Taste/Gustation Detection of Chemicals and Regulation of Ingestion

Chemical Sensation Oldest sensory system Bacteria detect and move toward chemical food source We taste chemicals in food Our cells bind and respond to chemicals within our bodies

Chemosensation Taste & Smell = conscious awareness of chemicals Nerve endings in skin and in mucous membranes react to irritating chemicals Nerve endings in digestive tract respond to chemicals Receptors in aorta measure carbon dioxide and oxygen

Organs of Taste Tongue, epiglottis, palate, pharynx Taste is due to chemicals, texture, temperature and pain and smell Taste cells and somatosensory receptors

Combination of Receptors Complex tastes arise from activation of multiple receptors at once Smell of food contributes to distinction of taste Texture and temperature and pain—capsaicin from hot peppers Vision also participates in food selection and in enjoyment and expectation-emotional response to food

Cravings Body can detect the absence of certain chemicals and create cravings for them Food Allergies: allergic to foods you crave or “can’t live without” Due to abnormal flora in gut that creates craving for energy source for that bacteria

Chemotransduction Detection of chemicals in the environment (food) Chemicals activate chemoreceptors that transiently alter membrane potential of taste cell Called a receptor potential Can be depolarizing or hyperpolarizing

Papillae On tongue surface are protrusions (bumps) with different shapes (ridges, pimples, mushrooms) Each papilla is a collection of buds Each taste bud has taste receptor cells arranged as orange sections Taste cells=1% of tongue epithelium:rest is basal cells and gustatory afferent axons

Organization of Taste Organ Papillae (contain taste buds; 100s) –Vallate (pimple) –Fungiform (mushroom) –Foliate (ridges) Taste buds (contain taste cells; ) Taste Cells (innervated by gustatory afferent axons of CN 7, 9, 10) Basal cells synapse with axons & taste cells

Taste Buds Normal range is taste buds Can be as little as 500 or as many as 20,000 90% of taste cells respond to 2 or more chemicals Allow for population coding

Taste Cells Do not have axons—are like hair cells that are innervated by sensory axons which receive excitatory input from taste receptor cells within taste bud

Taste Cell Life Cycle 2 weeks—growth, death, regeneration Requires afferent innervation If axon is damaged, then taste cell degenerates

Taste Cell Anatomy Apical End-membrane region near tongue surface Has microvilli that project into the taste pore Taste cells have synapses with endings of gustatory afferents near bottom of taste cell

Taste Cells Taste bud contains 100 taste receptor cells Saliva has low Na+ concentration –microvilli on apical end of taste cell detect chemicals in the aqueous (saliva) environment

Taste Cells –Basal Cells Taste cells have electrical and chemical synapse with basal cells Basal cells can synapse with gustatory afferents Form information processing circuit within taste bud

Modalities of Taste Only 4 components to taste –Salty=High sodium ions –Sour=acidic compounds=high protons –Bitter=amino acids & other organics, K+, caffeine –Sweet=sugars s.a. sucrose 5 th Taste: Umami=japanese for “delicious”= MSG or taste of glutamate

Bitter Sour Salty Sweet

Transduction Tastant: taste stimuli Transduce the taste by –Directly passing through ion channel (salt & sour) –Bind and block K ion channel (sour & bitter) –Bind and open channel (amino acids) –Bind receptors that activate 2 nd messengers that open or close ion channels (sweet, bitter umami)

Saltiness Taste of Na+ Na+ selective ion channel blocked by amilioride, insensitive to voltage; always open As you eat salty food the external Na+ increases and Na+ flows into cell through channel Directly depolarized membrane

Sourness High acid foods taste sour (low pH) HCl generates H+ ions Transduced by –H+ passing through amilioride sensitive Na Channel, Depolarizes cell (can’t tell salt from sour) –H+ binds weakly & blocks K+ channels & causes depolarization; at normal ph channel open

Sweetness Sweet transduced by –Binding specific receptors & activate 2 nd messenger cascades –G protein triggers formation of cAMP, activation of PKA, phosphorylation of K+ channel (not sour channel) and closes it leading to depolarization –Cation channels directly gated by sugars

Bitterness Bitter receptors detect poisons Transduced in many ways –Quinine (bitter,tonic) & Ca++ bind to K+ channel and block them –Bitter receptors that activate G proteins that lead to increased IP3 levels & modulates NT release without depolarizing cell—directly causes Ca++ release from intracellular stores

Amino Acids Umami—glutamate, aspartate Glutamate transduced by –Permeating Na/Ca ion channel, depolarizes, opens voltage gated Ca channel that triggers NT release –Binds G-protein coupled, decreases cAMP –Arginine and proline gate their own channels

Receptor Potential Hyperpolarization or Depolarization caused by activation of taste cell Depolarization causes calcium channel opening Triggers NT release at synapse with afferent neuron (unknown NT) Causes AP in afferent sensory axon

Threshold Concentration Concentration of a basic chemical that registers a perception of taste At low concentration, papilla are very sensitive but at high concentration they respond to all stimuli

Perception of Taste One afferent axon gets input from many different taste cells each maximally responsive to combinations of taste Population Coding: Groups of broadly tuned neurons specify taste rather than single finely tuned taste cells and neurons.

Population Coding Analysis of the response of population of cells to particular food Some nerve cells will increase or decrease the rate of firing Cortex discerns what the overall pattern of activation is and decides you ate chocolate

CNS Pathways

Central Taste Pathways Taste bud- brain stem-thalamus-cerebral cx 3 CN carry taste –Anterior 2/3 of tongue have afferents in CN7 facial nerve –Posterior 1/3 of tongue have afferents in CN9, the glossopharyngeal –Epiglottis, pharynx, glottis have axons in CN10 vagus

Gustatory-Solitary Nucleus In Medulla-first synapse for taste afferents is the gustatory nucleus that is part of nucleus solitary

Thalamus-CNS From Gustatory nucleus to ventral posterior medial (VPM) nucleus of thalamus (sensory for head) To Broadman area 36 above temporal lobe = Primary Gustatory Cortex To insula cortex Uncrossed & Crossed pathways from CN to CX

Gustatory Projections Projects to nuclei in medulla involved in swallowing, salivation gagging, vomiting, digestion and respiration Hypothalamus & amygdala involved in controlling eating Lesions to amygdala can cause animals to ignore food or overeat

Somatosensory Inputs The tongue in also innervated by afferents for touch temperature and pain that contribute to recognition of foods by texture and heat Travel to primary somatosensory cortex in post central gyrus

Additional CNS Circuit Nucleus Solitary to Pons –Pontine Taste Nucleus to Hypothalamus For feeding regulation To Amygdala for emotional connections To Thalamus for Taste perception Primitive Pathway