Human Physiology, 14th edition (2016)

278 Chapter 10

Evidently, the brain must somehow integrate the information

from many different receptor inputs and then interpret the pat-

tern as a characteristic “fingerprint” for a particular odor.

The mitral and tufted neurons of the olfactory glomeruli

in the olfactory bulb send axons through the lateral olfactory

tracts to numerous brain regions of the frontal and medial tem-

poral lobes that comprise the primary olfactory cortex. There

are interconnections between these regions and the amygdala,

hippocampus, and other structures of the limbic system. For

example, the piriform cortex, a pear-shaped region at the medial

junction of the frontal and temporal lobes, receives projections

from the olfactory bulb and makes reciprocal connections with

the prefrontal cortex and amygdala, among other structures.

The prefrontal cortex receives information regarding taste

as well as smell; perhaps this is why olfactory stimulation dur-

ing eating can be perceived as taste rather than smell. The struc-

tures of the limbic system were described in chapter 8 as having

important roles in emotion and memory. The interconnections

between the olfactory and limbic system may explain the close

relationship between the sense of smell and emotions, and how

a particular odor can evoke emotionally charged memories.

The processing of olfactory information begins in the
olfactory bulb, where the bipolar sensory neurons synapse
with neurons located in spherically shaped arrangements
called glomeruli (see fig.  10.9 ). Evidence suggests that each
glomerulus receives input from one type of olfactory receptor.
The smell of a flower, which releases many different molec-
ular odorants, may be identified by the pattern of excitation
it produces in the glomeruli of the olfactory bulb. Identifica-
tion of an odor is improved by inhibition provided by GABA
released from periglomerular cells that surround the glomeru-
lus and make dendrodendritic synapses with the second-order
neurons within the glomerulus (termed mitral and tufted cells;
see fig.  10.9 ). Inhibitory GABA effects are also produced by
dendrodendritic synapses with interneurons within the glom-
erulus. This is a type of lateral inhibition (see fig.  10.6 ) and
helps to sharpen odor perception.
How can the human brain perceive as many as the esti-
mated 10,000 different odors, if each sensory axon carries
information relating to only one of about 380 olfactory recep-
tor proteins? One reason is that a particular odorant may bind
to a particular olfactory receptor protein with a high affinity,
but it may also bind less avidly to other receptor proteins. In
that way, a particular odorant may be perceived by the pattern
of activity it produces in the glomeruli of the olfactory bulb.

Figure 10.11 How an odorant molecule depolarizes
an olfactory neuron. The olfactory receptor is coupled to many
G-proteins, which dissociate upon binding of the receptor to the
odorant. The a subunit of the G-proteins activates the enzyme
adenylate cyclase, which catalyzes the production of cyclic AMP
(cAMP). Cyclic AMP acts as a second messenger, opening cation
channels. The inward diffusion of Na^1 and Ca^2 1 then produces
depolarization.

cAMP

cAMP

(a)

(b)

Odorant

Odorant

Odorant

receptor

Odorant

receptor

G-proteins

Adenylate

cyclase

Adenylate

cyclase

Na+/Ca2+

channel

Na+/Ca2+

channel

Ca2+ Na+

AT P

α

β

γ

α

γ

β

| CHECKPOINT

5. Explain how the mechanisms for sour and salty
   tastes are similar to each other, and how these differ
   from the mechanisms responsible for sweet and
   bitter tastes.


6. Explain how odorant molecules stimulate the
   olfactory receptors. Why is our sense of smell so
   keen?

10.4 Vestibular Apparatus and Equilibrium

The sense of equilibrium is provided by structures in the

inner ear collectively known as the vestibular apparatus.

Movements of the head cause fluid within these structures

to bend extensions of sensory hair cells, and this bending

results in the production of action potentials.

LEARNING OUTCOMES

After studying this section, you should be able to:

7. Describe the structures of the vestibular apparatus
   and explain how they function to produce a sense of
   equilibrium.

The sense of equilibrium, which provides orientation with

respect to gravity, is due to the function of an organ called

the vestibular apparatus. The vestibular apparatus and a

snail-shaped structure called the cochlea, which is involved