C H A P T E R 5 6
Introduction to the gastrointestinal system
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Peritoneointestinal reflex
: Irritation of the peritoneum
as a result of inflammation or injury leads to a
cessation of GI activity, preventing continued
movement of the GI tract and thus further irritation
of the peritoneum.
•
Renointestinal reflex
: Irritation or swelling of the
renal capsule causes a cessation of movement in the GI
tract, again to prevent further irritation to the capsule.
•
Vesicointestinal reflex
: Irritation or overstretching of
the bladder can cause a reflex cessation of movement
in the GI tract, again to prevent further irritation to
the bladder from the GI movement. Many people with
cystitis or overstretched bladders from occupational
constraints or neurological problems complain of
constipation, which can be attributable to this reflex.
•
Somatointestinal reflex
: Taut stretching of the skin
and muscles over the abdomen irritates the nerve
plexus and causes a slowing or cessation of GI activity
to prevent further irritation. During the era when
tight girdles were commonly worn, this reflex was
often seen among women, and constipation was a
serious problem for many women who wore such
constraining garments. Tight-fitting clothing (e.g.
jeans) can have the same effect. People who complain
of chronic constipation may be suffering from
overactivity of the somatointestinal reflex.
Central reflexes
Two centrally mediated reflexes—swallowing and
vomiting—are very important to the functioning of the
GI tract.
Swallowing
The swallowing reflex is stimulated whenever a food
bolus stimulates pressure receptors in the back of the
throat and pharynx. These receptors send impulses to
the medulla, which stimulates a series of nerves that
cause the following actions: the soft palate elevates and
seals off the nasal cavity; respirations cease in order to
protect the lungs; the larynx rises and the glottis closes
to seal off the airway; and the pharyngeal constrictor
muscles contract and force the food bolus into the top
of the oesophagus, where pairs of muscles contract in
turn to move the bolus down the oesophagus into the
stomach. This reflex is complex, involving more than
25 pairs of muscles.
This reflex can be facilitated in a number of ways
if swallowing (food or medication) is a problem. Icing
the tongue by sucking on ice cube blocks external nerve
impulses and allows this more basic reflex to respond.
Icing the sternal notch or the back of the neck, although
not as appealing, has also proved effective in stimu-
lating the swallowing reflex. In addition, keeping the
head straight (not turned to one side) allows the muscle
pairs to work together and helps the process. Providing
stimulation of the receptors in the mouth through tem-
perature variations and textured foods helps to initiate
the reflex. People who do not produce their own saliva
can be given artificial saliva to increase digestion and to
lubricate the food bolus, which also helps the swallow-
ing reflex.
Vomiting
The
vomiting
reflex is another basic reflex that is cen-
trally mediated and important in protecting the system
from unwanted irritants. The vomiting reflex is stimu-
lated by two centres in the medulla. The more primitive
centre is called the emetic zone. When stimulated, it ini-
tiates a projectile vomiting. This type of intense reaction
is seen in young children and whenever increased
pressure in the brain or brain damage allows the more
primitive centre to override the more mature chemo
receptor trigger zone (CTZ). The CTZ is stimulated in
several ways:
• Tactile stimulation of the back of the throat, a reflex
to get rid of something that is too big or too irritating
to be swallowed
• Excessive stomach distension
• Increasing intracranial pressure by direct stimulation
• Stimulation of the vestibular receptors in the inner ear
(a reaction often seen with dizziness after “wild” rides
in amusement parks)
• Stimulation of stretch receptors in the uterus and
bladder (a possible explanation for vomiting in early
pregnancy and before delivery)
• Intense pain fibre stimulation
• Direct stimulation by various chemicals, including
fumes, certain drugs and debris from cellular
death (a reason for vomiting after chemotherapy or
radiation therapy that results in cell death)
Once the CTZ is stimulated, a series of reflexes occurs.
Salivation increases, and there is a large increase in the
production of mucus in the upper GI tract, which is
accompanied by a decrease in gastric acid production.
This action protects the lining of the GI tract from poten-
tial damage by the acidic stomach contents. (Nauseated
people who start swallowing repeatedly or complain
about secretions in their throat are in the process of
preparing for vomiting.) The sympathetic system is stim-
ulated, with a resultant increase in sweating, increased
heart rate, deeper respirations and nausea. This prepares
the body for fight or flight and the insult of vomiting. The
oesophagus then relaxes and becomes distended and the
gastric sphincter relaxes. The person takes one deep res-
piration; the glottis closes and the palate rises, trapping
the air in the lungs and sealing off entry to the lungs.
The abdominal and thoracic muscles contract, increas-
ing intra-abdominal pressure. The stomach then relaxes,
and the lower section of the stomach contracts in waves,
approximately six times per minute. With nothing in the
