THE SCIENCE OF TASTE AND SMELL
2.6 GUSTATION: SIGNALING—RECEPTORS, CELLS, AND TISSUE Each tastant will diffuse through the fluids of the mouth and bind to protein receptors
embedded in the surface of the taste receptor cells clustered in our taste buds. Taste buds are small groups of taste receptor cells found throughout the tongue. The taste receptor cells are located primarily on the top or dorsal surface of the tongue. The taste receptors are localized into tissues projections called papillae. The surface of the tongue has three distinct types of papillae that are located in three different areas. The papillae are small
Acidic Neutral Alkaline
pH = –log [H+]
(a) (b)
pH = 7 Neutral pH [H+] = [OH–] pH < 7 Acidic pH [H+] > [OH–] Increase [H+] decrease pH pH > 7 Basic pH [H+] < [OH–]
Decrease [H+] increase pH
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
12,6
11.6 13.0 10.5 9.0 7.0 5.5 4.5 6.4 2.0 0.1
0.8 2.4
8.5 10.0
FIGURE 2.6 The pH scale. (a) Equation for calculating pH and pH concepts. (b) The pH scale and the pH of some different common household items, foods, and drink.
pimple‐, ridge‐, or mushroom‐shaped structures each a couple of millimeters in size.
The three types of papillae are directly involved in taste sensations (Fig. 2.7). Fungiform papillae are pimple‐shaped structures located on the surface primarily down the sides of the tongue. As you move down the side of the tongue, the next set of papillae is foliate papillae that are a series of ridges with one distinct set on each side of the tongue. Finally, in the back center of the tongue are the circumvallate papillae that present functionally as a row of mushroom‐shape structures at the back of the tongue.
Each of these types of papillae contains multiple taste buds (Table 2.6).
Each taste bud is composed of 50–150 taste receptor cells, a substantial series of basal and support cells that surround the receptor cells, and a set of sensory afferent neurons that carry sensory inputs from the receptor to the brain. The number of each type of papillae and the number of taste buds in each type of papillae vary.
Table 2.6 gives average values for these numbers. A fourth type of papillae called the filiform papillae, which are long cone‐shape structures, are present on the tongue.
The filiform papillae are the most numerous papillae on the tongue. The filiform papillae don’t participate in gustation and serve a mechanical function.
On average, people have 2000–5000 taste buds with the extremes of the range being from 500 to 20,000. As stated earlier, each of the taste buds has 50–150 taste
Epiglottis Palatine tonsil
Lingual tonsil Terminal sulcus
Midline groove of tongue
Filiform papillae
Fungiform papillae Vallate papillae Palatoglossal arch Palatopharyngeal arch Median glossoepiglottic fold
FIGURE 2.7 Anatomy of the tongue.
TABLE 2.6 Numbers of Papillae and Taste Buds per Papillae.
Papillae
Papillae/Tongue
(Average) Taste Buds/Papillae Total Taste Buds
Fungiform papillae 200 1–18 Front of tongue 1120
1–9 Middle of tongue
Foliate papillae 11 117 1280
Circumvallate papillae 8 252 2200
receptor cells. People at the very high end of the range of number of taste buds have been identified as supertasters. It is estimated that 35% of women and 15% of men are categorized as supertasters. The evolutionary value of being a supertaster is unclear. Supertasters would have an advantage in avoiding noxious or toxic sub-stances because of the increased ability to taste them in potential food items. It is difficult to identify other evolutionary advantages. In fact, many supertasters are rather picky eaters as the tastes of many foods and beverages can be overwhelming to them. A few bites of a rich desert or a salty main dish are typically sufficient to satiate the desire for that taste. But that does not mean that all picky eaters are super-tasters or that all supersuper-tasters are picky eaters.
The structure of the taste bud lies beneath the surface of the tongue (Fig. 2.7). Thus contrary to popular belief, you cannot see your taste buds. What you see are the papillae on the surface of your tongue. The opening between the surface of the tongue and the taste bud is called the taste pore. The taste pores are not located directly on the top of the gustatory papillae but typically located near the sides and in the crevasses of the papillae. The taste receptor cells are modified epithelial cells with microvilli that project into the taste pore. The microvilli contain the membrane receptors that bind the tastants to initiate the taste process. The taste cells are surrounded by basal cells that support the structure of the taste bud and taste cells but do not directly participate in gustation. The taste cells form a chemical synapse with the sensory neurons leaving the taste buds. When activated, the taste cells release neurotransmitter activating the sensory neuron that carries electrical impulses into the brain.
As scientists learn more about the nature of the taste receptors, taste receptor cells, and the anatomy of taste buds, they have found the old model of taste map of the tongue (where different regions of the tongue were thought to be responsible for a specific taste) is wrong (Fig. 2.8). This map did not include umami or account for all sorts of types of taste receptor cells for each of the flavor types. As stated earlier, each taste bud contains 50–100 taste receptor cells. And each taste receptor cell is unique and a taste bud is comprised of a diverse collection of taste receptor cells with several kinds of taste receptors. Each taste receptor cell will only have one kind of taste receptor and is associated with its own afferent nerve that signals to the thalamus and neocortex. This means that each taste bud is wired and can recognize a variety of different tastes and flavors. The makeup of each bud and density of the buds can vary leading to the incorrect mythological taste map of the tongue.
Complicating matters is that some of the taste receptor cells express only one type of taste receptor (e.g., the sweet taste receptors) and bitter taste receptor cells have many different receptors produced on a single taste receptor cell. Thus for each cell with only one type of receptor, those cells with its associated nerve signal to the neocortex can distinguish between different types of flavors such as sweet. While the several bitter compounds will each bind and activate the same taste cell and its pathway to the brain. The bottom line is that we can tell the difference between different sweet flavors but even though there are thousands of different bitter compounds, we cannot tell the differences between them (Box 2.1).
They are all signaling through the same taste receptor cell and nerve to the same place in our brain!
Zone of the
salty taste Zone of the
sweet taste Zone of the
bitter taste Zone of the
sour taste
FIGURE 2.8 The mythical tongue map.
BOx 2.1 ARE YOU A SUPERTASTER?
The first step to determining whether you are a supertaster is to evaluate the number of fungiform papillae present on your tongue. The fungiform papillae are small pink bumps on the surface of the tongue. The following process will make it easier to count the papillae and determine whether you are a super taster.
Supplies
• Vial of blue food coloring
• 2 paper towels per person
• 2 cotton swabs per person
• 2 cotton balls per person
• 2 sticky notebook hole reinforcement circles per student
• A magnifying glass
• A ruler with a millimeter scale Procedure
• Work with a partner.
• Use the cotton swab as an applicator to paint the surface of the tip of your partner’s tongue with the blue dye.
• Put a few drops of the food coloring on the cotton swab.
• Paint the tip of the tongue with the dyed cotton swab.
• Move your tongue around in your mouth to make sure the dye covers the entire tongue. The dye is safe to swallow.
• Pat your tongue dry with one paper towel.
• Place the reinforcement circle on the tip of your partner’s blue‐dyed tongue.
This circle defines the sample area on the tongue.
2.7 GUSTATION: MEMBRANE PROTEINS, MEMBRANE