Gastrointestinal Motility and Its Regulation: Study Notes

The Gastrointestinal System

Gastrointestinal Motility and Its Regulation

The gastrointestinal (GI) system relies on coordinated movements of its muscular walls to mix and propel contents, facilitating digestion and absorption. These movements are regulated by neural, hormonal, and intrinsic pacemaker mechanisms.

• GI Motility: Refers to the movements of the GI tract wall, primarily due to contractions of the muscularis externa.

• Main Functions: Mixing and propelling GI tract contents.

Electrical Activity in GI Smooth Muscle

GI smooth muscle exhibits spontaneous electrical activity, which underlies its contractile patterns.

• Pacemaker Activity: GI smooth muscle generates slow waves of depolarization, which may or may not reach threshold to trigger contractions.

• Basic Electrical Rhythm (BER): The frequency of slow waves varies throughout the GI tract and determines the timing of contractions.

• Amplitude Modulation: Neural and hormonal inputs affect the amplitude of slow waves:

  • Parasympathetic stimulation: Increases amplitude (excitation).

  • Sympathetic stimulation: Decreases amplitude (inhibition).

• Stomach: Larger depolarizations lead to stronger contractions; action potentials are not required for force development, but their presence increases contraction strength.

• Intestines: Action potentials are required for force; contraction strength varies with action potential frequency.

Peristalsis and Segmentation

Two primary patterns of GI motility are peristalsis and segmentation, each serving distinct roles in digestion.

• Peristalsis: Propels contents from oral to aboral direction (mouth to anus).

  • Proximal segment: Circular muscle contracts, longitudinal muscle relaxes, diameter decreases.

  • Distal segment: Circular muscle relaxes, longitudinal muscle contracts, diameter increases.

  • Contents are propelled forward from areas of small diameter to large diameter.

  • Occurs in esophagus, stomach, and intestines.

• Segmentation: Alternating contractions of the circular muscle layer between intestinal segments.

  • Mixes chyme and brings it into contact with mucosa for absorption.

  • Overall movement is oral to aboral, but primarily serves mixing rather than propulsion.

  • Occurs mainly in the small intestine.

Chewing and Swallowing

Chewing and swallowing are the initial steps in GI motility, involving both voluntary and involuntary control.

• Chewing: Involves voluntary and involuntary components; the chewing reflex is cyclical, with jaw muscles alternately contracting and relaxing in response to food pressure.

• Swallowing (Deglutition):

  • Bolus: Chewed food mixed with saliva.

  • Tongue: Forms and moves bolus to pharynx, initiating the swallowing reflex.

  • Swallowing center: Located in the medulla oblongata; coordinates the swallowing reflex.

  • Receptive relaxation: Swallowing center also initiates relaxation of stomach smooth muscle prior to bolus arrival.

Events of the Swallowing Reflex

1. Bolus pushed into pharynx, pressing on epiglottis; glottis moves upward and closes, epiglottis covers glottis to prevent food entry into trachea; inspiration is inhibited.

2. Upper esophageal sphincter relaxes, bolus enters esophagus, sphincter closes behind bolus.

3. Wave of peristalsis travels along esophagus, propelling bolus toward stomach; travel time is approximately 9 seconds.

4. Lower esophageal sphincter opens, bolus enters stomach.

5. If necessary, secondary peristaltic waves move bolus into stomach.

Gastric Motility

Gastric motility refers to the movement patterns of the stomach, regulated by the enteric nervous system (ENS) and various hormones.

• Peristaltic Waves: Occur at a rate of 3 per minute, moving from the upper body to the pylorus and strengthening as they approach the pylorus.

• Mixing of Chyme: Pyloric sphincter closure propels chyme backward for mixing.

• Gastric Emptying: Stronger contractions open the pyloric sphincter; rate is governed by:

  • Volume of chyme in stomach

  • Strength of gastric peristalsis

  • Contents of chyme:

    • Liquids: ~3 min after ingestion

    • Small particles (<1mm): 20-30 min after ingestion

    • Large particles: up to 9 hours after ingestion

• Migrating Motility Complex: Wave of intense contractions that travels short distances between meals to clear the stomach.

Regulation of Gastric Motility

• Activation of stomach: Promotes contractions; gastrin increases force.

• Activation of small intestine: Inhibits contractions; CCK, secretin, and GIP decrease force.

Phases of Regulation

• Cephalic-phase: Excitations (anger, aggression) and inhibitions (pain, fear, depression).

• Gastric-phase: Excitatory stimulus is distension of the stomach.

• Intestinal-phase: Inhibitory stimuli include distension of the duodenum and chyme contents (osmolarity, acidity, fat).

Vomiting

• Stimuli: Illness, emotions, pain, excessive GI distension, motion sickness, ingestion of emetics.

• Vomiting center: Located in the medulla oblongata; initiates strong abdominal contractions and retrograde movement of chyme from stomach and duodenum.

Motility of the Small Intestine

The small intestine exhibits segmentation and peristalsis, regulated by neural and hormonal mechanisms.

• Segmentation: Mixes chyme.

• Peristalsis: Propels chyme forward.

• Regulation:

  • Distension increases motility.

  • Autonomic nervous system:

    • Parasympathetic: excites motility.

    • Sympathetic: inhibits motility.

  • Hormones: gastrin stimulates motility.

Motility Reflexes (CNS)

• Intestino-intestinal reflex: Injury or severe distension inhibits intestinal contractions.

• Ileogastric reflex: Distension of ileum inhibits gastric motility, reducing stomach emptying rate.

• Gastroileal reflex: Presence of chyme in stomach increases motility in ileum.

Motility of the Colon

The large intestine displays unique motility patterns to facilitate water absorption and fecal elimination.

• Haustration: Occurs in the proximal colon; similar to segmentation but slower (about 2 per hour).

• Mass Movement: Propels contents toward rectum; similar to peristalsis but contractions last longer than relaxation.

Motility Reflexes (CNS)

• Colonocolonic reflex: Distension of one area of colon causes relaxation of other areas.

• Gastrocolic reflex: Food in stomach increases colonic motility and frequency of mass movement.

Defecation Reflex

• Control: Both voluntary and involuntary.

• Mechanism:

  • Distension of rectum activates stretch receptors, causing smooth muscle contraction and increased rectal pressure.

  • Peristaltic contractions of sigmoid colon propel more fecal material into rectum, further increasing pressure.

  • Internal anal sphincter relaxes as external anal sphincter contracts; with continued pressure, both sphincters relax, resulting in defecation.

Key Terms Table

Term	Definition	Role in Digestion
Peristalsis	Wave-like contractions propelling contents	Moves food through GI tract
Segmentation	Alternating contractions mixing contents	Mixes chyme, enhances absorption
Migrating Motility Complex	Intense contractions between meals	Clears stomach and intestines
Haustration	Segmental contractions in colon	Mixes and moves contents slowly
Mass Movement	Strong contractions in colon	Propels fecal material toward rectum
Basic Electrical Rhythm (BER)	Spontaneous slow waves in smooth muscle	Coordinates timing of contractions

Example: During a meal, peristalsis in the esophagus propels the swallowed bolus toward the stomach, while segmentation in the small intestine mixes the chyme for optimal nutrient absorption.