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 100-200 buds• Each taste bud has 50-150 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; 50-150)• Taste Cells (innervated by gustatory

afferent axons of CN 7, 9, 10)• Basal cells synapse with axons & taste cells

Taste Buds

• Normal range is 2000-5000 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

• 5th 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 2nd 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 2nd

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