General Principles of GASTROINTESTINAL SYSTEM Dr . Moizuddin Khan Dr. Beenish Mukhtar

Learning objectives:- Physiologic anatomy of gastrointestinal wall The general characteristics of smooth muscle The specific characteristics of smooth muscle Control of gastrointestinal function (ENS) Functional types of movements in the gastrointestinal tract Gastrointestinal blood flow (Splanchnic circulation) Effects of gut activity and metabolic factors on GI blood flow

اسم ورقم المقرر – Course Name and No. 4/19/2019

Special characteristics of GI smooth muscles:- Contracts as a syncytium:- Muscle fibres of GIT are 200-500 micrometres in length and 2-10 micrometres in breadth. Muscle bundles are connected by gap junctions Each muscle layer represents a branching latticework of smooth muscle bundles, functioning as syncytium. Action potential travels in all directions

Electrical activity of GIT muscle Slow waves:- undulating changes in RMP 5-15 Mv Frequency is 3 waves per min in stomach,12 in duodenum and 8 in ileum By interstitial cells of cajal….electrical pace makers Slow waves excite appearance of spike potentials

Spike potentials:- True action potentials Occur at RMP of -40mV Higher the slow wave pot. Greater is the frequency of spike pot. Frequency is 1-10 spikes per second Ca-Na channles are responsible At peak of slow waves increased quantities of Ca enters the fibers to cause contraction

Types of Smooth Muscle contraction in GIT Phasic contractions - Periodic contractions followed by relaxation; such as in gastric antrum, small intestine and esophagus Tonic contractions - Maintained contraction without relaxation; such as in orad region of the stomach, lower esophageal, ileocecal and internal anal sphincter - not associated with slow waves

Neural supply of GIT Intrinsic (Enteric) Myenteric or Auerbachs plexus and Submucosal or Meissners plexus Extrinsic Sympathetic and parasympathetic

Myenteric or Auerbach s plexus Linear chains of interconnecting neurons extending up to the entire length of GIT On stimulation it causes increased tone of gut wall Increased intensity, velocity and rate of contraction Both excitatory and inhibitory depending on type of neurotransmitter secreted Sub-mucosal or Meissner's plexus Controls secretion, absorption and local contraction of sub- mucosal muscle and blood flow

Important neurotransmitters of enteric neurons are Ach Norepinephrine Serotonin Dopamine Cholecystokinin VIP (Vasoactive intestinal peptide), Substance P Leu-enkephalin met-enkephalin

Parasympathetic innervation Parasympathetic supply to gut is divided into cranial and sacral divisions Cranial part - Almost entirely in vagus nerve providing innervation to esophagus, stomach and pancreas Sacral part- originate in 2nd ,3rd, 4th sacral segments of spinal cord and pass thru pelvic nerves to large intestines, rectum and anus. Post ganglionic neurons of parasympathetic are located in Submucosal and Myenteric plexuses

Sympathetic Innervation From T-5 to L-2 Fibers go to symp. ganglionic chain from where they relay in separate ganglions e.g. celiac and mesenteric Post gang fibers supply whole GIT more extensively in oral cavity and anus Neuro transmitter is mainly norepinephrine. Functions by inhibiting intestinal smooth muscle On strong stimulation motor activity is blocked hence causing stasis of food in gut

LAW OF GUT The contractile ring appears on the orad side of the distended segment pushing the intestinal contents in anal direction for 5-10 cms before dying out and simultaneously the gut relaxes towards anus….Receptive relaxation. This is termed as myenteric reflex or peristaltic reflex along with anal direction of movement is called law of gut

Splanchnic circulation It is the blood supply of:- Intestine Pancreas Spleen Liver

Blood flow to gut increases eight fold during active absorption but decreases back to the resting level over another 2 to 4 hours. Increased blood flow is due to hormones VIP, Gastrin, Secretin, Cholecystokinin Increased metabolism leads to decreased oxygenation causing increase in Adenosine that leads to increased blood flow

COUNTER CURRENT FLOW IN VILLI Arterial and venous flow around the villi are in opposite directions 80% arteriolar oxygen diffuses out to the venules without being carried to villous tips Normally this shunting is not harmful But in conditions like circ. Shock villi suffer ischemic death. اسم ورقم المقرر – Course Name and No. 4/19/2019

Parasympathetic stimulation causes vasodilation Sympathetic stimulation causes vasoconstriction. After few mins of vasoconstriction , blood flow returns to normal by means of auto regulatory escape. This occurs due to metabolic vasodilators which are released due to fall in oxygen conc. Vasoconstriction is imp during heavy exercise ,conditions like shock so that blood is deviated to more vital areas.

THANK YOU اسم ورقم المقرر – Course Name and No. 4/19/2019