Taste System

Reception

receptor absorbs stimuli

Transduction

stimuli converted from physical energy to neural activity (change in receptor cell potential)

Coding

action potentials represent change in patterns of environmental energy and carry info to the brain

Stimulus meaning

identification of stimuli (sensation to perception)

Four basic functions of each system

1. Reception

2. Transduction

3. Coding

   1. Stimulus Meaning

Stimulus energy physical

vision, audition, somatosensation

Chemical energy

taste, smell, pain/chemesthesis

each system contains multiple

synaptic relay nuclei (CNS and PNS)

each system has several

distinct sub-pathways (what/where)

each pathway is

topographically organized

most pathways cross

the midline

Type I Taste Cells

glial-like, express glial markers

Type II Taste Cells

Taste Receptor Cells (release NT-like substance), express G-protein-coupled receptors, molecules involved in non-classical synaptic transmission

Type III Taste Cells

presynaptic cell, express classical synaptic transmission markers (contain vesicles, Ca2+ channels, SNARE, etc)

taste cells aggregated into

taste buds

taste bud

about 70-150 individual taste cells, aggregated into onion-like configuration

taste buds by the teeth

nasoincisal taste buds

taste buds by esophagus

oropharynx/oropharyngeal taste buds (solitary buds)

taste buds in the middle of the tongue

circumvallate papillae and foliate papillae (500 TB/papilla)

taste buds at tip of tongue

fungiform papillae (1-2 TB/papilla; 500 papillae)

papilla around each taste bud

filiform papillae

Taste Cell Microvilli

hair-like extensions off of taste buds

lingual epithelium

cells that line the tongue

axons that connect to taste bud nuclei

taste primary afferent axons

palatal TB nerve

Greater Superficial Petrosal Nerve (GSP)

oropharyngeal TB nerve

vagus (CN X)

foliate and circumvallate papillae TB nerve

Lingual-Tonsilar Glossopharyngeal (CN IX)

fungiform papillae TB nerve

Chordatympani Branch of Facial Nerve (CN VII)

Where do the nerves all meet?

Floor of 4th ventricle in the nucleus of the solitary tract in the medulla

perceptual taste

detection of type of food

visceral taste

flavor

CT and GSP axons respond to

appetitive

LT-IX axons respond to

aversive

vagus axons respond to

intense salt and sour

Sweet

saccharides, calories from carbohydrates (macronutrient)

bitter

plant alkaloids, ions, terrenes; poison, vitamins (micronutrient)

salty

NaCl and other cations, esp. monovalent; electrolytes and water balance

sour

H+, spoilage, vitamins/carbs

Umami

L-amino acids, esp. glutamate, protein (macronutrient)

Fat

triglycerides, lipid calories

Sweet Receptor

Type II cells that express TAS 1 R2 and R3

Umami Receptor

TAS 1 R1 and R3; evidence also that isoform of MGLM (metabotropic glutamate receptor type 4) R4 sense amino acids; most in anterior part of the oral cavity

IP3R

ligand-gated Ca2+ channel

TRPM5

Transient Receptor Potential, Melastatin 5 cation- selective (Na+ and Ca2+)

Bitter Cells

express in single member of the TAS 2 R family (about 40 receptors in humans)

Salt

appears to be subtype of Type II cells that do not expressTRPM5