Taste

Question 1

what is the special sense of taste?

Answer 1

Special sense

Mammalian taste receptors are located in different locations throughout the oral cavity

Respond to chemical signals that are dissolved in the saliva

Taste recognition probably used as a defense –protective mechanism

Question 2

What are the 5 qualities of taste? is that all there are?

Answer 2

Qualities of taste can be categorized into at least 5 distinct classes:
Sweet, sour, bitter, acid, salt

We now know there are at least several other categories:
metalic, umami, ???

Question 3

What is the location of taste receptors?

Answer 3

All receptors have basically the same type of structural arrangement

• most found in what are termed taste buds on dorsal surface of tongue

exist in papillae on surface of tongue

There are 4 types of papillae (3 have taste receptors)
     –  filiform 
  fungiform*
  circumvallate*
  folliate*			*contain taste r’s

Question 4

What are filiform papillae?

Answer 4

have NO (zero) taste buds on their surfaces
-are the most numerous
these are the papillae that help you lick ice cream (grooming in some animals)

Question 5

What are fungiform papillae?

Answer 5

The fungiform papillae (about 200 in humans) have taste buds (4-6) located on their dorsal surfaces

– located in the anterior two thirds of the tongue (Fig 6-1A)

Question 6

What are circumvallate papillae?

Answer 6

The circumvallate papillae (10-12):

located at the junction between the body and base of the tongue (sulcus terminalis)

have numerous taste buds (upwards of 250 each papillae) located along their walls.

The serous glands of von Ebner that empty into the crypt at the base of each papilla provide some of the media for dissolving chemicals (Fig. 14.37, Avery)

Question 7

What cranial nerve supplies the taste for the front two thirds of the tongue? posterior 1/3?

Answer 7

facial nerve (VII)

IX

Question 8

What are the folliate papillae?

Answer 8

The folliate papillae are located on the posterior sides of the tongue.

consist of a series of folds (like an accordion) on the lateral aspects of the tongue

have many taste buds located in these walls (estimated that there are 1200-1300 per papillae). Fig 6-1C

Question 9

where are other locations of taste buds in the oral cavity?

Answer 9

There are taste buds in other locations of the oral cavity - Lie on surface epithelium

• epiglottis(~950),
• soft palate(~400),
  larynx

These taste buds are more right out on the surface of the epithelium rather than tucked away.

Question 10

What is the variation of taste buds?

Answer 10

• young > old

- varied proportions of receptors between mammals

Question 11

What is the structure of taste receptors?

Answer 11

All taste buds are essentially built on the same structural framework:

4 different types of cells are found in a taste bud

• 3 open to the oral cavity through the taste pore and are joined together by tight junctions to seal off the interior of the taste bud (taste cells)

– the 4th type is located along the basement membrane and is probably the progenitor cell for the rest (basal cells)

• are like epithelium because they replicate, but like a neuron because they have a receptor component

right on the border between epithelium and neuron.

Question 12

What are the 4 types of cells in the taste bud?

Answer 12

Type 1 cells: located mainly around the peripheral walls of a taste bud. “glial like” ?

Type II cells: express G-prt coupled receptors and are selective for sweet, bitter, and umami

Type III cells: most neural like (chemical synapses; synaptic vesicles, voltage gated Ca2+ channels) –sour

Type IV cells: (called the basal cell) is considered to be the progenitor cell

Not sure which cell type detects salt.

If different taste buds have different amounts of type II vs. type III receptors, this may underlie some of the selectivity of the five different tastes in one taste bud to another

Question 13

What is the innervation of a taste bud?

Answer 13

Myelinated nerves penetrate the basement membrane – their endings ramify throughout the taste bud. About 50 fibres enter the taste bud & break up into ~ 250 fibres within the bud

Seem to spiral around all types of taste cells, but only see recognizable synapses on Type III cells.

Information from Type II cells is transmitted via ATP (transported through gap junctions) (how that actually results in neurotransmission, we don’t know)

Question 14

What about the unmyelinated nerve fibers penetrating the basement membrane?

Answer 14

Lots of unmyelinated nerve fibres penetrate the basement membrane as well. Thought to be involved in pain perception – BUT… some evidence that these are involved in some sort of taste discrimination.

