Why do we have a sense of taste?

• protection

palatable or poisonous?

• pleasure

taste and olfaction = flavour

• communication

olfaction important, taste less so

From Gray’s Anatomy

Sensors (taste buds) located in immediate vicinity of mouth

Tongue

Palate

Pharynx, oesophagus, epiglottis

Papillae on tongue bear taste buds

Usually 2000-5000 taste buds in humans

LM section of taste buds

From UT Houston Medical School

light, dark and basal cells

Taste buds

structure

cell types

cell lineage

cell turnover

cell number

Orange/onion structure

EM appearance - four types of cells

3H thymidine studies for determining cell “birthdays”

Very high10-13 days most species

Originally thought to be lowNow thought to be 50-100 cells per taste bud

Type I (dark cells) 55-75%

Extend base to apex of bud

Have dense granules in cytoplasm, partciularly apically

Irregular nucleus

Rough endoplasmic reticulum

Long branched apical microvilli

Type II (light cells) 20%

Extend base to apex of bud

Large oval nucleus

Smooth endoplasmic reticulum

Short apical microvilli

Type IV (dark cells)

Do not reach taste pore

Dense core vesicles

Rough endoplasmic reticulum

Precursor cell for other types? interneuron? or mechanoreceptor?

Type III (intermediate cells) 5-15%

Extend base to apex of bud

Similar to Type II, with dense cored vesicles in cytoplasm particularly at base of cell

Synapses with afferent nerves

Cell types in taste buds

How do we taste?

1. Transduction of taste stimuli by taste cells.

Taste buds found throughout oral cavity, oropharynx and epiglottis.

Processing of taste stimuli not simple - different tastes detected by different mechanisms, and processed in the taste bud before afferent nerves are stimulated.

Actually at least 5 tastes (if not more) and some are detected in more than one way.

Two new tastes? Or more?

umami - “deliciousness”

glutamate acts on a mutant glutamate receptor found in taste cells (found February 2000)

first identified as a taste in 1908

Fats

free fatty acids in mouth close K+ channel

specific fatty acid transporters directly increase [Ca2+]I

“thermal” tastes (2000)

How do we taste?

2. Communication of taste signals to the nervous system.

Innervation of taste buds

Anterior 2/3 - VII (in chorda tympani)

Posterior 1/3 - IX

Oropharynx, epiglottis - X

Somatosensory afferents - VEfferent innervation

Innervation of taste buds

primary afferent

papilla

Taste bud

Taste cellsTaste cells

papilla

Taste bud - 50-100 cells, 5-15 afferents

Each afferent innervates 5-10 taste cells

Afferents may innervate multiple taste buds, in multiple papillae.

How is the information decoded into taste?

Complex signalling in taste transduction

Taste cells - don’t know specificity of taste transduction

Taste buds - single buds respond to more than one taste stimulus

Primary afferents - respond best to one taste but may respond to all

NTS and thalamic neurons - also multiresponsive, but may be more “tuned” to one stimulus

Tastes 1o afferent NTS Thalamus Cortex1 30% 30% 40% 48%2 44% 44% 25%

3/4 26% 26% 35%52%

Olfaction

Olfactory epithelium

specialised epithelium above turbinate cartilage

receptor cells:

bipolar neurons - unmyelinated axons

cilia form dense mat above cells

new cells generated every 60 days from basal cells

Odorant detection

molecules dissolve in mucus film

diffuse to cilia/bind to carrier protein

bind to specific receptors on olfactory neurons (cells may respond to more than one odorant)

L-carvone = carraway

D-carvone = spearmint

specific anosmias

Odorant binds to receptor = cAMP generation leading to receptor potential due to direct opening of Na+ channels

Ability to cAMP directly related to ability to generate action potentials

Olfaction

Specific stimuli activate specfic receptors

One cell = one stimulus = one response - probably not

Projection to higher centres -

directly to cortex, then to limbic regions and other cortical areas