GI motility

Stimulus

• Irritant molecules
• Radial stretch of gut lumen

Sensor

• Intrinsic primary afferent neurons (stretch and chemoreceptors)

Controller

• Interneurons in the myenteric plexus

Efferent

• Inhibitory (NO) and excitatory (ACh) motor neurons

Effector

• Circular and longidudenal smooth muscle

Mechanical features of content

• Stretch (myenteric reflex) \(\uparrow\)

Chemical features of chyme

• Acidic pH \(\uparrow\)
• Hyperosmolar \(\uparrow\)
• Lipids \(\downarrow\) (so there's time to absorb them)

Serum electrolyte disturbances \(\downarrow\) peristalsis

• HypoK, hypoCa, hypoMg
• Calcium channel blockers

Excitatory hormones

• ACh (+ nicotine, neostigmine)
• Motilin (+ erythromycin)
• Serotonin

Inhibitory hormones:

• NO
• VIP
• Catecholamines

Process

1. The fundus and body relax to store food bolus
2. The antrum tonically contracts to create a 5mmHg pressure gradient with the duodenum
3. The antrum then produces peristaltic waves, pulverizing larger particles against the closed pylorus + pusing small particles through

Solids

1. Receptive relaxation of fundus and body (stores food)
2. Lag phase of ~1 hour as antrum grinds food
3. Linear emptying phase as particles becomes small enough, with half-time of ~1 hour
4. Overall takes 3-4 hours for stomach to empty

Liquids

1. \(\uparrow \Delta V\ \ \to \ \uparrow \Delta P\)
2. Therefore exponential emptying with \(t_{\frac{1}{2}} = 30\text{mins}\)

Fed or fasted?

• MMCs in fasted state sweep stomach of secretons
• Abolished in fed state

Position

• Left lateral or supine \(\downarrow\)

Mechanical features of content

• Solids slower than liquids (lag phase \(\to\) linear emptying vs exponential emptying)
• For liquids, \(\uparrow V \ \to \ \uparrow\) rate of emptying due to stretch
• For solids, stomach stretching depresses peristalsis

Chemical features of content (mostly opposite of the intestinal effects!)

• Acidic pH \(\downarrow\)
• Hypertonicity \(\downarrow\)
• Lipids \(\downarrow\) (same as intestine)

Excitatory hormones

• Motilin
• Ghrelin
• ACh
• Circulating catecholamines

Inhibatory hormones

• CCK and secretin
• NO
• VIP
• Glucagon, GLP1

Position

• Upright posture (gravity)
• Right lateral (pylorus dependent)

Anatomical features

• His angle forms a mucosal flap (one-way valve)

Sphincters

• LOS; smooth muscle, around lower oesophagus. Tonically contracted.
• Diaphragmatic crura; skeletal muscle, sides of the oesophageal hiatus
• LOS tone > gastric pressure (usually 20mmHg)

Determinants of LOS tone

Neruogenic (vagal inhibition \(\to\) NO \(\to\) relaxation)

Myogenic (tonic contraction)

Hormones that increase rate of gastric emptying also increase sphincter tone (to maintain difference)

• Gastrin
• Motilin

And vice versa

• Secretin
• Glucagon
• CCK
• VIP