavity

how is taste divided on the tongue?

it’s not! the whole tongue is sensitive to all tastes; some areas are more sensitive to some tastes, but all tastes are everywhere to at least some extent

what are papillae

protrusions on the tongue (ridge-like) containing taste buds that contain receptor cells

how many taste buds on a papillae? Taste receptor cells in a taste bud?

1-100 taste buds/papillae

50-150 rec cells/taste bud

where are taste chemical receptors

apical end of taste cells in microvilli (in taste buds)

how are taste receptors activated

substance binding to the receptor depolarizes it, causing Ca2+ influx and release of neurotransmitters

neurotransmitter for sour and salty tastes

serotonin

neurotransmitter for bitter, umami, and sweet tastes

ATP

what are tastants

molecules producing taste stimuli

how/when do tastants evoke taste stimuli

they must reach minimum concentration (threshold) to elicit stimulus evoking taste perception. A single cell may response more or less selectively to a taste depending on concentration

when are taste receptor cells more/less likely to distinguish a single or multiple tastes

when tastant concentration is just above threshold will likely perceive 1 basic taste. at higher conentration, less selective

how selective are taste receptor cells? ~ how many of them respond to multiple basic tastes?

partially selective

over 90% of taste receptor cells respond to 2+ basic tastes, and some respond to multiple chemicals

transduction mechanism in taste receptor cells for salt, sour

tastant passes directly through ion channel

transduction mechanism in taste receptor cells for just sour

tastant binds to and blocks ion channel

transduction mechanism in taste receptor cells for bitter, sweet, umami

tastant binds to GPCRs in membrane that acts as a second messenger system that opens ion channels

transduction of saltiness/Na+

taste receptors detect Na+; have Na+ channels that are amiloride-sensitive, can detect at low concentrations

high Na+ concentration can also activate receptor cells for sour/bitter

activation triggers serotonin nt release!

transduction of sourness/acids

taste receptors detect H+ from acids

express special type of K+ channels that is blocked by H+

nt released is serotonin

receptor proteins for bitter/sweet/umami transduction

T1R & T2R GPCR families

second messenger cascade for bitter/sweet/umami transduction

GPCR activated cascade initiated by IP3 (inositol triphosphate)

how are bitter/sweet/umami differentiated if they use the same GPCRs and 2nd messenger cascade?

GPCRs for these three tastes are expressed in different receptor cells, which are connected to different gustatory axons

parallel processing of bitter, sweet, umami

each flavor is transmitted preferentially in different transmission lines

bitter receptors at T2Rs and highly expressed in bitter receptor cells

sweet and umami receptors express different combinations of T1R proteins; different combos allow sweet v. umami distinction

sensitivity to bitter taste, which receptors?

30 bitter/T2R receptors; have low discrimination capatcity between different bitter subtypes, but a high sensitivity to detect presence of bitter and signal potential risk

poison receptors are T2R

T1R family of receptors signals what

attractive tastes (R1, R2, R3)

sweet receptor cells’ channels

T1R2 and T1R3 (calcium), required for sweet taste

umami receptor cells’ channels

T1R1 and T1R3

taste neural pathway from receptors to primary neuron

input from tongue receptors → solitary nucleus via nerves VII, IX, X (SVA)

taste neural pathway from primary neuron to secondary neuron

goes to gustatory nucleus (part of solitary nucleus processing taste) and projects to VPM in the thalamus

taste neural pathway from secondary neuron to 3rd neuron

in gustatory part of VPM, connects to insular cortex

general taste neural pathway

anterior/posterior tongue & epiglottis → gustatory nucleus → VPM → gustatory cortex

besides VPM, where else does gustatory nucleus project to?

other parts of solitary nucleus and other cranial nerve nuclei

hypothalamus and limbic structures

gustatory nucleus projecting to solitary nucleus/cranial nerve nuclei used for what?

swallowing, gagging, vomiting, salivation, digestion, respiration

gustatory nucleus projecting to hypothalamus and other limbic structures goes where, used for what?

to lateral hypothalamic area and tuberal area; controls food intake and motivation to eat

why is olfaction generally important

it gathers info from the environment and is a mode of communication between individuals - important in reproductive behavior, territorial boundaries, identification, and aggression

3 main types of cells in olfactory epithelium

olfactory receptor neurons

supporting cells (similar to glia, they produce mucus where odorants dissolve)

basal cells, which are the source of new receptor cells/stem cell

steps for olfactory receptor cell transduction

1) odorant binds to receptor protein in membrane (GPCR)

2) GPCR stimulated

3) activates adenylyl cyclase

4) activates cAMP

5) activates cAMP-gated cation channels

6) Na+ and Ca+ flow into cells through those channels

7) Ca2+-depending Cl- channels open, Cl flows out, depolarization (olfactory receptor cells have unusually high interior Cl- concentration)

adaptation of odor receptor cells

response of receptor cell itself decreases despite continuing presence of odorant

ways to terminate olfactory responses

1) odorant diffuses away

2) enzymes in mucus break down odorant molecules

3) signal termination by activating other mechanism (such as cAMP activating other pathways that stop the olfactory transduction signal)

convergence of primary neurons to glomeruli → pathway, #s?

olfactory receptor cells = primary neurons, they synapse onto second order neurons (mitral cells) in glomeruli.

about 25,000 primary neurons project onto 100 mitral cells

what kind of correspondance of axons from receptors into glomeruli?

precise correspondance; receptors expressing specific receptor genes all converge onto same glomeruli

ex: cells expressing P2 (mouse receptor gene) have synapses specifically in 2 glomeruli of olfactory bulb

what does it mean that odor receptors are broadly tuned

they respond to many odorants but show preferential response to some odorants → each glomeruli in olfactory bulb receives inputs from cells expressing the same type of olfactory receptor proteins

what is population coding

taste and olfactory systems use response of large population of receptors and neurons to encode specific stimuli

where do mitral cells in glomeruli project to

they project broadly; go to many neurons in pyriform (primary olfactory) cortex

neural pathways for olfactory inputs

olfactory bulb → secondary sensory neuron cell bodies in glomeruli

olfactory tract → connects olf bulb to primary olfaction area

system is connected to limbic structures; mediates emotional response to smell. olfactory cortex of amygdala, entorhinal cortex

olfactory cortices and temporal lobe structures

primary olfactory cortex and EC → primitive cortices evolutionarily → involved in odor detection, olfactory-related emotion, motivation, aggression

additional pathways for olfactory perception

olfactory tubercle → medial dorsal nucleus → orbitofrontal cortex

what is flavor

integration of olfactory and taste

how does flavor happen

each food activates different combinations of basic tastes

simultaneous taste and smell, integration occurs at insular cortex

other sensory modalities (texture, temperature) contributes to tasting experience

integration of odor and teste sensations in brain enables flavor detection