GIS19 Motility Of The GI Tract Flashcards
1
Q
Importance of GI motility
A
- Retain food at different compartments of GI tract in an appropriate time
- Mixing food with digestive secretions —> better digestion
- Controlled by autonomic reflexes
2
Q
Swallowing
A
3 phases:
- Oral phase:
- voluntary
- transfer of food to pharynx by **tongue (only Cerebral cortex involved)
- food bolus activates cells in the opening of pharynx
- -> stimulate **swallowing centre in the medulla oblongata
- -> initiate involuntary swallowing reflex - Pharyngeal and 3. Esophageal phase:
- involuntary
- efferent signals trigger motor responses from pharynx, larynx, esophagus and respiratory muscles
- prevent entrance of food into trachea
- also need to ***inhibit respiration
ALL controlled by Swallowing centre in medulla
3
Q
The 3 phases of swallowing
A
- Oral phase / buccal phase
- Transfer of food to pharynx by tongue - Pharyngeal phase
- **Elevation of soft palate to block nasopharynx to prevent food from entering nasal cavity
- **Tongue blocks oral cavity
- **Vocal folds close the glottis; **Larynx raised; **Epiglottis swings backward —> prevents food from entering trachea
- **Respiratory centre in the brainstem is inhibited by swallowing - Esophageal phase
- Esophageal peristalsis
- ***Relaxation of upper esophageal sphincter
- Closure of sphincter after passage of food
- Glottis opens
- Breathing resumes
4
Q
Esophagus
A
- Upper esophageal sphincter (below pharynx) protects airway from swallowed material and gastric reflux
- Lower esophageal sphincter (entrance of stomach) prevents reflux of gastric content into esophagus
- Primary peristaltic contractions of the esophagus triggered by:
—> Food bolus in the esophagus after swallowing - Secondary peristaltic contractions triggered by
—> Distension of esophagus by retained food (to clear any leftover material and reflux from stomach) - Swallowing + Esophageal distension also stimulate:
—> Relaxation (opening) of lower esophageal sphincter
—> Receptive relaxation of stomach - Esophageal peristalsis + Sphincters are regulated ***entirely by Neural control (Vagus + Myenteric NS of esophagus)
5
Q
Dysphagia
A
- Difficulties in swallowing
- causes:
1. Painful disease of mouth and pharynx e.g. stomatitis, ulcers
- Disorders in mechanisms of swallowing
- Neural (damage of swallowing centre in encephalitis, damage in nerves involved in swallowing)
- Muscular (muscle dystrophy)
- Neuromuscular transmission (myasthenia gravis)
- Motility disorder (Achalasia: LES fail to open) - External compression on esophagus (goitre, enlarged lymph nodes)
- Intrinsic disease of esophagus (stricture, esophageal cancer)
6
Q
Vomiting
A
- Forceful expulsion of contents from the stomach and upper intestinal GI tract
- Protective value: removal of toxic ingested substance before they are absorbed
- excessive vomiting can lead to
—> **Large losses of water + salt (K and Cl)
—> **Alkalosis
- Vomiting reflex: Stimuli —> Afferent pathway —> Vomiting centre (***Medulla oblongata) —> Efferent pathway —> Vomit
7
Q
Stimuli that cause vomiting
A
- ***Irritation of the GI mucosa:
- distension
- acute infections (gastroenteritis)
- chemotherapy / radiation - Tactile stimuli at the back of the throat
- Toxins in blood
—> activation of ***chemoreceptor trigger zone (area postrema) on the wall of the 4th ventricle (outside BBB) - Rotating movement of the head (motion sickness) detected by Vestibular system
—> sends information to the brain via CN8 - Emotional stimuli (afferents from the limbic system)
8
Q
Vomiting reflex
A
Receptors sensing the emetic stimuli can be found in the brain / GI tract
- Afferent:
—> **Vagal + **Sympathetic
—> excites vomiting centre in medulla - Efferent:
—> **Cranial nerves to upper GI tract
—> **Spinal nerves to diaphragm and abdominal muscles
9
Q
Vomiting act
A
- Early events in the vomiting reflex: ***Reverse peristalsis (as far down as ileum)
- Preceding:
