Taste and Olfaction

Anatomy and Physiology II

BIO 232

Chemical Senses

• Gustation (Taste) and Olfaction

• Chemoreceptors

• Taste - food chemicals dissolved in saliva

• Smell - airborne chemicals that dissolve in fluid coating the nasal membranes

Taste Buds and Gustation

• Sweet - sucrose, glucose, fructose, aspartame & etc.

• Sour - most acids

• Salty - most salts

• Bitter - quinine, caffeine, K⁺ & Mg⁺⁺

• Umami - (delicious in Japanese) - savory - MSG or arginine

Distribution of Taste Receptors

Papillae of the Tongue

• Types

• Filiform- ridge-like structure

• Virtually no taste buds

• Vallate - pimple-like structure

• Fungiform - mushroom-like structure

• Each has 1 to several hundred taste buds

• 50-150 taste receptors/taste bud

• Typically - 2000 to 5000 taste buds

• Range: 500 - 20,000

Taste Buds on the Tongue

Taste Receptor Cells

• Taste pore - opening that exposes the receptor to the contents of the mouth

• Apical end - microvilli

• Synapse with CN VII (distal) & CN IX (proximal)

• CN X - innervates soft palate

• Life span about two weeks

• 90% of the taste receptors respond to more than one taste quality

• Receptor potential - in response to appropriate chemical stimulus

Taste Receptors Response

Mechanism of Taste Transduction

• Salt taste - Na⁺ influx through sodium channels followed by Ca⁺⁺ influx

• Sour taste - mediated by H⁺ ions and blockade of K⁺ (or Na⁺)

^• Bitter and sweet - are mediated by G protein mediated mechanism that act via second messengers to promote depolarization by increasing intracellular Ca⁺⁺

Gustatory Pathway

Taste is Multimodal

• Taste is 80% smell

• Blocked olfactory receptors (e.g. stuffy noise) makes food taste bland

• Also involved are thermoreceptors, mechanoreceptors and even nocioceptors

Olfactory Receptors

Olfactory Transduction

Olfactory Pathway

• In the olfactory bulb - glomeruli which are globular tangles of dendrites & axons

• Input - axons of olfactory cells - each terminates in only a single glomerulus

• Typically tens of millions of olfactory cells - only 2000 glomeruli

• 5000 to 10,000 olfactory neurons converge on a single glomerulus

• Glomeruli receive input from olfactory neurons which respond to similar odors

Olfactory Pathway

• Therefore different odors excite different receptors which activated different sets of glomeruli

• The output cells of the olfactory bulb are mainly Mitral cells - form olfactory tracts

• Then via the thalamus to the olfactory cortex

• Also pathway that connects to the hypothalmus and limbic system - emotional and autonomic response to odors