GI Flashcards
(188 cards)
What are the 2 patterns of movement in the GI tract
Peristalsis - reflex response -result of wall stretch
segmentation - mixes lumen content
Electrical activity of GI tract
spontaneous rhythmic fluctuation with MP -64 and -45
specialised cells aka interstitial cells of cajal
spike potential→ increase tension→ cause contraction
Depo→ ca2+ influx
Repo→ K+ efflux
stimulated by stretch, ACh, Parasympathetic
3 ways GI motility is regulated
- Reflexes from outside the digestive system
- Reflex from inside digestive system (enteric ns)
- GI peptides
The extrinsic nervous system (Sympathetic)
originate between T5 &L2 of the spinal cord
innervated all of GI tract
Nerve ending secrete mainly NE and epinephrine
Stimulation = inhabits GI activity
The extrinsic nervous system (parasympathetic)
divided into cranial and sacral
cranial PN fibres are almost in the vagus nerves except to mouth and pharyngeal
sacral originates from 2-2 sacral segment of spinal cord
it passes through pelvic nerve to distal half of large intestine
stimulation causes increased activity
ENS (layers of cells)
-Lumen
-Mucosa (epithelium, lamina propria, muscularis mucosa)
-Submucosa (submucosal plexus)
- Muscularis propria (circular muscle, myenteric plexus, longitudinal plexus)
- Serosa
6 main polypeptides of GI
Cholecystokinin
Secretin
GIP
GLP-1
Gastrin
Motilin
CCK
- produced by I cells in duodenum and jejunum
- also secreted by neurons in the brain
- stimulates contraction of gall bladder for bile release
- Stimulation of pancreatic enzyme release
- relaxes Sphincter of Oddi
- inhibits gastric emptying by slowing contraction,
- promotes intestinal motility
Secretin
- produced by S cells of upper small intestine mucosa.
- mildly affects gastric emptying,
- promotes pancreatic secretion of HCO3
GIP
- secreted by “K cells” mainly in duodenum
- slows the rate of stomach emptying when SI is overloaded.
GLP-1
- produced by “L cells” in distal small intestine mucosa.
- delays gastric emptying
Gastrin
- released from the gastric antrum G cells by stomach distension.
- increases motility in the stomach
Motilin
- secreted by “M-cells” in the upper small intestine.
- secreted during fasting, and the only known function of this hormone is to increase gastrointestinal motility - migrating motor complex
3 segments of the Oesophagus
cervical
thoracic
Abdominal
Innervation and muscle types of the oesophagus
Proximal 1/3 → striated:
- innervated by somatic motor neurons of vagus nerve
distal 2/3 → smooth muscle
- innervated by visceral motor neurons of vagus nerve
- synapse with postganglionic neurons
Lower oesophageal sphincter (LOS)
- Mainly closed
- ACH causes intrinsic sphincter to contract (closes)
- NO &VIP → inhibitory (opens)
- Function: prevent reflux
3 phases of swallowing
- Oral → voluntary
- Pharyngeal
- Oesophageal
5 features that make up the anti- reflux barrier
- LOS
- normal oesophageal peristalsis
- Crural fibres of the diaphragm
- Length of the abdo oesophagus
- Gravity (small effect)
4 aspects of gastric motility
- Filling
accommodates 20-fold change in its volume by receptive relaxation: - vago-vagal reflex from stomach to brainstem and back; reduces tone of muscular wall.
- the proximal stomach relaxes to store food at low
pressure whilst it is acted upon by acid and enzymes.- Storage
- Mixing
Peristaltic contraction (PC) usually begin in body of stomach down
PC becomes more vigorous as it reaches the antrum.
This propels the chyme forward - Emptying
A small portion of the chyme is pushed through the “partially” open sphincter into the duodenum
When PC reaches the pyloric sphincter, the sphincter closes tightly→ No further emptying
Chyme not delivered into duodenum is forced backward into the stomach – “retropulsion.”
3 parts of the stomach
- Body normally stores food; has weak contraction & numerous oxyntic glands
- Antrum has thick muscularis externa, vigorous contractions & numerous pyloric glands - (secrete gastrin)
- Pylorus regulates passage of chyme into duodenum
Entry of food into duodenum depends on:
- Hypertonic chyme, low pH
- Presence of a.a and peptides
- Fatty acids and monoglycerides → slows gastric emptying
Emesis
- A regulation of the gut motility function following excessive irritation or distension of any part of upper GI
- signals initiating vomiting mainly originate from upper GI - pharynx, oesophagus, stomach and duodenum.
- nerve impulses are transmitted by vagal and sympathetic afferent nerve to v. centre.
- sufficient stimulation of v. centre leads to vomiting act;
- involves forceful expulsion of gastric (and duodenal) contents through the mouth;
- often preceded by salivation, nausea, rapid irregular heart beat, dizziness, retching.
- Vomiting can also be elicited by drugs or by rapid change in direction
- some emetics stimulate duodenal receptors, others act on receptors on floor of 4th ventricles called CTZ
- motion stimulates receptors in vestibular labyrinth centre, then to CTZ.
- It represents a defence reaction to protect the body against intake of dangerous agents.
- Toxins in the GI tract can be recognised either
- before absorption via visceral parasympathetic and sympathetic afferent fibres.
- after absorption into the blood via the chemoreceptor trigger zone (CTZ) located in the area postrema.
- Toxins in the GI tract can be recognised either
- Thus CTZ integrates both the afferent signals from the gastrointestinal tract and the chemical signals from the blood.
Motility of the Small intestine (SI)
- SI is 5m in length, has 3 sections
- It takes 2-4 hrs for chyme to traverse it
- Basic electrical rhythm is entirely intrinsic.
- Movements of SI can be
- peristaltic (propulsive contractions)
- mixing (segmentation) contractions
- migrating contractions
- Segmentation is characterised by closely spaced contractions of circular muscles
- Slow waves of smooth muscle causes segmental contractions backed by myenteric nerve plexus.
- Peristaltic contraction progressive, moves content in orthograde direction, about 2 cm/sec
- net movement along SI averages 1cm/min
Regulation of SI
- Peristaltic contraction
- Neural by local reflex mediated via ENS, but can be extrinsic innervation
- Hormonal factors:
- gastrin, CCK, insulin, motilin, and serotonin enhance intestinal motility
- Secretin, VIP and glucagon inhibit
- Drugs such as codeine & other opiates decrease motility