Gastrointestinal System: Anatomy and Function

• Functional Anatomy and General Principles of Functions of Gastrointestinal System

Functional Anatomy

• Functional organization involves: mouth, pharynx, esophagus, stomach, small intestine, large intestine
• Structural characteristics of the GIT wall are important
• Innervation of the GIT includes intrinsic and extrinsic components
• Gastrointestinal blood flow sustains function

General Principles of Gastrointestinal Functions

• Motility, secretion
• Gastrointestinal motility has unique characteristics
• Gastrointestinal hormones play a key role

Functional Organization

• Digestive system consists of the gastrointestinal tract (GIT) and accessory digestive organs: teeth, tongue, salivary glands, liver and exocrine pancreas
• Alimentary canal is a muscular tube from mouth to anus, around 10m long
• Mouth leads into an oral cavity, also called the buccal cavity that contains the tongue and teeth
• Oral cavity extends from lips to the oropharyngeal isthmus, the junction with the pharynx, and subdivides into vestibule and oral cavity proper
• Vestibule lies between lips/cheeks (external) and gums/teeth (internal)
• Oral cavity proper lies within alveolar arches, gums, and teeth

Accessory Organs

• Tongue performs a role in tasting food and chewing/swallowing
• Incisors cut, while canines tear, and pre-molars/molars grind food
• Pharynx serves as passage common to gastrointestinal and respiratory systems
• Esophagus is a fibromuscular tube, about 25 cm long, with upper/lower sphincters present
• Stomach: hollow bag connecting the esophagus to the duodenum
• Small intestine (long tubular structure): duodenum (~25 cm), jejunum (~2.5 m), ileum (~3.5 m)
• Large intestine: encloses the small intestine with the caecum, colon, anal canal, appendix and rectum

Structural Features of the GIT Wall

• Intestinal wall typically consists of the mucosa, submucosa, muscle coat, and serosa
• Mucosa: the innermost coat, consisting of epithelium, lamina propria (connective tissue, glands, vessels, nerve fibers) and muscularis mucosae
• Submucosa: connective tissue layer with vessels, lymphatics, and nerve fibers (submucosal plexus)
• Muscle coat: smooth muscle fibers in circular and longitudinal layers, contains Auerbach's plexus (myenteric plexus)
• Serosa: outermost connective tissue layer, aids attachment

Innervation of the GIT

• The GIT includes an intrinsic (enteric nervous system) and extrinsic system
• Intrinsic nervous system: nerve cells/fibers within intestinal wall, controls muscle contraction

Enteric Nervous System

• ANS is a network of nerve cells and fibers within the intestinal wall itself for smooth muscles of musculature, secretion, and motility
• Auerbach's plexus (between muscle layers) modulates the tone/contractions/rate/velocity
• Meissner's plexus: in submucosa, and controls blood flow/secretory activity
• The Auerbach's and Meissner's plexuses interconnect and receive parasympathetic/sympathetic input

Extrinsic Innervation & Divisions

• The extrinsic system nerves comes primarily from the parasympathetic and sympathetic
• Parasympathetic division: cranial (vagus) and sacral (pelvic) outflow
• Parasympathetic stimulation excites muscle/secretion, inhibits sphincters
• Sympathetic innervation from T8-L2 spinal segments innervates all GIT portions
• Sympathetic vasoconstriction, excitation of sphincters, inhibition of gut motility

Gastrointestinal Blood Flow

• Proportional to local activity

Gastrointestinal Motility

• Gut's smooth muscle acts as syncytium

• Resting membrane potential (RMP) shows undulating slow waves -50 to -60mV

• The slow waves may determine contractions

• Factors, like stretching, acetylcholine, parasympathetic nerves, and gastrointestinal hormones, depolarize the membrane.

• Factors that hyperpolarize the membrane: norepinephrine, sympathetic nerves at their nerve endings

• Spike potentials (action potentials) occur above -40mV using calcium-sodium channels

• Peristalsis includes propulsive and mixing

Gastrointestinal Hormones

• Regulate secretion and sometimes motility
• Glandular cells are scattered, targeting nearby tissues via portal circulation
• Their effects persist after nerve connections are severed
• Specific Features:
  • Each hormone affects multiple target tissues
  • Each target tissue responds to more than one hormone

Hormone Families

• Gastrin family: gastrin, cholecystokinin
• Secretin family: secretin, GIP, VIP, glucagon
• Other hormones: motilin, neurotensin, substance P, GRP, somatostatin