Gastrointestinal Motility
GASTROINTESTINAL MOTILITY
Basic Electrical Rhythm (BER):
- Are “Spontaneous rhythmic fluctuations of GI tract’s smooth muscle cells producing membrane potential between about -65 and -45 mV.
- Initiated by Pacemaker cells/“Interstitial cells of Cajal”.
Events:
- During these cyclic fluctuations, if resting membrane potential crosses threshold for excitation, an action potential or a burst of action potential may develop.
- Hence, BER itself does not cause muscle contraction.
- But when spike potentials (action potentials) superimposed on most depolarizing portions of BER, muscle contraction occurs.
- Tone of contraction depends on,
- Frequency of stimulus rather than amplitude.
Metrics:
- Threshold for action potential is -50 mV.
- Rate of BER:
- About 4/min in stomach.
- About 12/min in duodenum
- About 8/min – distal ileum (falls down).
- In Colon:
- Rate rises from about 2/min at Cecum → about 6 min at Sigmoid colon
Pacemaker cells –
- Interstitial cells of Cajal.
- Stellate mesenchymal cells with smooth muscle-like features.
Location:
- Are absent in esophagus & proximal stomach.
- In distal stomach & small intestine – Located in outer circular muscle layer near myenteric plexus.
- In colon – Located at submucosal border of circular muscle layer.
Frequency:
- In Stomach & small intestine –
- Pacemaker frequency is in descending gradient.
- Pacemaker with the highest frequency usually dominates (Eg., In Heart).
MOTILITY ACROSS GI TRACT:
1. MOTILITY IN STOMACH:
1a) Gastric contraction:
- 2 phases –
- Digestive phase – Type 1 & Type 2 waves.
- Interdigestive phase – 2 contractions types – Migratory Motor Complexes & Hunger Contractions.
1b) Gastric emptying:
- Rate of stomach emptying its contents into duodenum.
- Via 2 mechansims – Hormonal mechanism & Enterogastric reflex.
2. MOTILITY IN SMALL INTESTINE:
- Propulsive movement (Peristalsis).
- Mixing movement (Segmentation).
1. Propulsive movement:
- Small intestinal propulsive movement referred as “Peristalsis”.
- Stimulus– Distention of gut
2.Mixing movements:
- Also referred “Segmentation”.
- Contraction rings appear at irregular intervals of distance.
- Dividing intestine into series of segments.
Functions:
- Provides for ample mixing of intestinal contents.
- Works along intestinal tract length providing time for digestion & absorption.
Reflexes involved:
- Gastroileal reflex.
- Ileogastric reflex.
- Intestointestinal reflex.
3. MOTILITY IN LARGE INTESTINE:
- Propulsive movements
- Mixing movements / Haustrations
Reflexes involved:
1. Gastrocolic & Duodenum reflex:
- Facilitates appearance of mass movements after a meal.
- Ex., For gastrocolic reflex –
- Seen in early infancy.
- Where defecation often follows a meal.
- Maximum post-prandial motility seen in proximal part of descending colon.
2. Defecation reflex / Rectoanal inhibitory reflex:
- Initiated by distension of rectum with feces.
- Leads to propulsive wave extending from descending colon through sigmoid colon to rectum
- Internal sphincter relaxes reflexly – “Rectoanal inhibitory reflex”.
Transit time:
- From sigmoid colon to anus, slower transport & quarter of test meal residue retained in rectum for upto 3 days.
- Food residue spends more than 20 hrs in colon before lost in feces.
Exam Question
GASTROINTESTINAL MOTILITY
- BER initiated by Pacemaker cells called “Interstitial cells of Cajal”.
- Threshold for action potential is -50 mV.
- Rate of BER:
- About 4/min in stomach.
- About 12/min in duodenum.
- About 8/min – distal ileum (falls down).
- In Colon – Rate rises from about 2/min at Cecum → about 6 min at Sigmoid colon
Pacemaker cells –
- Interstitial cells of Cajal.
- Are absent in esophagus & proximal stomach.
- In distal stomach & small intestine – Located in outer circular muscle layer near myenteric plexus.
1. MOTILITY IN STOMACH:
- Gastric contraction.
- Gastric emptying.
2. MOTILITY IN SMALL INTESTINE:
- Propulsive movement (Peristalsis).
- Mixing movement/Segmentation.
3. MOTILITY IN LARGE INTESTINE:
- Propulsive movements.
- Mixing movements / Haustrations
Reflexes involved:
1. Gastrocolic & Duodenum reflex:
- Facilitates appearance of mass movements after a meal.
- Ex., For gastrocolic reflex –
- Seen in early infancy.
- Where defecation often follows a meal.
- Maximum post-prandial motility seen in proximal part of descending colon.
2. Defecation reflex / Rectoanal inhibitory reflex:
- Initiated by distension of rectum with feces.
- Leads to propulsive wave extending from descending colon through sigmoid colon to rectum.
- Also referred”Rectoanal inhibitory reflex”.
Transit time:
- From sigmoid colon to anus, slower transport & quarter of test meal residue retained in rectum upto 3 days.
- Food residue spends more than 20 hrs in colon before lost in feces.
Don’t Forget to Solve all the previous Year Question asked on Gastrointestinal Motility
