Human Physiology, 14th edition (2016)

Sensory Physiology 275

somatosensory cortex of the postcentral gyrus devoted to the

tongue. Information is also sent to the prefrontal (orbitofron-

tal) cortex, which is important for taste associations and the

perception of flavor.

The specialized epithelial cells of the taste bud are known

as taste cells. The different categories of taste are produced

by different chemicals that come into contact with the micro-

villi of these cells ( fig. 10.8 ). Four different categories of taste

are traditionally recognized: salty, sour, sweet, and bitter.

There is also a more recently discovered fifth category of

taste, termed umami (a Japanese term for “savory,” related to

a meaty flavor), for the amino acid glutamate (and stimulated

by the flavor-enhancer monosodium glutamate). Although sci-

entists long believed that different regions of the tongue were

specialized for different tastes, this is no longer believed to be

true. All areas of the tongue are able to respond to all five cat-

egories of taste. This is true even for a single taste bud, which

can contain taste cells sensitive to each category of taste. How-

ever, a particular taste cell is sensitive to only one category of

taste and activates a sensory neuron that transmits information

regarding that specific taste to the brain.

For example, the sweet taste evoked by sugar is carried to

the brain on sensory neurons devoted only to the sweet taste.

Saccharin in low concentrations stimulates only the sweet

receptors, but at higher concentrations can also stimulate bit-

ter receptors and give saccharine an “aftertaste.” The com-

plex tastes we can perceive depend on the relative activities of

the sensory neurons from each of the five categories of taste,

together with information conveying the sense of smell from

olfactory receptors. Taste is also influenced by the temperature

nucleus of second-order neurons in the medulla oblongata.

From there, the second-order neurons project to the thala-

mus, which serves as a switchboard for directing sensory

information to the cerebral cortex (chapter 8, section 8.2).

Third-order neurons from the thalamus convey taste informa-

tion to the primary gustatory cortex in the insula, and to the

Figure 10.7 A taste bud. Chemicals dissolved in the fluid

at the pore bind to receptor proteins in the microvilli of the sensory

cells. This ultimately leads to the release of neurotransmitter, which

activates the associated sensory neuron.

Sensory

nerve fiber

Supporting

cell

Gustatory

(taste) cell

Taste hair

Taste

bud

Tongue surface Taste pore

Figure 10.8 The five major categories of taste. Each category of taste activates specific taste cells by different means.

Notice that taste cells for salty and sour are depolarized by ions (Na^1 and H^1 , respectively) in the food, whereas taste cells for sweet,

umami, and bitter are depolarized by sugars, the amino acids glutamate and aspartate (not shown), and quinine, respectively, by

means of G-protein-coupled receptors and the actions of second messengers.

x

Na+ H+ Sugars or

amino

acids

Salty Sour Sweet and Umami Bitter

Na+ through ion channel H+ through ion channel

(and other effects)

Binds to membrane receptors

for sweet or umami

Binds to membrane

receptor

Depolarization Depolarization

Depolarization

Opens
Ca2+
channels Ca

2+ released from

endoplasmic

reticulum

Opens

Ca2+

channels

Sensory neuron stimulated Sensory neuron stimulated Sensory neuron stimulated

Neurotransmitter

released

Neurotransmitter

released

Neurotransmitter

released

Neurotransmitter

released

Sensory neuron stimulated

G-proteins

Second

messenger

G-proteins

Second

messenger

Quinine

Close K+

channels

+

+ +

+++

++ ++

+

+

++

Ca2+ Ca2+ Ca2+ Ca2+

+

+

Ca2+