Families with a genetic mutation (familial dysautonomia) don’t have developed taste buds. If you inject methacholine into these patients, they can now taste sweet/salt food types with almost normal thresholds.
(taste exists in the absence of taste buds!).

Question 15

What about the turn-over of taste buds?

Answer 15

Tritiated thymidine studies show that these taste bud cells have a half-life of about 7-10 days, so each cell in a taste bud turns over in 2-3 weeks.

Taste buds also have the ability to regenerate following injury – can burn (scald) off the taste buds associated with the fungiform papillae; once reinnervation occurs, re-establish new taste buds

Question 16

How is the transduction of taste information carried out?

Answer 16

Sensitivity of taste cells to presence of chemicals measured by two different methods

1) Detection threshold:
- The concentration of the chemical that the individual can recognize as different from water (still can’t identify the taste though).

– flood the receptor with water; now put very weak concentration of chemical onto taste cell. Note the even lower concentrations can be used if subject swills the solution around their oral cavity.

2) Recognition threshold
– the concentration of a chemical required to be recognized by the subject as specific taste (i.e., sweet & not salty)

Should be obvious that recognition thresholds are normally greater than detection thresholds.

Question 17

What is the basic receptor structure?

Answer 17

With the advent of patch clamp techniques, physiology of taste receptors worked out (review by Kinnamon, S. 1988 TINS 11(11): 491-496).

Different transduction mechanisms underlie each of the primary taste sensations;

• Sour taste depends on acid block of K+ channels
• Salt taste on amiloride-sensitive Na+ channels (apical surface)
• Sweet taste depends on receptor mediated cAMP (dependent phosphorylation of K+ channels)

Question 18

What are the two types of signalling pathways?

Answer 18

Direct activation of ion channels
sour (type III cells)
Salt (? Type I cell ?)

G-protein mediated signalling pathways
sweet, bitter, umami
thought to only code for one of these three senses in a given type II cell

taste is really controled at the recetpro level.

Question 19

What is an example of direct activation of ion channels

Answer 19

Type III cells: Sour taste depends on acid block of K+ channels

Acid/ quinine binds to K+ channel 
K+ channel is “activated”
Channel closes
 [K+ ] inside cell increases
Cell depolarizes
Activation of Ca2+ dep. channels
Ca2+ floods  into cell
Exocytosis of neurotransmitter- 
       containing vesicles into cleft (5-HT, NE)
Activation of postsynaptic receptor 
        on sensory nerve ending

Question 20

What is an example of G-protein linked receptor signalling?

Answer 20

Type II cells: Sweet taste depends on receptor mediated cAMP –dependent phosphorylation of K+ channels

Saccharide binds to G-protein linked receptor.
G-prt complex activates adenylate cyclase
ATP  cAMP
cAMP activates PKA
PKA phosphorylates K+ channel
K+ channel closes
Depolarization of cell
Activation of Ca2+ dep. channels
Ca2+ floods into cell
Gap junctions open
ATP is released as the “neurotransmitter”

Question 21

What are the pathways conveying taste information?

Answer 21

Taste information is conveyed by three main nerves

anterior two thirds tongue = facial

Post. one third of the tongue = glossopharyngeal

epiglottis and larynx = vagus

(note: attributes of food seeming “hot” or cold may actually be modulated by CN V)

The facial nerve carries most of the information from the fungiform papillae

The glossopharyngeal nerve carries info from the circumvallate and folliate papillae

The vagus nerve carries from the taste buds on the laryngeal, epiglottal surfaces

These sensory nerve fibres convey the taste information to the gustatory nucleus.
This is located at the rostral end of the nucleus tractus solitarius (solitary nucleus) located within the medulla/pons.

Question 22

What are three taste disorders?

Answer 22

Mainly three types

dysgeusia – distortion of taste sensation

hypogeusia – loss of sensitivity to taste

ageusia - no taste sensation at all

Question 23

why do we have taste disorders?

Answer 23

Can be due to a variety of causes ranging from 
drugs - vitamin A deficiency leading to keratinosis; upper respiratory tract infections; 
brain tumors (pituitary pressing on the amygdala leading to uncinate fits, etc);  
genetic mutations – familial dysautonomia 

Olfaction can also play a role in helping us determine the “taste” of a food, but “taste” is strictly defined as what is detected via the gustatory nucleus