Introduction to GIT Slidelearn Team

Function of GIT Tract Main functions of GIT is that it Provides continual supply of water, electrolytes and nutrients to the body: All this could be accomplished by Movement of food Secretion of enzymes and digestion Absorption of digestive products, water & electrolytes Circulation of blood through GI organs to carry away absorbed substances Control of all these functions by nervous and hormonal systems

Individual Functions Each part of the GIT is adapted to its specific function: Simple passage of food – Esophagus Storage of food – Stomach Digestion and absorption – Small intestine

PHYSIOLOGIC ANATOMY OF GASTROINTESTINAL WALL Layers of GI wall: From inner layer outward Mucosa - with muscularis mucosae Submucosa Circular smooth muscle layer Longitudinal smooth muscle layer Serosa – visceral peritoneal lining Peritoneum – mesentery – attached to posterior abdominal wall Adventitia organs that are not covered by peritoneum i.e. retroperitoneal organs posses adventitia

FUNCTIONAL SYNCYTIUM GI smooth muscles act as a Functional syncytium Arrangement in bundles Longitudinal and circular layers Fibers in each bundle ---– electrically connected by gap junctions

Action Potential Gap junctions – allow low resistance – movement of ions from one cell to the next Electrical signals travel readily from one fiber to the next within each bundle Conduction is more rapid lengthwise than sideways Each bundle is partly separated by loose connective tissue Each bundle is partly connected at many points so each muscle layer represents a branching latticework of smooth muscle bundles

Action Potential (Cont...) Muscle Sheath acts as a functional syncytium Therefore electrical signals initiated anywhere can travel in all directions within the muscle Distance traversed depends on excitability Few connections between circular and longitudinal layers So excitation of one layer leads to excitation of other layer.

ELECTRICAL ACTIVITY OF GI SMOOTH MUSCLES Resting Membrane Potential: -50 to -60 mV - changeable to different levels Avg. __ - 56mv Slow continual intrinsic electrical activity Two basic types of electrical waves 1. Slow waves ( basic electrical rhythm) 2. Spike potentials

SLOW WAVES Slow undulating changes in RMP They are not action potentials Frequency of slow waves = rhythmicity of GI contraction Intensity varies between 5-15 mv Frequency ranges between 3-12/minute in different parts of GIT Unknown mechanism of production but believed to result from complex interactions among the smooth muscle cells and specialized cells called interstitial cells of Cajal. Interstitial cells of Cajal---- electrical pacemakers Unique ion channels which open periodically and produce pacemaker currents Do not cause muscle contraction except in stomach Control appearance of intermittent spike potentials

SPIKE POTENTIALS True action potentials Generated automatically when RMP rises to -40mV When peaks of slow waves rise above – 40 mV spike potentials appear on these peaks Frequency: 1 – 10/second Lasts for 10 – 20 milliseconds

COMPARISON OF ACTION POTENTIALS GI smooth muscle Duration: 10-20msec Generated by entry of Ca++ and Na+ Nerve fibre Duration: 0.3 – 0.4 msec Generated by rapid entry of Na+ ions through rapid Na+ channels

Depolarization: When membrane potential becomes less negative (more excitable) Hyperpolarization: When membrane potential becomes more negative (less excitable)

FACTORS THAT DEPOLARIZE MEMBRANES Stretching of muscle Stimulation by acetylcholine Parasympathetic stimulation Stimulation by GI hormones FACTORS THAT HYPERPOLARIZE MEMBRANE Effect of epinephrine and nor-epinephrine Stimulation of sympathetic nerves

CALCIUM IONS AND MUSCLE CONTRACTION Entry of Ca++ ions acting through a calmodulin control mechanism Interaction between actin and myosin In slow waves there is no Ca++ entry – only entry of Na+ ions In spikes – large number of Ca++ and small number of Na+ ions enter

TONIC CONTRACTION Some smooth muscles exhibit tonic contraction/rhythmical contractions Not associated with slow waves – continuous in nature Last several minutes to hours Intensity may vary Sometimes produced by continuous repetitive spikes Sometimes caused by hormones Sometimes by continuous entry of Calcium ions----indifferent of changes in membrane potential.