Gastrointestinal Control
Control of GI Function
GI function is controlled by neural and chemical regulation. There are three phases of GI regulation - cephalic, gastric, and intestinal.
Neural Regulation
The extrinsic, or autonomic, system consists of the parasympathetic system, represented by the vagus and pelvic splanchnic nerves. The sympathetic system originates from the celiac, superior mesenteric, and inferior mesenteric ganglia.
The gut also contains the enteric nervous system, a discrete web of 100 million neurons that can control gut function without extrinsic control. It consists of the myenteric plexus, located between longitudinal and circular muscle layers, and the submucosal plexus.
Reflexes mediate motility, secretions, hormone release, and vasculature. Long reflexes go from gut mechano- and chemoreceptors along the vagus to the brain, and back. Short reflexes are mediated exclusively by the enteric nervous system.
Short reflexes include the gastroenteric, enterogastric, and gastrocolic reflexes.
Stomach Pacemaker Cells
Gut smooth muscle has a basic electrical rhythm, where once every 3-5 minutes there is a change in resting potential in pacemakers cells in the stomach.
Normally these changes do not reach the threshold for contraction. However, with stimulation from the vagus or hormones such as gastrin, the threshold is crossed and contraction occurs.
Contraction strength increases with stimuli strength, but there is no change in contraction frequency.
Chemical Regulation
The GI system is also regulated by both paracrine and endocrine hormones. Most of these originate from within the GI system.
GI hormones are expressed at various points during feeding to regulate digestion and GI motility.
Most GI hormones are polypeptides and therefore have a short half-life.
| Hormone | Source | Stimulus | Major Actions |
|---|---|---|---|
| Gastrin | G cells in antrum and duodenum | ACh, antral distension, digested protein | acid secretion, mucosal growth, antral motility |
| Cholecystokinin (CCK) | I cells in SI | fat, digested protein | pancreatic enzyme and bile secretion, relaxation of Sphincter of Oddi, decreased gastric emptying |
| Secretin | S cells in duodenum | pH < 4.5 | pancreatic and bile bicarbonate secretion, decreased acid secretion |
| Glucose-dependent Insulinotropic peptide (GIP) | K cells in duodenum and jejunum | glucose, fat | insulin secretion |
| motilin | endocrine cells in duodenum, jejunum | neural stimulation | migrating motility complex |
| Molecule | Source | Stimulus | Major Actions |
|---|---|---|---|
| somatostatin | D cells in stomach and SI | ph < 3, digested fat | decreased gastrin and gastric acid release |
| histamine | gastric mast cells | ACh, gastrin | potentiates parietal cells |
Hormone Deregulation
Zollinger-Ellis Syndrome is a gastrin secreting tumour. As gastrin causes mucosal growth, the stomach of patients can become quite overgrown.