Neurology 400 - Special Senses: Taste

How long does adaptation to taste take to occur?

w/ continuous stimulation, adaptation occurs in *1-5 min*

What NT do Gustatory receptor cells use in synapse w/ 1st order neuron?

*Adenosine Triphosphate (ATP)*

Which papillae *w/ taste buds* makes up most of what we see on the surface of the tongue?

*Fungiform* papillae

What type of potential do Gustatory receptor cells generate?

*Receptor* Potential

What is the purpose of the different taste receptors?

*What we should eat and not eat* 1) *Sweet* tastes indicate *energy-rich* foods 2) *Salty* foods indicate *electrolyte-rich* foods 3) *Bitter/Sour* are considered *aversive* tastes (foods are either toxic or "gone bad") 4) *Umami* indicates food high in *amino acids* for building proteins

Where is the primary gustatory area of the brain?

*parietal lobe* of cerebral cortex (Brodmann's #43)

If all tastants cause release of NT from many gustatory receptor cells, why do foods taste different?

1) *Different tastes* arise from activation of *different groups of taste neurons* 2) Each individual gustatory receptor cell responds to more than one of 5 primary tastes and may respond more strongly to some tastants than to others

Where is the Gustatory nucleus located?

1) *Medulla* 2) rostral part of *solitary nucleus* (aka: nucleus solitarius/ nucleus of tractus solitarius in medulla)

Types of taste receptors

1) *Sweet:* Activated by *sugars (carbs), alcohol, some amino acids* 2) *Salt:* Activated by metal ions, commonly *Na+* 3) *Sour:* Activated by *H+ ions*, and therefore by *acids*. Stronger acid means lower pH and more sour taste 4) *Bitter:* Activated by *alkaloids such as caffeine, nicotine*, etc 5) *Umami:* Activated by *glutamate, as well as MSG and aspartate*. Meaty/Savory

What cranial nerves are responsible for taste?

1) *V (trigeminal)* serves *hot/cold, texture* 2)* VII (facial)* serves taste *anterior 2/3 of tongue* 3)* IX (glossopharyngeal)* serves taste *posterior 1/3 of tongue* 4) *X (vagus)* serves taste *palate & epiglottis*

How does *Sweet, Bitter & Umami* tastants enter gustatory receptors cells?

1) *do not enter* gustatory receptor cells 2) *bind to receptors in plasma membrane* 3) activate G proteins 4) activate 2nd messengers 5) cause depolarization & release of NT (ATP)

Circumvallate papillae

1) 12 very large structures form an inverted V-shaped row at back of the tongue 2) each houses 100-300 taste buds

What are the 3 types of papillae *containing taste buds*?

1) Circumvallate papillae 2) Fungiform papillae 3) Foliate papillae

Specific molecules that activate Umami

1) L-Glutamate 2) MSG

*Filiform* papillae

1) Papillae w/ *NO TASTE BUDS* 2) pointed, threadlike structures of tactile receptors 3) increase friction B/W tongue/food 4) allow ease of tongue to move food into oral cavity

What are the possible responses in the brain to taste?

1) Signals travel to *thalamus or limbic system & hypothalamus* 2) Taste fibers extend from thalamus to *primary gustatory area* on *parietal lobe* of cerebral cortex 3) provides conscious perception of taste

Taste overview

1) Taste is a chemical sense 2) To be detected, molecules must be dissolved 3) Taste stimuli classes include *sour, sweet, bitter, salty, and umami* (meaty or savory) - the latter stimulated by L-glutamate 4) All other flavours, e.g. chocolate, pepper and coffee, are combinations of the five primary tastes, plus accompanying olfactory and tactile sensations 5) Odours of food can pass upward from the mouth into the nasal cavity - stimulate olfaction (which is more sensitive than taste) - think of when you have a cold or allergies - cannot taste food because olfaction is blocked, not taste

What other senses are used to "taste"?

1) Temperature (Hot/Cold) 2) Texture 3) Smell

Describe the threshold for various tastes

1) Threshold for bitter (e.g. quinine) is lowest b/c poisonous substances are often bitter - low threshold (high sensitivity) to have protective function 2) Threshold for sour substances (e.g. lemon) measured by HCl - higher threshold 3) Thresholds for salty (NaCl) and sweet (sucrose) are similar and higher threshold than those for bitter or sour substances

What percentage of taste is actually smell?

1) Up to 80% 2) Loss of smell can lead to depression

How much of a molecule do you need in order to be able to taste it?

1) Variable 2) Bitter substances tend to be lowest (poisons are often bitter) 3) quinine (bitter) = 0.0000004 M 4) sucrose (sweet) = 0.02 M

Supporting cells

1) cells which surround 50 gustatory receptor cells in each taste bud 2) gustatory hair projects from each gustatory receptor cell to external surface through the taste pore

What contributes to taste adaptation?

1) changes that occur in taste receptors 2) olfactory receptors 3) neurons of gustatory pathway in CNS

How does H+ in sour tastants enter gustatory receptors cells?

1) flow into gustatory receptor cells via *H+ channels* 2) influence opening and closing of other types of ion channels (K+)

*Foliate* papillae

1) located in the small trenches of the lateral margins of the tongue 2) most *degenerate in early childhood* (QUIZ)

Basal cells

1) stem cells found at periphery of taste bud near connective tissue layer 2) develop into gustatory receptor cells or supporting cells

Each taste bud is an oval body consisting of three kinds of epithelial cells

1) supporting cells 2) gustatory receptor cells 3) basal cells

Classic distribution of taste receptors in humans is under debate

All parts of the tongue sense all diff tastes

Gustatory receptor cells

1) synapse with dendrites of 1st order neurons that form first part of gustatory pathway 2) dendrites of each 1st order neuron branch profusely and contact other cells in several taste buds

How does tastant trigger a nerve impulse?

1) tastant dissolves in saliva 2) makes contact with gustatory hair plasma membrane 3) result is *receptor potentials developed in gustatory hairs* 4) cause release of NT (ATP) that gives rise to nerve impulses formed in 1st order neurons

Where are taste buds found?

1) tongue 2) soft palate 3) epiglottis 4) pharynx 5) *lungs*

How does Na+ in salty food enter gustatory receptors cells?

1) via Na+ channels in plasma membrane 2) accumulation of ions cause depolarization 3) leads to release of NT

What is the life span of Basal cells?

10 days

Approx how many taste buds do we have?

10,000

Which cranial nerve is involved in the taste process, but does not actually "taste"

CN V: Trigeminal (hot/cold, texture)

What pathology is *Quinine* used to treat? (QUIZ)

Malaria

Will you achieve full adaptation to taste?

Most likely, never

Taste buds are found b/w elevations on tongue called?

Papillae

How to describe Umami taste senses?

Savoury/Meaty

What type of nerve cell are gustatory receptor cells?

Separate Cell's

T/F? Receptor potentials arise differently for different tastants?

True

What is the threshold for tastes?

Varies among 5 primary tastes: 1)*most sensitive to bitter (poisons*) = lower threshold/ incr sensitivity 2)*least sensitive to salty & sweet* = higher threshold/ decr sensitivity

*Tastant*

chemicals that stimulate gustatory receptor cells

What happens to number taste buds as we age?

decline

Where exactly are the taste buds located?

in the groove b/w the papillae

Fungiform papillae

mushroom shaped elevations scattered over *entire surface* of tongue with 5 taste buds each