Taste and Olfactory Systems

Transduction Mechanism Saltiness

taste of salt is mostly due to the detection of Na+. Saltiness receptors express Na+ channels (Amiloride- sensitive channels). Detect Na+ at low concentrations High concentration of salt can activate also other receptor cells (for sour and bitter)

Neural pathways for Taste

primarily ipsilateral -*primary neuron* carries input form the tongue to Solitary Nucleus (afferent fibers carried in by VII, IX and X, SVA component) Secondary -*secondary neuron* In the Gustatory nucleus (special part of the Solitary Nucleus that process Taste) projecting to the VPM in the thalamus -*third neuron* In the VPM, connects to the insular cortex

5 basic Taste modalities

*Sweet*: sugars (fructose, sucrose, etc.) *Bitter*: (quinine or caffeine) - can be detected at nanomolar concentrations. Poisonous substances are bitter (triggered by K+, Mg2+, and complex organic molecules) *Umami*: taste of the amino acid glutamate (monosodium glutamate) *Sour*: acids, vinegar, Citrics (triggered by H+ ions) *Salty*: Salts, (triggered by Na+ ions)

Taste Receptor Proteins G-protein coupled receptors

-30 bitter receptors - T2R -Attractive tastes - T1R family (R1,2, 3) • Sweet Receptor cells express: T1R2 + T1R3 receptors required for sweet taste - T1R3 - calcium • Umami receptor cells express a combination of T1R1 +T1R3

Integration of Olfaction and Taste

-each food activates a different combination of the basic tastes - combined taste and smell occurring simultaneously -other sensory modalities contribute to a unique food- tasting (flavor) experience. Texture, Temperature

Transduction Mechanisms

Bitter, Sweet and Umami- share the same transduction mechanism, using receptor proteins of the T1R and T2R families (G-protein coupled receptors), and use ATP as neurotransmitter. Sweet and Umami receptor expressed different combinations of T1Rs proteins: this difference allows distinction of Sweet vs Umami

Ensemble coding

Cells that respond to different odorants are diffusely distributed in pyiriform cortex - no spatial organization

Taste receptors cells and signal transduction

Chemoreceptors for taste are located on the apical end of taste cells (in the microvilli) Binding of substance to receptor depolarizes the taste receptor, either allowing Ca2+ influx, or causes release of Ca2+ from intracellular stores (depending on the transduction mechanism) *Serotonin is neurotransmitter for Sour and Salty ATP for bitter, Umami and Sweet*

Input convergence in the Glomeruli

Convergence of primary neurons Synapsing onto second-order neurons in glomeruli 25000 primary onto 100 second- order- mitral and tufted cells

cortical processing of odors

Different odorants can activate unique pattern of neurons in Pyriform cortex (Olfactory cortex) Activation patterns of different odorants can overlap, since one neuron can respond to multiple odorants

Neural pathways for Olfactory inputs

LOOK @ SLIDES *Olfactory Cortices (temporal lobe)* Primary Olfactory cortex, and Entorhinal cortex (primitive cortices from evolutionary perspective) - involved in odor detection, motivation, emotion, memory Other olfactory areas: Amygdala *Conscious perception* Olfactory tubercle Medial dorsal nucleus Medial dorsal nucleus Orbitofrontal cortex

Location of Olfactory receptors

Olfactory epithelium - stratified - 3 main cell types *Olfactory receptor neurons* - Supporting cells (similar to glia produce mucus) * Basal cells* - source of new receptor cells (stem cells). Replenish the Olfactory receptors cells every 1-2 months *Mucus *- odorants dissolve in mucus before reaching receptor cells - water base with muco-polysaccharides, proteins - antibodies, enzymes, odorant binding proteins and salts

Transmission and termination of Signal

Olfactory response terminates: 1. odorant diffuses away 2. enzymes in mucus break them down odorant molecules 3. termination of signaling by activation of other mechanisms (e.g. cAMP can activate other pathways that stop transduction signal) 4. adaptation (the response of the receptor cell itself decreases despite the continuing presence of the odoran

Taste receptors

Papillae are protrusions in the tongue shaped like ridges, or pimples. Each papillae has from 1-100 taste buds. Taste buds have 50- 150 receptor cells

Location of Taste receptor organ

Primarily located in the Papillae of the Tongue, in receptor organs called Taste buds In addition, there are receptors located in the Palate, Pharynx, Epiglottis, and Nasal cavity The whole tongue is sensitive to all basic tastes

different types of Odorant receptor proteins

Receptors broadly-tuned - responds to many odorants, but show preferential response to some odorants. Each glomerulus receives input from cells expressing the same olfactory receptor proteins

Taste receptors are partially selective for particular chemicals

Tastants (molecules producing taste stimuli) must be reach a minimum concentration of (Threshold concentration) to elicit and taste stimulus that will evoke a perception of taste. (Too low concentration will not produce taste sensation) *Receptors could respond more or less selectively to a taste depending on the concentration.* At concentrations just above threshold, most papillae tend to be sensitive to only one basic taste. At higher concentrations, papillae become less selective

Coding of taste/olfactory modalities: Population coding

Taste and Olfactory systems uses the responses of a large population of receptors and neurons to encode a specific stimulus (*Population coding*) receptors are broadly tuned (can respond to multiple stimulus) a particular smell (or taste) is encoded by the combinatorial responses of many different receptor cells, and processed in the cortex by a large ensemble of neurons allows to detect an almost unlimited amount of chemicals (odors and tastes) with a limited amount of receptors

smell

besides gathering information from the environment is a mode of communication between individuals Important signals: Reproductive behaviour, Territorial boundaries, Identification, Aggression

Signal transduction in olfactory receptor cel

odorant binds to receptor proteins in the membrane (G-protein couple recptors)*->* stimulate G-protein couple *->*activates adenylyl cyclase*->* cAMP *->* activates cAMP gated cation chanels *->* Na+ and Ca 2+ flow inside *->* Ca2+ dependent CL- channels open *->* depolarization

Transduction Mechanism sourness

sour taste is mostly due to the detection of H+ from Acids (HCl, Acetic acid,etc). Sour receptors express PKD2L1 (of the TRP receptor superfamily), special type of K+ channel blocked by H+