23. Gastric Regulation Flashcards
Describe what happens in the GI tract when a meal is anticipated
3 phases
.There are three main phases of gastric regulation:
> Cephalic phase
> Gastric phase
> Intestinal phase.
Cephalic phase
These reflexes are decreased by stimulation
of the sympathetic nervous
system with,
for example, pain, fear and anxiety.
Thought, sight and smell of food
/ Stimulates
Cerebral cortex and hypothalamus
/
Medulla oblongata
/
Stimulates
/ Impulses from medulla travel via vagus
Vagus nerve
/
Submucosal plexus
/ /
Stomach – ^ Gastric Juices Blood – ^ Gastrin
Gastric phase
> As food and fluid enter the stomach,
stretch and chemoreceptors are
activated.
> This leads to a further increase
in gastric secretions and
increases peristalsis.
> The tone of the lower oesophageal sphincter
is increased to prevent reflux of acid.
> Once the pH has reached 2, gastrin
begins to exert a negative feedback
to inhibit further acid secretion.
Intestinal phase
This begins when chyme
(food mixed with gastric juices)
enters the duodenum,
causing the secretion of three main gut hormones:
> Gastric inhibitory peptide (GIP),
which inhibits further gastric secretions
and motility
> Secretin, which inhibits further gastric secretions
> Cholecystokinin (CCK),
which inhibits stomach emptying.
Describe the function of these gut hormones.
There are four main hormones involved: gastrin, GIP, secretin and CCK.
Table 23.1 Functions of gut hormones
Gastrin
Release stimulated by:
Cephalic phase
Stomach distension
Proteins in stomach
Actions ↑ pH of chyme in stomach ↑ Secretion of gastric juices ↑ Motility Encourages growth of mucosa Constricts LOS Relaxes pyloric and ileocaecal sphincters
GIP
Release stimulated by:
Fatty acids in small intestine
Actions
↑ Insulin release
Inhibits secretion of gastric juices
Slows gastric emptying
Secretin
Stim by
Acidic chyme in small intestine
Action
Stimulates contraction of gallbladder to release bile
Stimulates release of pancreatic enzymes
Augments effect of CCK
CCK
Stim
Amino acids in small intestine
Fatty acids in small intestine
Actions Stimulates contraction of gallbladder to release bile Stimulates release of pancreatic enzymes Induces feeling of satiety Inhibits gastric emptying Enhances actions of secretin
Describe the sphincters present in the gastrointestinal tract.
A sphincter is a structure, usually made up of circular muscle, that surrounds the opening of a hollow organ or body and constricts to close it.
Sphincters can be
anatomical,
where they are clearly different
from the surrounding tissue,
e.g. the anus,
or
functional
where the histological distinction
is not so clear,
e.g. lower oesophageal sphincter.
Sphincters can be under voluntary
or involuntary control.
There are many sphincters in the gastrointestinal tract: > Upper oesophageal > Lower oesophageal > Pyloric > Ileocaecal > Sphincter of Oddi > Anus
Upper oesophageal sphincter:
This is at the level of the C5–6 vertebrae
and is made up of the cricopharyngeal
part of the inferior pharyngeal constrictor muscle.
It is under conscious control and in its resting state it is usually constricted to avoid air being drawn into the stomach during breathing.
Lower oesophageal sphincter
(also called the ‘cardiac’ sphincter):
This is a functional sphincter,
found at the junction between
the non-keratinised squamous epithelium
of the oesophagus and
the simple columnar epithelium
of the stomach.
Its function is to prevent reflux of the acidic stomach contents into the oesophagus and so it is constricted at rest and has a pressure of 15–20 mmHg.
The sphincter opens ahead of peristalsis
during the process of swallowing
to allow food and fluid to enter the stomach.
It is supplied by the vagus nerve.
‘Barrier pressure’
Describes the difference between
LOS pressure and intragastric pressure.
The closer the barrier pressure is to zero,
the more likely it is that reflux will occur.
So, reducing LOS tone
or increasing intragastric pressure
(e.g. pregnancy, full stomach, abdominal distension)
makes reflux more likely.
Pyloric sphincter:
This is an anatomical sphincter
found at the junction of the
stomach and duodenum.
Its ring of muscle relaxes to allow
chyme to pass out of the stomach.
It is supplied by the coeliac ganglion.
Ileocaecal sphincter:
An anatomical sphincter
found at the junction
of the small and large bowels.
It prevents reflux of colonic material into the ileum.
