DIGESTION IN SMALL INTESTINE – MOTILITY, SECRETION & CONTROL
DIGESTION IN SMALL INTESTINE – MOTILITY, SECRETION & CONTROL
- Motility in small intestine,
– Propulsive movement (Peristalsis).
– Mixing movement (Segmentation).
1.PROPULSIVE MOVEMENT/PERISTALSIS:
- Stimulus for peristalsis – Distention of gut.
- Local stretch reflex – Reflex contraction of gut wall to stretch.
Mechanism of action:
- Peristalsis consists contraction ring in one segment & its receptive relaxation in immediate distal segment.
- Results in content propulsion from contracted to relaxed segment.
- Cycle repeats, resulting in contraction wave traveling distally resulting in content propulsion.
Activation:
1. Neuronal activation factors:
- Requires an active myenteric plexus.
- Local stretch by chyme releases serotonin activating myenteric plexus.
- Plexus’ cholinergic neurons passing in retrograde/antegrade direction release substance causing smooth muscle contraction.
– Acetylcholine & substance P.
- Activate neurons secrete substances producing relaxation ahead of stimulus.
– VIP, NO & ATP,
2. Hormonal activation factors:
- Gastrin, Cholecystokinin, insulin, motilin & serotonin enhance intestinal motility.
3. Hormonal inhibiting factors:
- Secretin & glucagon inhibit small intestinal motility.
Factors influencing:
- Can be increased/decreased by autonomic input to gut.
- Occurrence is independent of extrinsic innervation.
Peristaltic rush –
- Very intense peristaltic wave.
- Not seen in normal individuals.
- Occurs in obstrcted intestine.
2. MIXING MOVEMENTS/SEGMENTATION:
- With presence of meal, enteric nervous system promotes peristalsis.
– Retards intestinal contents movement.
- Contraction ring appear at irregular intervals of distance.
– Divides 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:
1. Gastroileal reflex:
- Increased secretory & motor activity of stomach.
- Occurring normally after a meal,
– Increases motility of ileum.
– Relaxes ileocaecal sphincter.
- Mediated by gastrin.
- Designed to empty small intestine in anticipation of filing process.
2. Ileogastric reflex:
- Distension of ileum decreasing gastric motility.
- Reflex prevents further ileum distension.
3. Intestointestinal reflex:
- Distension of one segment of ileum leads to relaxing rest of intestine.
- Either relieve distension/stop further movements till distension cause is removed.
- Dependent on extrinsic innervation.
Exam Important
DIGESTION IN SMALL INTESTINE – MOTILITY, SECRETION & CONTROL
Propulsive movement:
- Referred as “Peristalsis”.
- Stimulus – Distention of gut
Activation:
1. Neuronal activation factors:
- Requires an active myenteric plexus.
- Local stretch by chyme releases serotonin, which activates myenteric plexus.
- VIP, NO & ATP → Producing relaxation ahead of stimulus
- Acetylcholine & substance P → Causing smooth muscle contraction.
2. Hormonal activation factors:
- Gastrin, Cholecystokinin, insulin, motilin & serotonin enhance intestinal motility.
3. Hormonal inhibiting factors:
- Secretin & glucagon inhibit small intestinal motility.
Factors influencing:
- Can be increased/decreased by autonomic input to gut.
- Occurrence is independent of extrinsic innervation.
Peristaltic rush –
- Very intense peristaltic wave.
- Not seen in normal individuals.
- Occurs on intestinal obstruction.
2. MIXING MOVEMENTS:
- Also referred “Segmentation“.
REFLEXES INVOLVED:
1. Gastroileal reflex:
- Increased secretory & motor activity of the stomach.
- Occurring normally after a meal,
– Increases motility of ileum.
– Mediated by gastrin.
2. Ileogastric reflex & Intestointestinal reflex.
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