Human Physiology, 14th edition (2016)

274 Chapter 10

Chemoreceptors that respond to chemical changes in the inter-

nal environment are called interoceptors; those that respond

to chemical changes in the external environment are extero-

ceptors. Included in the latter category are taste (gustatory)

receptors, which respond to chemicals dissolved in food or

drink, and smell (olfactory) receptors, which respond to

gaseous molecules in the air. This distinction is somewhat

arbitrary, however, because odorant molecules in air must

first dissolve in fluid within the olfactory mucosa before the

sense of smell can be stimulated. Also, the sense of olfaction

strongly influences the sense of taste, as can easily be verified

by eating an onion (or almost anything else) with the nostrils

pinched together.

Taste

Gustation, the sense of taste, is evoked by receptors that con-

sist of barrel-shaped taste buds ( fig.  10.7 ). Located primarily

on the dorsal surface of the tongue, each taste bud consists of

50 to 100 specialized epithelial cells with long microvilli that

extend through a pore in the taste bud to the external environ-

ment, where they are bathed in saliva. Although these sensory

epithelial cells are not neurons, they behave like neurons; they

become depolarized when stimulated appropriately, produce

action potentials, and release neurotransmitters that stimulate

sensory neurons associated with the taste buds. Because of this,

some scientists classify the taste cells as neuroepithelial cells.

Taste buds are located mainly within epithelial papil-

lae. These include fungiform papillae on the anterior surface

of the tongue; circumvallate papillae on the posterior sur-

face of the tongue; and foliate papillae on the sides of the

tongue. Information regarding taste is transmitted from the

taste buds on the fungiform papillae via the chorda tympani

branch of the facial nerve ( VII ) and from the taste buds on

the circumvallate and foliate papillae via the glossopharyn-

geal nerve ( IX ). These nerves carry taste information to a

10.3 Taste and Smell

The receptors for taste and smell respond to dissolved

molecules; hence, they are classified as chemoreceptors.

Although there are only five basic modalities of taste, they

combine in various ways and are influenced by the sense

of smell, permitting a wide variety of sensory experiences.

Figure 10.6 Lateral inhibition. When an object touches
the skin ( a ) , receptors in the central area of the touched skin are
stimulated more than neighboring receptors ( b ). Lateral inhibition
within the central nervous system reduces the input from these
neighboring sensory neurons. Sensation, as a result, is sharpened
within the area of skin that was stimulated the most ( c ).

Degree ofstimulation

Stimulation

Skin locations stimulated

(a)

(b)

(c)

Amount ofsensation

Sensation

Skin locations of

perceived touch

Lateral inhibition

sharpens perception

Blunt object

Skin

Lateral inhibition

within central nervous system

| CHECKPOINT

3. Using a flow diagram, describe the neural pathways
   leading from cutaneous pain and pressure receptors
   to the postcentral gyrus. Indicate where crossing-
   over occurs.
   4a. Define the term sensory acuity and explain how
   acuity is related to the density of receptive fields in
   different parts of the body.
   4b. Explain the mechanism of lateral inhibition in
   cutaneous sensory perception and discuss its
   significance.

LEARNING OUTCOMES

After studying this section, you should be able to:

5. Identify the modalities of taste and explain how they
   are produced.


6. Explain how odorant molecules stimulate their
   receptors and describe how the information is
   conveyed to the brain.