Introduction to GIT

Slidelearn Team

Function of GIT TractMain 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 digestionAbsorption of digestive products, water & electrolytesCirculation of blood through GI organs to carry away absorbed substances Control of all these functions by nervous and hormonal systems

Individual FunctionsEach 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 -40mVWhen 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 POTENTIALSGI 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.