1. Introduction to GIT
Slidelearn Team

2. 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

4. 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

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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.