GI Motility & Regulation

Question 1

Mechanism of GI muscle contraction (basic)

Answer 1

• GI muscle = smooth muscle = arranged actin (thin) and myosin (thick) filaments
  
  • myosin forms cross bridges with actin
• Ca²⁺ entry into cell (via VSCC or sarcoplasmic reticulum) ==> myosin-actin interactions ==> muscle contraction
  
  • Ca²⁺ complexes w/calmodulin ==> activates myosin light chain kinase ==> phosphorylates myosin ==> cycling (cross-bridge formation) to occur

Question 2

Characteristics of basic electrical rhythm (BER)

Answer 2

• membrane potential of GI smooth muscle = ~-60mV
• • spontaneous rhythmic depolarizations = “slow waves” = “basic electrical rhythm” (BER)
    
    • voltage = 10-15 mV
    • rate = 12/min
    • rapid upstroke depolarization, slight repolarization, then plateau, complete repolarization
    • BER is myogenic = muscle-derived/intrinsic
• depolarizations do not cause contractions

Question 3

BER regulation of muscle contraction

Answer 3

• BER alone does not induce muscle contraction
• contraction occurs when sufficient depolarization is reached during one or more action potentials (“spike potentials”) at peak of slow wave
• addition of ACh to muscle region ==> muscle contraction @ frequency set by BER

Question 4

Characteristics of segmentation (GI movement)

Answer 4

• mechanism
  
  • isolated contraction ==> propulsion of contents in both directions
  • relaxation ==> contents flow back to originial segment ==> mixing w/out net propulsion
• fxn
  
  • ensures proper digestion and absorption
  • predominant @ intestines

Question 5

Mechanism of peristalsis

Answer 5

1. Bolus induces distention ==> contractile ring forms proximal to bolus ==>
2. Contactile ring pushes bolus distally
3. Intestine distal to bolus undergoes receptive relaxation
4. Contraction of next segment follows

• coordination requires myenteric plexus input
• coordination ==> polarized propulsion

Question 6

Food travel from mouth ==> stomach

Answer 6

1. food enters mouth ==> back of oral cavity toards pharynx
2. swallowing - voluntary phase
3. swallowing reflex
   
   1. swallowing - pharyngeal phase
   2. swallowing - esophogeal phase
      
      1. peristatlic wave sends bolus ==> stomach

Question 7

Mechanism of swallowing

Answer 7

• voluntary, pharyngeal, and esophageal phases

1. tongue pushes bolus backward into pharynx (voluntary)
2. soft palate rises + upper constrictor m. contraction ==> nasopharynx closes (voluntary)
3. respiration centrally inhibited + larynx rises + glottis closes ==> protects trachea from bolus entry (involuntary)
4. upper esophageal sphincter relaxes (involuntary)
5. coordinated contraction of the middle and lower constrictor muscles ==> bolus into the esophagus (involuntary)

Question 8

Characteristics of esophageal peristalsis

Answer 8

• structure = upper 1/3 is skeletal m.; lower 2/3 primarily smooth muscle
• peristaltic wave of contraction ==> bolus to lower esophageal sphincter (LES)
  
  • controlled by vagus nerve
• LES relaxes and bolus ==> stomach

Question 9

Major fxns of the stomach

Answer 9

• storage
• mixing
• slow, controlled emptying

Question 10

Mechanism of peristalsis in stomach

Answer 10

1. bolus arrives ==> contractions start around mid-stomach @ ~3/min
2. peristaltic waves push a bolus toward the antrum
3. contractions become stronger and faster in the antrum and begin to outrun the bolus
4. Since pyloric opening small, most content reflected backward toward the body of the stomach.

• reflection of contents = “retropulslon”
• serves to break up the food + mix it with digestive juices so that absorption can take place later in the gut. The digestive juice mixture is called chyme.

Question 11

Mechanism of gastric emptying

Answer 11

• Transient opening of the pylorus allows smaller particles and chyme to leave the stomach and enter the duodenum.
• Gastric emptying is controlled by pyloric sphincter tone; sphincter = normally under high tone
• autonomic/hormonal controls impact sphincter tone

Question 12

Control of gastric emptying

Answer 12

• @ stomach
  
  • increased stretch ==> increased peristalsis + decreased pyloric tone (via vagal and myenteric reflexes)
  • food presence ==> gastrin release ==> inncreased peristalsis + decreased pyloric tone
• @ duodenum
  
  • arrival of food ==> irritation via acidity + high osmolarity ==> reflex inhibition of gastric peristalsis + increased pyloric tone
  • arrival of fats ==> cholecystokinin (CCK) release by enteric endocrine cells ==> decreased gastric motility/emptying

Question 13

Mechanism of intestinal motility @ small intestine (absorptive phase)

Answer 13

• Segmentation
  
  • rhythm of segmentation = same as BER
  • fxn = chyme mixed w/digestive enzymes and kneaded ==> absorptive surfaces continually exposed to new contents
• Peristalsis
  
  • Peristaltic movements occur over short distances ==> propel the digestive chyme @ ~1cm/min in an aboral (distal) direction
  • 3-5 hrs to transit the intestin

Question 14

Mechanism of intestinal motility @ small intestine (between meals)

Answer 14

• MMC = migrating myoelectric motor complexes
• MMCs migrate down gastric antrum and along small intestines
• occurs ~every 90 mins
• fxn = housekeeping role to remove bacteria and indigestible material
• corresponds to rise in plasma concentration of motilin (released by small intestines)

Question 15

Characteristics of large intestine motility

Answer 15

• segmentation contractions ==> sacculations = “hausta”
  
  • fxn = mix and dry chyme to reduce fluid lost in feces
• propulsion forward = “mass movement”
  
  • very intense, prolonged peristaltic contraction
  • strips are of intestine clear of contents
  • segmental activity stops = no haustra

Question 16

Mechanism of defecation

Answer 16

• after meal ==> gastrocolic and duodenocolic reflexes increase colonic motility ==> push feces into rectum
• ==> stimulation of defecation rflex mediated by pelvic nerves
  
  • ==> relaxation of internal anal sphincter (involuntary)
  • ==> relaxation of external anal sphincter (voluntary)