HUMAN BIOLOGY

270 Chapter 14

taste

bud

taste buds

tonsil

A

hairlike

ending

of taste

receptor

BC D sensory nerve

chemical senses Senses

that detect substances dis-

solved in fluid that is in con-

tact with chemoreceptors.

olfactory receptors

Receptors in the nose’s

olfactory epithelium that

detect water-soluble or

vaporized substances.

taste receptors Chemo-

receptors in taste buds.

taste and smell: Chemical senses

neuron, which conveys the message to centers in the brain

where the stimulus is interpreted. Every perceived taste is

some combination of five primary tastes: sweet, sour, salty,

bitter, and umami (the savory taste associated with meats

or aged cheese).

The flavors of most foods are some combination of the

five basic tastes, plus information from olfactory receptors

in the nose. Simple as this sounds, scientists now know

that our taste sense involves complex genetic mechanisms.

Science Comes to Life on the facing page examines some of

these findings.

The olfactory element of taste is extremely important.

In addition to odor molecules in inhaled air, molecules of

volatile chemicals are released as you chew food. These

waft up into the nasal passages. There, the “smell” inputs

contribute to the perception of complex flavors. This is why

anything that dulls your sense of smell—such as a head

cold—also seems to diminish food’s flavor.

Olfaction is the sense of smell

Olfactory receptors detect substances that dissolve in

watery mucus on the surface of olfactory epithelium in

the upper nasal passages (Figure 14.7). When odor mol-

ecules bind to receptors on olfactory neurons in cells of the

olfactory epithelium, the resulting nerve impulse travels

directly to olfactory bulbs in the frontal area of the brain.

There, other neurons forward the message to a center in the

cerebral cortex, which interprets it as “fresh bread,” “pine

tree,” or some other substance.

From an evolutionary perspective, olfaction is an ancient

sense—and for good reason. Food, potential mates, and

predators give off substances that can diffuse through air

(or water) and so give clues or warnings of their where-

abouts. Even with our rather insensitive sense of smell,

we humans have about 10 million olfactory receptors in

patches of olfactory epithelium in the upper nasal passages.

Just inside your nose, next

to the vomer bone (Section 5.3),

is a tiny vomeronasal organ, or

“sexual nose.” (Some other mam-

mals also have one.) Receptors

in this organ can detect phero-

mones, which are chemicals that

influence social interactions in

many animal species. Phero-

mones can affect the behavior—

and maybe the physiology—

Figure 14.6 Animated! Taste

receptors are present in several

areas of the tongue. Each taste

bud may contain receptors for

several “tastes.” (© Cengage Learning)

n    Receptors sensitive to chemicals are responsible for the

special senses of taste and smell.

n Links to Facial bones 5.3, Sensory centers in the forebrain

13.7, Sensory areas of the parietal lobe 13.8

Taste and smell are chemical senses. They begin at

chemo receptors, which are activated when they bind a

chemical that is dissolved in fluid

around them. Although these recep-

tors wear out, new ones replace

them. In both cases, sensory infor-

mation travels from the receptors

through the thalamus and on to the

cerebral cortex, where perceptions

of the stimulus form. The input also

travels to the limbic system, which

can integrate it with emotional states

and stored memories.

Gustation is the sense of taste

The technical term for taste is gustation. Sensory organs

called taste buds hold the taste receptors (Figure 14.6).

About 10,000 taste buds are scattered over your tongue, the

roof of your mouth (the palate), and your throat.

A taste bud has a pore through which saliva and

other fluids in the mouth contact the surface of receptors.

The stimulated receptor in turn stimulates a sensory

Science Source

14.3

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