CONTROL MECHANISMS IN GASTROINTESTINAL SYSTEM
CONTROL MECHANISMS IN GASTROINTESTINAL SYSTEM
- Successful assimilation of nutrients requires the coordination of
multiple organs, and is achieved by a series of overlapping controls
that are applied through neural, hormonal, and paracrine
mechanisms.
- The major regulated processes that can generate change (effectors)
in the gastrointestinal physiology are gut motility, epithelial
secretion, and blood flow.
- Each of these processes has a level of intrinsic activity, referred to
as gastrointestinal tone.
1. ENTERIC NERVOUS SYSTEM
The ENS is a division of the autonomic nervous system. It is a large neural network located within the wall of the gastrointestinal tract.
The ENS can be thought of as a
“minibrain” with independent
information processing capability. It
contains a “program library” that
initiates patterns of gastrointestinal
activity (e.g., interdigestive,
digestive, and emetic activity). The
ENS is responsible for much of the
control of gut motility and secretion.
The ENS is arranged as myenteric and submucosal nerve plexuses.
- The myenteric plexus is mainly involved with control of gut motility and innervates the longitudinal and circular smooth muscle layers.
- The submucosal plexus coordinates intestinal absorption and secretion through its innervation of the glandular epithelium, intestinal endocrine cells, and submucosal blood vessels.
2. GUT-BRAIN AXIS
Although the ENS can
function without other
connections, it is
linked with the CNS
via the
parasympathetic and
sympathetic nerves,
giving rise to the
concept of
a gut-brain axis.
The link between the ENS and the CNS is through the
parasympathetic and sympathetic divisions of the autonomic nervous
system.
Parasympathetic
innervation generally causes
excitation (more secretion,
more propulsive motility).
Parasympathetic nerves
contain sensory afferent fibers,
which convey nonpainful
distension and nausea. For
example, distension of the
stomach during a meal gives
rise to an afferent signal,
which results in stimulation of
gastric acid secretion.
The sympathetic
nerves innervates the
vascular smooth muscle,
which results in
vasoconstriction;
motility and secretion
are decreased. Sensory
afferent signals within
the sympathetic nerves
convey the sensations of
pain and nausea to the
CNS.
The CNS may send
“command signals” to the
ENS, which link
gastrointestinal activity with
behavior; for example,
salivation and gastric
secretion occur in response
to the thought or smell of
food.
Emotions can also have
a profound effect on gut
physiology via the gutbrain axis (e.g., diarrhea
induced by anxiety).
The link between the
gut and the brain also
conveys sensory input
to the CNS, giving rise
to the conscious
sensations of fullness,
satiety, nausea, and
pain.
3. GASTROINTESTINAL HORMONES
Hormones play a prominent role in the control of gastrointestinal
function, and in most cases these hormones are secreted by the
gastrointestinal mucosa itself.
The release of hormones is influenced by the chemical environment
in the lumen of the gastrointestinal tract; enteroendocrine cells have
microvillibearing receptors that “taste” the gut lumen, allowing the
cells to secrete hormone at the appropriate time.
Other Endocrine Organs
