Digestive system I

Alimentary tract

• Continuous provision– 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– 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

SecretionLocalBlood flow

AfferentFibers(local and other reflexes)

• 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• 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

• 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

• Excitation

– Norepinephrine/epinephrine• 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

• 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

• 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