- feeling of nausea, rapid heart beat, increased salivation, sweating - Starting:
- deep breath, closure of glottis, lifting soft palate - **Lowering of diaphragm and **forceful contraction of abdominal muscle —> strongly compressing stomach —> ***building up stomach pressure
- ***Lower esophageal sphincter relaxes
- Gastric content forced out of stomach into the esophagus
—> can occur repeatedly without expulsion of content through the mouth (Retching)
Following a sequence of retches, content is expelled from esophagus into the mouth upon
- Relaxation of upper esophageal sphincter
- Closure of glottis + Inhibition of respiration prevents vomitus to enter trachea
10
Q
Motor functions of the stomach
A
Different parts of stomach show different contractions
- Proximal (Reservoir): **storage: **Tonic contractions
- Distal (Antral pump): **mixing: **Phasic contractions
- Storage: receptive relaxation
- Mixing and grinding
- contractions mix ingested food with gastric juice
—> solubilise some constituents + reduce particle size
—> facilitate digestion - Controlled emptying
- controlled emptying of food to the duodenum
—> providing optimal time for intestinal digestion and absorption
Presence of food in stomach and duodenum provide both **Mechanical + **Chemical stimuli to regulate motor function
11
Q
Storage function of the stomach
A
Esophagus distension
—> **Vagovagal reflex
—> esophageal peristalsis begins and LES relaxes
—> **Reduces tonic contraction of the stomach wall —> increase volume
—> Receptive relaxation of proximal stomach (fundus and body)
—> accommodate ingested material with negligible increase in pressure
- Food forms concentric circles in Body of stomach: newest food nearest esophageal opening and oldest food nearest the wall
- Volume of empty stomach about 50ml, ↑ to 1.5L when a meal is swallowed
12
Q
Mixing and grinding of food in the stomach
A
When food enters stomach:
- Induce peristaltic contraction of Body of stomach
- **Weak contractions in **Body (3x per min)
- -> determined by ***“slow wave” of stomach smooth muscle
- -> cause mixing of gastric content with gastric secretions
- -> weak mixing effect
- Major mixing in ***Antrum
- -> strong antral peristaltic contractions **against closed pylorus (*Retropulsion)
- -> propel mixture back towards Body
- -> mixing + breaking down food particles (***Grinding)
- -> during onset of terminal antral contraction: pylorus closING
- -> complete terminal antral contraction: pylorus closED
- Resulting mixture: ***Chyme
13
Q
Contraction of stomach during hunger
A
Fasted state:
- Antral contraction occurs in a different pattern when stomach emptied for several hours
- **Strong contraction of antrum (5-10 mins, then 75-90 mins quiescence) with a **relaxed pylorus
- -> Migrating myoelectric complex (MMC)
- -> sweep from stomach to ileum
- Function: ***remove remaining materials in stomach
- usually associated with ***hunger feeling –> intensifies drive to acquire food
14
Q
***Regulation of gastric emptying
A
- ensure gastric contents not emptied into duodenum at a rate > small intestine can neutralize gastric acid + process the chyme
- regulated through ***1. Antral contraction + 2. Tone of pyloric sphincter
***- Neural + Hormonal mechanisms
- **Weak gastric factors promote emptying:
1. Distension of stomach wall
2. Gastrin - **Strong duodenal factors inhibit emptying:
1. Distension of duodenum
2. Presence of hypertonic solution, low pH, fatty acids, monoglycerides in duodenum - -> Neural (extrinsic + intrinsic)
- -> Hormonal (Secretin, CCK, GIP)
15
Q
Regulation of GI smooth muscle contraction
A
Smooth muscle of GI tract: Muscularis externa
- **Frequency of muscle remains **CONSTANT
- -> determined by ***“slow wave” depolarisation
- -> intrinsic pattern/frequency
- **Force of muscle contraction can be CHANGED by **Neural + Hormonal mechanisms
- Neural / Hormonal stimuli:
- -> change baseline of depolarisation
- -> regulates ***number of action potential
- -> hence ***contraction force
