Page 603 - Fundamentals of anatomy physiology

Clinical Case she collects water from a deep hole.
A Chance to See
Makena is a 12-year-old Kenyan girl living with
her family in a small village in sub-Saharan Africa. wears eyeglasses, and most do not
Their village is miles away from roads, electricity, and health
care. Her family is very poor, and lives on less than $1.25 per day.
Makena’s daily chores include walking two hours to collect water,
every morning before school and again after school. Water is so
precious in this part of the world that nobody in Makena’s village
has ever taken a shower.
Makena is a bright girl, but she does not do well in school.
She can’t see the chalkboard in her classroom. She can’t see the
soccer ball when she tries to play with her classmates. She even
has trouble seeing the path that leads to the dry riverbed where
An Introduction to the Special Senses
17 Our knowledge of the world around us is limited to those
characteristics that stimulate our sensory receptors. We may
not realize it, but our picture of the environment is incomplete.
Colors we cannot see guide insects to flowers. Sounds we can-
not hear and smells we cannot detect give dolphins, dogs, and
cats key information about their surroundings.
What we do perceive varies considerably with the state of
our nervous system. For example, during sympathetic activa-
tion, you experience a heightened awareness of sensory infor-
mation. You hear sounds that normally you would not notice.
Yet, when concentrating on a difficult problem, you may be
unaware of fairly loud noises.
Finally, our perception of any stimulus reflects activity in
the cerebral cortex, but that activity can be inappropriate. In
cases of phantom limb pain, for example, a person feels pain
in a missing limb. During an epileptic seizure, a person may
experience sights, sounds, or smells that have no physical basis.
In our discussion of the general senses and sensory path-
ways in Chapter 15, we introduced basic principles of receptor
function and sensory processing. We now turn to the five special
senses: olfaction (smell), gustation (taste), vision, equilibrium
(balance), and hearing. The sense organs involved are structur-
ally more complex than those of the general senses, but the
same basic principles of receptor function apply. ATLAS: Embryol-
ogy Summary 13: The Development of Special Sense Organs
17-1 Olfaction, the sense of smell,
involves olfactory receptors responding
to chemical stimuli
Learning Outcome Describe the sensory organs of smell, trace the
olfactory pathways to their destinations in the brain, and explain the thelial surface, exposing their considerable surface area to dis-
physiological basis of olfactory discrimination.
590
She has no idea that other people
can see these things better.
No one in Makena’s village
even know what they are. Makena’s
teacher, however, studied at the
University of Nairobi and knows that
glasses can correct vision problems.
The teacher thinks perhaps nearsightedness is a problem for
Makena. Can a desperately poor child, many miles away from
health care of any kind, obtain glasses to correct her poor vi-
sion? To find out, turn to the Clinical Case Wrap-Up on p. 633.
The sense of smell, called olfaction, is made possible by paired
olfactory organs. These organs are located in the nasal cavity
on either side of the nasal septum (Figure 17–1a). The olfactory
organs are made up of two layers: the olfactory epithelium and
the lamina propria.
The olfactory epithelium contains the olfactory recep-
tor cells, supporting cells, and regenerative basal cells (stem
cells) (Figure 17–1b). This epithelium covers the inferior surface
of the cribriform plate, the superior portion of the perpen-
dicular plate, and the superior nasal conchae of the ethmoid.
p. 244
The second layer, the underlying lamina propria, consists
of areolar tissue, numerous blood vessels, and nerves. This layer
also contains olfactory glands. Their secretions absorb water
and form thick, pigmented mucus.
What happens when you inhale through your nose? The
air swirls within your nasal cavity. This turbulence brings
airborne compounds to your olfactory organs. A normal, re-
laxed inhalation carries a small sample (about 2 percent) of
the inhaled air to the olfactory organs. If you sniff repeatedly,
you increase the flow of air across the olfactory epithelium,
increasing the stimulation of the olfactory receptors. However,
only the molecules of water-soluble and lipid-soluble materials
that can diffuse into the overlying mucus can stimulate those
receptors.
Olfactory Receptors
Olfactory receptor cells are highly modified neurons. The ex-
posed tip of each receptor cell forms a prominent knob that
projects beyond the epithelial surface (Figure 17–1b). The knob
is a base for up to 20 cilia-shaped dendrites that extend into
the surrounding mucus. These dendrites lie parallel to the epi-
solved compounds called odorants. Olfactory reception begins