Digestion and Liver Function

• GI motility involves the movement of the GI tract.
• Digestion, absorption, and defecation are some of the basic processes of the GI system.

Role of the Gastrointestinal System

• Mechanically and chemically breaks down food into absorbable components
• Absorption and assimilation of nutrients
• Stores and converts excess glucose to glycogen and fat
• Manufactures materials
• Excretes waste products
• Facilitates speech
• Stores blood
• Regulates blood component
• Produces hormones, making it also an endocrine organ

Functional Anatomy

• The GI tract includes the esophagus, stomach, small intestines, and colon.
• Accessory organs aid in digestion.
• Physiologic barriers or partitions include sphincters.

Feces

• Feces have excretory products like heavy metals, organic ions, cell debris, and dead white blood cells.
• The brown color results from bacterial action on bilirubin (stercobilin).
• Odor is caused by bacterial action (indole, skatole, hydrogen sulfide, mercaptans).
• Feces composition: 75% water, 25% solid particles (fiber, bacteria, cholesterol/fat, inorganic substances, protein).

General Organization

• The mucosa absorbs and secretes
• The submucosa supports the mucosa with blood and lymph vessels
• The muscularis externa propels food via inner circular and outer longitudinal layers

Regulation of GI Activities

• GI activity regulation: communication through endocrine, paracrine, and neurocrine mechanisms.
• Stimuli from the GI tract activate intrinsic and extrinsic sensory afferent pathways.
• The brain and spinal cord influence the enteric nervous system affecting motility, secretion, and blood flow.

Control of GI Function

• The digestive system performs six processes: ingestion, secretion, motility, digestion, absorption, and defecation.
• GI motility aids in mixing and propulsion of food, coordinated with secretion and absorption

GI Smooth Muscle

• GI smooth muscle is involuntary but can be contracted voluntarily in some parts (muscles in GIT).
• The muscular layer has inner circular and outer longitudinal layers.
• Exceptions: The esophagus has varying muscle types and the external anal sphincter is striated

Electrical Activity of GIT

• Interstitial Cells of Cajal (ICC) act as pacemaker cells, generating electrical impulses for muscle contraction
• ICCs produce slow waves leading to depolarization and action potentials.
• Slow waves are undulating changes in resting membrane potential generated by ICC
• Spike potentials are true action potentials that cause muscle contraction.
• Depolarization of slow waves, from calcium influx, causes spike potentials and contraction

Smooth Muscles

• Slow waves are affected by stretch, acetylcholine, and parasympathetics
• Hyperpolarization is stimulated by norepinephrine and sympathetics

Types of Movement

• Phasic movements are short and rhythmic that mix and transmit chyme.
• Tonic movements sustain long constrictions for limiting flow or providing reservoirs
• Types of movement are peristaltic and segmentation.

Regulation of GI Motility

• Extrinsic: Involves the autonomic nervous system.
• Intrinsic: Involves the enteric nervous system

Specific Motility Patterns

• Esophagus: 6-8 seconds to traverse
• Stomach (fundus): 20-30 minutes, quiescent
• Small bowel: 100-150 minutes, active
• Colon: 12-30 hours, minimally active

Mastication

• Mastication: food bolus is broken.
• Increases pleasure
• Taste receptors contacted
• Teeth chew

Deglutition

• Oral Phase: voluntary, bolus propelled to pharynx.
• Pharyngeal Phase: reflex, respiration inhibited, bolus moves into esophagus.
• Esophageal Phase: bolus moves from pharynx to stomach.

Esophagus

• Primary peristalsis: initiates with swallowing triggering mechanoreceptors
• Secondary peristalsis: triggered by esophageal distention

Pressures

• UES/Pharyngoesophageal Sphincter Pressure: 50-150 mmHg.
• LES/Gastroesophageal Sphincter Pressure: 15-50 mmHg.

Gastroesophageal Reflux

• Protective effects: mechanics avoid reflux during decreased pressure

Gastric Motility

• Storage: Fundus.
• Mixing: Distal corpus and antrum.
• Emptying: Pyloric sphincter.

Enterogastric Reflex

• Prevents food from entering to duodenum

Gastric Emptying

• Gastric emptying is dependent on the type of consistency of food.
• The pylorus has limited capacity to empty the food.
• Gastric factors include increased volume and gastrin promoting emptying
• Duodenal factors include high H+, high lipid, increased osmolality, and tryptophan inhibiting emptying.

Retropulsion

• Chemical receptors detect protein digestion products and mechanoreceptors detect distention of the gastric walls
• Activity in smooth muscles and pyloric sphincters lead to grinding in the gastric antrum

Vomiting

• Forceful expulsion of contents of the stomach and upper intestinal.
• Coordinated in the vomiting center found in the medulla

Small Intestine Motility

• Fed State: Mixing or segmentation contractions and Peristalsis or propulsive contraction
• Fasted State: Migratory Motor Complex (MCC)

Ileocecal Valve

• Prevents backflow from the colon to the small intestine.
• Fluidity of contents promote colon emptying
• Pressure and chemical irritation promotes colon emptying
• Colon emptying inhibited by pressure and chemical irritation of the cecum

Large Intestine Motility

• Haustrations mix colonic contents
• Mass movements are propulsive
• Defecation: Expulsion of feces is voluntary

Rectoanal Inhibitory Reflex

• The anus undergoes relaxation in response to rectal distension

Gallbladder Motility

• The gallbladder stores, concentrates, and excretes bile
• Empties 25% during Phase II and 75% during fatty meals
• Stimulated by cholecystokinin (CCK)

Sphincter of Oddi

• Prevents and allows bile and pancreatic enzymes to the duodenum
• Relaxes by VIP
• Tonically contracted which prevents the gallbladder from filling when fasting