The Digestion and Absorption of Food
543
also have trophic (growth-promoting) effects on various tis-
sues, including the gastric and intestinal mucosa and the
exocrine portions of the pancreas. Certain gastrointestinal
hormones (like GIP) also infl uence the endocrine pancreas,
thereby amplifying the insulin response to eating and absorb-
ing a meal (Chapter 16).
Phases of Gastrointestinal Control
The neural and hormonal control of the gastrointestinal system
is, in large part, divisible into three phases—cephalic, gastric,
and intestinal—according to where the stimulus is perceived.
The
cephalic phase
is initiated when receptors in the
head (
cephalic
) are stimulated by sight, smell, taste, and chew-
ing. Various emotional states can also initiate this phase. The
efferent pathways for these refl exes are primarily mediated
by parasympathetic fi bers carried in the vagus nerves. These
fi bers activate neurons in the gastrointestinal nerve plexuses,
which in turn affect secretory and contractile activity.
Four types of stimuli in the stomach initiate the refl exes
that constitute the
gastric phase
of regulation: distension,
acidity, amino acids, and peptides formed during the diges-
tion of ingested protein. The responses to these stimuli are
mediated by short and long neural refl exes and by release of
the hormone gastrin.
Finally, the
intestinal phase
is initiated by stimuli in
the intestinal tract: distension, acidity, osmolarity, and various
digestive products. The intestinal phase is mediated by both
short and long neural refl exes and by the gastrointestinal hor-
mones secretin, CCK, and GIP, all of which are secreted by
endocrine cells in the small intestine.
We reemphasize that each of these phases is named for
the site at which the various stimuli initiate the refl ex and
not for the sites of effector activity. Each phase is character-
ized by efferent output to virtually all organs in the gastro-
intestinal tract. Also, these phases do not occur in temporal
sequence except at the very beginning of a meal. Rather,
during ingestion and the much longer absorptive period,
refl exes characteristic of all three phases may be occurring
simultaneously.
Keeping in mind the neural and hormonal mechanisms
available for regulating gastrointestinal activity, we can now
examine the specifi c contractile and secretory processes that
occur in each segment of the gastrointestinal system.
Mouth, Pharynx, and Esophagus
Chewing
Chewing is controlled by the somatic nerves to the skeletal
muscles of the mouth and jaw. In addition to the voluntary
control of these muscles, rhythmical chewing motions are
refl exly activated by the pressure of food against the gums,
hard palate at the roof of the mouth, and tongue. Activation
of these mechanoreceptors leads to refl exive inhibition of the
muscles holding the jaw closed. The resulting relaxation of
the jaw reduces the pressure on the various mechanoreceptors,
leading to a new cycle of contraction and relaxation.
Although chewing prolongs the subjective pleasure
of taste, it does not appreciably alter the rate at which food
is digested and absorbed. On the other hand, attempting to
swallow a large particle of food can lead to choking if the par-
ticle lodges over the trachea, blocking the entry of air into the
lungs.
Saliva
The secretion of saliva is controlled by both sympathetic and
parasympathetic neurons. Unlike their antagonistic activity in
most organs, both systems stimulate salivary secretion, with
the parasympathetics producing the greater response. There is
no hormonal regulation of salivary secretion. In the absence
of ingested material, a low rate of salivary secretion keeps the
mouth moist. The smell or sight of food induces a cephalic
phase of salivary secretion. This refl ex can be conditioned to
other cues, a phenomenon made famous by Pavlov. Salivary
secretion can increase markedly in response to a meal. This
refl ex response is initiated by chemoreceptors (acidic fruit
juices are a particularly strong stimulus) and pressure recep-
tors in the walls of the mouth and on the tongue.
Increased saliva secretion is accomplished by a large
increase in blood fl ow to the salivary glands, which is medi-
ated primarily by an increase in parasympathetic neural activ-
ity. The volume of saliva secreted per gram of tissue is the
largest secretion of any of the body’s exocrine glands.
Sjögren’s syndrome
is a fascinating immune disorder in
which many different exocrine glands are rendered nonfunc-
tional by the infi ltration of white blood cells and immune
complexes. The loss of salivary gland function frequently
occurs and can be treated by taking frequent sips of water and
with oral fl uoride treatment to prevent tooth decay.
Swallowing
Swallowing is a complex refl
ex initiated when pressure recep-
tors in the walls of the pharynx are stimulated by food or
drink forced into the rear of the mouth by the tongue. These
receptors send afferent impulses to the
swallowing center
in
the medulla oblongata of the brainstem. This center then elic-
its swallowing via efferent fi bers to the muscles in the pharynx
and esophagus as well as to the respiratory muscles.
As the ingested material moves into the pharynx,
the soft palate elevates and lodges against the back wall of
the pharynx, preventing food from entering the nasal cav-
ity (
Figure 15–14b
). Impulses from the swallowing center
inhibit respiration, raise the larynx, and close the
glottis
(the
area around the vocal cords and the space between them),
keeping food from moving into the trachea. As the tongue
forces the food farther back into the pharynx, the food tilts
a fl ap of tissue, the
epiglottis,
backward to cover the closed
glottis (
Figure 15–14c
), thereby preventing food from enter-
ing the trachea (
aspiration
).
The next stage of swallowing occurs in the esophagus,
the foot-long tube that passes through the thoracic cavity,
penetrates the diaphragm (which separates the thoracic cav-
ity from the abdominal cavity), and joins the stomach a few
centimeters below the diaphragm. Skeletal muscles surround
the upper third of the esophagus, smooth muscles the lower
two-thirds.
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