Neurology 400 - Special Senses: Taste

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

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

classic distribution of taste receptors in humans is under debate:

All parts of the tongue sense all diff tastes

Where are taste buds found?

-tongue -soft palate -epiglottis -pharynx *-lungs*

What are the possible responses in the brain to taste:

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

What cranial nerves are responsible for taste:

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

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

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

How long does adaptation to taste take to occur?

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

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

⬥*Different tastes* arise from activation of *different groups of taste neurons* ⬥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

Describe the threshold for various tastes:

⬥Threshold for bitter (e.g. quinine) is lowest b/c poisonous substances are often bitter - low threshold (high sensitivity) to have protective function ⬥Threshold for sour substances (e.g. lemon) measured by HCl - is higher ⬥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?

⬥Up to 80% ⬥ Loss of smell can lead to depression

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

*Adenosine Triphosphate (ATP)*

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

*Filiform* papillae

-located in the small trenches of the lateral margins of the tongue -most *degenerate in early childhood* QUIZ

*Foliate* papillae

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

*Fungiform* papillae

Where is the Gustatory nucleus located?

*MEDULLA* -rostral part of *solitary nucleus* aka: nucleus solitarius/ nucleus of tractus solitarius in medulla

Where is the primary gustatory area of the brain:

*PARIETAL lobe* (Brodmann's #43)

What type of potential do Gustatory receptor cells generate?

*Receptor* Potential

- chemicals that stimulate gustatory receptor cells:

*Tastant*

What contributes to taste adaptation?

- changes that occur in taste receptors - olfactory receptors - neurons of gustatory pathway in CNS

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

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

How does tastant trigger a nerve impulse?

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

What type of nerve cell are gustatory receptor cells?

-Separate Cell's

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

- via Na+ channels in plasma membrane - accumulation of ions cause depolarization - leads to release of NT

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

1. Circumvallate papillae 2. Fungiform papillae 3. Foliate papillae

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

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

What is the life span of Basal cells:

10 days

Approx how many taste buds do we have:

10,000

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

Basal cells

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

CN V: Trigeminal hot/cold, texture

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

Circumvallate papillae

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

Fungiform papillae

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

Gustatory receptor cells

Specific molecules that activate Umami:

L-Glutamate MSG

What pathology is *Quinine* used to treat? QUIZ

Malaria

Will you achieve full adaptation to taste?

Most likley, never

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

Papillae

How to describe Umami taste senses:

Savoury/Meaty

What other senses are used to "taste"?

Senses for: ⬥Temperature (Hot/Cold) ⬥Texture ⬥Smell

-cells which surround 50 gustatory receptor cells in each taste bud -gustatory hair projects from each gustatory receptor cell to external surface through taste pore:

Supporting cells

Taste overview:

Taste is a chemical sense ⬥ To be detected, molecules must be dissolved ⬥ Taste stimuli classes include sour, sweet, bitter, salty, and umami (meaty or savory) - the latter stimulated by L-glutamate ⬥ All other flavours, e.g. chocolate, pepper and coffee, are combinations of the five primary tastes, plus accompanying olfactory and tactile sensations ⬥ 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

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

True

What is the threshold for tastes?

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

What is the purpose of the diff taste receptors?

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

What happens to num taste buds as we age?

decline

Where exactly are the taste buds located?

in the groove b/w the papillae