GIS19 Motility Of The GI Tract

Question 1

Importance of GI motility

Answer 1

1. Retain food at different compartments of GI tract in an appropriate time
2. Mixing food with digestive secretions —> better digestion
3. Controlled by autonomic reflexes

Question 2

Swallowing

Answer 2

3 phases:

1. 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
2. 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

Question 3

The 3 phases of swallowing

Answer 3

1. Oral phase / buccal phase
   - Transfer of food to pharynx by tongue
2. 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
3. Esophageal phase
   - Esophageal peristalsis
   - ***Relaxation of upper esophageal sphincter
   - Closure of sphincter after passage of food
   - Glottis opens
   - Breathing resumes

Question 4

Esophagus

Answer 4

• 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)

Question 5

Dysphagia

Answer 5

• Difficulties in swallowing
• causes:
  1. Painful disease of mouth and pharynx e.g. stomatitis, ulcers

2. 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)
3. External compression on esophagus (goitre, enlarged lymph nodes)
4. Intrinsic disease of esophagus (stricture, esophageal cancer)

Question 6

Vomiting

Answer 6

• 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

Question 7

Stimuli that cause vomiting

Answer 7

1. ***Irritation of the GI mucosa:
   - distension
   - acute infections (gastroenteritis)
   - chemotherapy / radiation
2. Tactile stimuli at the back of the throat
3. Toxins in blood
   —> activation of ***chemoreceptor trigger zone (area postrema) on the wall of the 4th ventricle (outside BBB)
4. Rotating movement of the head (motion sickness) detected by Vestibular system
   —> sends information to the brain via CN8
5. Emotional stimuli (afferents from the limbic system)

Question 8

Vomiting reflex

Answer 8

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

Question 9

Vomiting act

Answer 9

1. Early events in the vomiting reflex: ***Reverse peristalsis (as far down as ileum)
2. Preceding:
   - feeling of nausea, rapid heart beat, increased salivation, sweating
3. Starting:
   - deep breath, closure of glottis, lifting soft palate
4. **Lowering of diaphragm and **forceful contraction of abdominal muscle —> strongly compressing stomach —> ***building up stomach pressure
5. ***Lower esophageal sphincter relaxes
6. 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

Question 10

Motor functions of the stomach

Answer 10

Different parts of stomach show different contractions

• Proximal (Reservoir): **storage: **Tonic contractions
• Distal (Antral pump): **mixing: **Phasic contractions

1. Storage: receptive relaxation
2. Mixing and grinding
   - contractions mix ingested food with gastric juice
   —> solubilise some constituents + reduce particle size
   —> facilitate digestion
3. 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

Question 11

Storage function of the stomach

Answer 11

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

Question 12

Mixing and grinding of food in the stomach

Answer 12

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

Question 13

Contraction of stomach during hunger

Answer 13

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

Question 14

***Regulation of gastric emptying

Answer 14

• 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)

Question 15

Regulation of GI smooth muscle contraction

Answer 15

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

Question 16

2 types of contractions in small intestine

Answer 16

1. Segmentation contractions (mixing)
   - concentric contractions spaced at intervals along small intestine
   —> divide intestine into segments
   - one set of contractions relax —> new set begins at new points **between previous contractions
   - determined by intrinsic **“slow wave” of smooth muscle of wall: ~12-20 per min —> **CONSTANT
   - Chyme is divided, pushed **back and forth
   —> ***mixes the luminal content
2. Peristaltic contractions (propulsive)
   - progressive contractions of successive sections of **Circular smooth muscle (contraction moves foward)
   - propels chyme slowly along
   - peristalsis ↑ **after a meal (distension of stomach + duodenum)
   - Long transit time (3-5 hrs from pylorus to ileocecal valve)
   —> allow time for efficient digestion + absorption

Question 17

Empty into large intestine

Answer 17

Pressure/distension + chemical irritation in ileum
—> Ileocecal sphincter relaxes + excites peristalsis

Pressure/distension + chemical irritation in colon/caecum
—> Ileocecal sphincter contracts + inhibit peristalsis

—> allow material to enter colon + prevent reflex of fecal content into small intestine

Question 18

Motility of large intestine

Answer 18

1. Combined ***Segmentation contraction of circular + longitudinal muscle strips (i.e. Taeniae coli)
   - -> unstimulated portions of large intestine to bulge outward into haustrations

• movement of haustrations along colon: ***Sluggish
• -> allow storage of contents + absorb most of remaining water

2. Peristaltic wave (mass movement; 1-3 times per day,) propels significant amount of material into distal colon / rectum
   - -> Distension of rectum
   - -> Rectosphincter reflex (desire for defecation)

Question 19

Anal sphincter

Answer 19

Anus normally closed by:

1. Internal anal sphincter (smooth muscle)
   - innervated by **Pelvic nerves (Parasympathetic) + **Myenteric plexus (Enteric NS)
2. External anal sphincter (skeletal muscle)
   - striated muscle under voluntary control by ***Pudendal nerve (skeletal motor nerve)

Question 20

Defecation reflex

Answer 20

• initiated by mass movement of faeces into rectum

1. ***Intrinsic reflex (local enteric NS):
   distension of rectal wall
   —> Reflex ↑ in peristalsis in descending, sigmoid colon + Relaxation of internal anal sphincter
2. ***Parasympathetic reflex:
   - enhances defecation reflex
   - involves Sacral spinal cord + Pelvic nerves
3. if external sphincter also voluntarily relaxed
   —> defecation
   —> toilet training —> CNS can voluntarily override reflex relaxation of external anal sphincter —> delaying defecation

Question 21

Summary

Answer 21

Control of esophagus: Neural ONLY
Control of stomach: Neural + Hormonal
Control of small + large intestine: Neural + Hormonal
Control of anal sphincter: Neural ONLY