Digestive system I

Alimentary tract Continuous provision Achieved by Water Electrolytes Nutrients Achieved by Movement of food Digestion Mechanical and chemical Absorption Transport

Anatomical structures

Smooth muscles within the GI tract Layers Longitudinal Length-wise Circular Formation of syncitium Each fiber within respective layer Connected via gap junctions Ion movement

Contraction of GI smooth muscles Continual, slow intrinsic electrical activity Slow waves Not action potentials Too low Generated by the interaction of interstitial cells of Cajal Periodic openings of channels Do not usually cause muscle contraction

Contraction of GI smooth muscles Continual, slow intrinsic electrical activity Spike potentials Action potentials Generated when the resting potential goes over -40 mV Greater the rise in resting potential, greater the frequency Lasts longer than normal action potential (10-20 mSec) Generated by the movement of calcium ions Slower channels

Changes in resting potentials Depolarization Stretching of muscle Acetylcholine Stimulation of parasympathetic nerves GI hormones Hyperpolarization Epinephrine and norepinephrine Stimulation of sympathetic nerves

Role of calcium ions Tonic Entrance to cells Slow waves No muscle contraction Spike potentials Tonic Continuous but not associated with slow waves Continuous repetitive spike potentials Hormones and other factors Continuous entry of calcium ions Not associated with changes in membrane potential

Enteric nervous system Regulation GI tract movement GI tract secretion

Movement Afferent Fibers (local and other reflexes) Secretion Local Blood flow

Myenteric plexus Mostly linear chain Extends entire length of the GI tract Controls muscle activity along the length of the GI tract Tonic contraction/tone of the wall Intensity Rhythm (slight)

Myenteric plexus Movement of peristalic wave Inhibitory neurons Increased conduction velocity of excitatory wave Inhibitory neurons Secretion of inhibitory peptide Inhibition of sphincters Inhibits food movement

Submucosal plexus Local functions Absorption Secretion Contraction

Role of ANS Parasympathetic Cranial Sacral Excitation Vagus Esophagus, stomach, and pancreas Sacral Large intestine and anus Defecation reflex Excitation Increased activity

Sympathetic T5 and L2 of spinal cord Celiac and mesenteric ganglia Essentially innervates entire GI tract Excitation Inhibition of activity Smooth muscle Neurons of enteric nervous system T5 L2

Neurotransmitters Aceylcholine Norepinephrine/epinephrine Excitation Inhibition

Afferent sensory nerve fibers Activation Irritation of mucosa Distention Chemicals Inhibition or activation Transmission of information to the CNS Afferent vagus nerves (80 %)

Role of enteric nervous system Generation of reflexes Integrated within the enteric nervous system Local reflex Loop between the prevertebral sympathetic ganglia and GI tract Signals from lower portion of the GI tract to regulate activity of the upper GI tract or vise versa

Loop between the spinal cord/brain stem and the GI tract Vagus nerves from the stomach to the brainstem Pain reflex (inhibitory) Defecation reflex

Movement within the GI tract Propulsive movement Peristalsis Generated in response to GI tract distension Requires active myenteric plexus Formation of the contractile rings Receptive relaxation Polarized movement Move in one direction

Mixing movement Inhibition of peristalisis forward movement Sphincter Churning of the content within the segment Local intermittent constrictive contractions

Splanchnic circulation Flow of blood Afferent flow The GI tract Pancreas Spleen Enters liver via the portal vein Flow through liver sinusoids Exits liver via hepatic veins Vena cava

Absorption of nutrients Water soluble molecules 75 % temporally stored in liver Fats Intestinal lymphatics Enters circulation via thoracic duct

Arterial supply to the GI tract Mesenteric arteries (superior and inferior) Intestines Celiac artery Stomach Branches of arteries Muscle bundles Intestinal villi Submucosal vessels

Rate of flow Proportional to activity levels Increased flow Active absorption increases flow by max. 8 X Increased flow Vasodilators Decreased tissue oxygen concentrations

Counter-current exchange of oxygen Diffusion of oxygen from arterioles to venules without going through circulation Bypassed oxygen is not available for tissue metabolism

Neural regulation Parasympathietic stimulation Sympathetic stimulation Increased flow Increased glandular secretions Cause of increased flow Sympathetic stimulation Decreased flow Vasoconstriction Overcome by local vasodilators Local ischemia Allows re-direction of blood