The Digestion and Absorption of Food
541
concentration); (3) chyme acidity; and (4) chyme concentra-
tions of specifi c digestion products like monosaccharides, fatty
acids, peptides, and amino acids. These stimuli act on mecha-
noreceptors, osmoreceptors, and chemoreceptors located in
the wall of the tract which trigger refl exes that infl uence the
effectors—the muscle layers in the wall of the tract and the
exocrine glands that secrete substances into its lumen.
Neural Regulation
The gastrointestinal tract has its own local nervous system,
known as the
enteric nervous system,
in the form of two nerve
networks, the myenteric plexus and the submucosal plexus (see
Figure 15–6). These neurons either synapse with other neu-
rons in the plexus or end near smooth muscles, glands, and epi-
thelial cells. Many axons leave the myenteric plexus and synapse
with neurons in the submucosal plexus, and vice versa, so that
neural activity in one plexus infl uences the activity in the other.
Moreover, stimulation at one point in the plexus can lead to
impulses that are conducted both up and down the tract. For
example, stimuli in the upper part of the small intestine may
affect smooth muscle and gland activity in the stomach as well
as in the lower part of the intestinal tract. In general, the myen-
teric plexus infl
uences smooth muscle activity whereas the sub-
mucosal plexus infl uences secretory activity.
The enteric nervous system contains adrenergic and cho-
linergic neurons as well as neurons that release other neurotrans-
mitters, such as nitric oxide, several neuropeptides, and ATP.
Many of the effectors mentioned earlier—muscle cells
and exocrine glands—are supplied by neurons that are part
of the enteric nervous system. This permits neural refl exes
that are completely within the tract—that is, independent of
the CNS. In addition, nerve fi bers from both the sympathetic
and parasympathetic branches of the autonomic nervous sys-
tem enter the intestinal tract and synapse with neurons in
both plexuses. Via these pathways, the CNS can infl uence the
motility and secretory activity of the gastrointestinal tract.
Thus, two types of neural refl ex arcs exist (
Figure 15–13
):
(1)
short refl
exes
from receptors through the nerve plexuses
to effector cells; and (2)
long refl
exes
from receptors in the
tract to the CNS by way of afferent nerves, and back to the
nerve plexuses and effector cells by way of autonomic nerve
fi bers.
Finally, it should be noted that not all neural refl exes are
initiated by signals
within
the tract. Hunger, the sight or smell
of food, and the emotional state of an individual can have sig-
nifi cant effects on the gastrointestinal tract, effects that are
mediated by the CNS via autonomic neurons.
Hormonal Regulation
The hormones that control the gastrointestinal system are
secreted mainly by endocrine cells scattered throughout the
epithelium of the stomach and small intestine. That is, these
cells are not clustered into discrete organs like the thyroid or
adrenal glands. One surface of each endocrine cell is exposed
to the lumen of the gastrointestinal tract. At this surface, vari-
ous chemical substances in the chyme stimulate the cell to
release its hormones from the opposite side of the cell into the
blood. The gastrointestinal hormones reach their target cells
primarily via the circulation.
The four best-understood gastrointestinal hormones are
secretin, cholecystokinin (CCK), gastrin,
and
glucose-
dependent insulinotropic peptide (GIP)
. They, as well
as several candidate hormones, also exist in the CNS and in
gastrointestinal plexus neurons, where they function as neu-
rotransmitters or neuromodulators.
Table 15–4
, which summarizes the characteristics of
these GI hormones, not only serves as a reference for future
discussions but also illustrates the following generalizations:
(1) each hormone participates in a feedback control system
that regulates some aspect of the GI luminal environment,
and (2) most GI hormones affect more than one type of tar-
get cell.
These two generalizations can be illustrated by CCK.
The presence of fatty acids and amino acids in the small intes-
tine triggers CCK secretion from cells in the small intestine
into the blood. Circulating CCK then stimulates the pan-
creas to increase the secretion of digestive enzymes. CCK also
causes the gallbladder to contract, delivering to the intestine
the bile salts required for micelle formation. As fat and amino
acids are absorbed, the stimuli (fatty acids and amino acids in
the lumen) for CCK release are removed.
In many cases, a single effector cell contains receptors
for more than one hormone, as well as receptors for neu-
rotransmitters and paracrine agents. The result is a variety of
inputs that can affect the cell’s response. One such event is the
phenomenon known as
potentiation,
which is exemplifi ed by
the interaction between secretin and CCK. Secretin strongly
Central nervous system
Emotional states / Hunger
Gastrointestinal lumen
Response
Short
reflexes
Long
reflexes
Afferent
neurons
Efferent
autonomic
neurons
Sight, smell,
taste of food
Stimulus
Nerve
plexus
Gastrointestinal walls
Chemoreceptors,
osmoreceptors, or
mechanoreceptors
Smooth
muscle
or gland
Figure 15–13
Long and short neural refl ex pathways activated by stimuli in the
gastrointestinal tract. The long refl
exes utilize neurons that link the
central nervous system to the gastrointestinal tract. Chemoreceptors
are stimulated by chemicals, osmoreceptors are sensitive to changes
in osmolarity (salt concentration), and mechanoreceptors respond to
distention of the gastrointestinal wall.
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