Lower GIT Physiology Lecture Notes PDF

PHYSIOLOGY LECTURE | TRANS #2 LE Lower GIT with Accessory Organs RALPH LOUIE BAUTISTA, MD | 01/15/2024...

PHYSIOLOGY LECTURE | TRANS #2 LE Lower GIT with Accessory Organs RALPH LOUIE BAUTISTA, MD | 01/15/2024 | Version #1 04 OUTLINE → Despite structural and functional differences, these regions exhibit similar motor characteristics. I. Lower Gastrointestinal II. Accessory Organs Tract A. Liver A. Functional Anatomy of the B. Gallbladder Lower Git C. Exocrine Pancreas B. Electrophysiology of the III. Review Questions GI Smooth Muscle IV. References C. Motility of Small Intestine D. Motility of Large Intestine E. Defecation F. Gastrointestinal Reflex G.Application Must Lecturer Book Previous Youtube Know Trans Video Figure 1. The Small Intestine and its segments[Lecturer’s PPT] SUMMARY OF ABBREVIATIONS PHYSIOLOGICAL SPHINCTERS nT Neurotransmitter Physiological sphincters are located on both ends of the small VIP Vasoactive intestinal polypeptide intestine (pylorus and ileocecal junction). ANS Autonomic Nervous System → Functions: ▪ Isolate one region from the next ENS Enteric Nervous System ▪ Provide selective retention of luminal contents BER Basic Electrical Rhythm ▪ Prevent backflow MMR Migrating Myoelectric Complex IAS Internal Anal Sphincter EAS External Anal Sphincter RMC Rectal Motor Complex CNS Central Nervous System IBS Irritable Bowel Syndrome MMC Migrating Myoelectric Complexes CCK Cholecystokinin CFTR Cystic Fibrosis Transmembrane Conductance Regulator LEARNING OBJECTIVES Discuss the physiological processes in the small intestine. → Contrast the motility of the small intestine during fasting and fed states. → Describe the intestinal reflexes. Discuss the physiological processes in the large intestine. → Describe the motility of the large intestine. → Correlate the anatomical features of the ileocecal junction with physiologic function. → Explain the processes in defecation reflex Discuss the physiological processes in the hepatobiliary system. → Summarize the functions of the liver and gallbladder. → Describe the function, composition, and formation of bile. → Trace the enterohepatic circulation of bile acids. Discuss the functions of the pancreas in digestion. → Summarize the exocrine secretions of the pancreas. → Describe the phases of pancreatic secretion. I. LOWER GASTROINTESTINAL TRACT Figure 2. Physiological Sphincters of the Small Intestine. Cyan A. FUNCTIONAL ANATOMY OF THE LOWER GIT Square: Pylorus Junction; Yellow Square: Ileocecal Junction SMALL INTESTINE (Bauhin’s Valve)[Lecturer’s PPT] Approx. 3-7 meters long; spans from the duodenal side of the pylorus down to the ileocecal valve PRIMARY FUNCTIONS OF THE SMALL INTESTINE Divided into three segments: Mix chyme with bile, intestinal juice, and pancreatic juice. → Duodenum Churn chyme and bring it in contact with mucosa for digestion ▪ Receives chyme from the stomach, and digestive juices and absorption from the liver and pancreas Move the residues/undigested food toward the large intestine → Jejunum ▪ Main site of chemical digestion and absorption → Ileum ▪ Main site of chemical digestion and absorption LE 4 TG 8 & 9 | Garcia, L., Gatchalian, Gayares, Genoveza, TE | Go, S., Gomez, L. AVPAA | J. Enriquez, T. PAGE 1 of TRANS 2 Gimao, Go, M., Go, S., Gomez, Gonzaga, Gonzales, Guzman 24 M.J., Gonzales, M.T., Gonzales, M.K., Gonzales, J.C., Gonzales, R.J., Guden, Gutay PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD Figure 3. Typical cross section of the gut[Lecturer’s PPT] FOUR HISTOLOGIC LAYERS OF THE LOWER GASTROINTESTINAL TRACT Mucosa Figure 4. Ileocecal Junction[Lecturer’s PPT] → Small finger-like projections called villi extends into the lumen increasing the surface area for optimal absorption of nutrients The physical angulation between the terminal ileum and the → Primarily provides protection from luminal matter and offer the cecum contributes to the continence of the ileocecal junction first barrier of support → Ileocecal valve (Bauhin’s/Tulp’s/Tulpius Valve) Submucosa ▪ Consists of two muscle layers of the ileum (upper and → Consists of loose connective tissue, blood vessels, lower lips) which protrude into the lumen of the cecum lymphatics, and submucosal plexus (aka Meissner’s plexus) ▪ Functions more like a “flutter valve” → Receives sensory information from mechanoreceptors and ▪ Allows antegrade flow when a peristaltic wave is strong chemoreceptors enough to overcome its resistance → Manipulates secretion and local blood flow ▪ Prevents retrograde flow of colonic contents into the small Muscularis Externa intestine → Inner Circular Layer: Wrapped around the long axis of the → Ileocecal sphincter tract ▪ Thickened circular muscle located centimeters upstream ▪ Consists of circular smooth muscles from the Ileocecal valve ▪ Contraction of these muscles decreases the diameter of ▪ Normally constricted due to high basal tone the bowel lumen ▪ Delays the emptying of the ileal contents into the cecum ▪ At the sphincters, the circular smooth muscles generate a sustained tonic contraction which keeps the lumen LARGE INTESTINE (COLON) completely or partially shut to prevent reflux About 1 meter long ▪ To open the sphincter, the circular muscle must relax Divided into the cecum, ascending, transverse, descending and → Outer Longitudinal Layer: Composed of longitudinal smooth sigmoid parts of the colon and the rectum muscle cells → Proximal half of the colon for absorption ▪ When these contract, there will be shortening of the bowel → Distal half of the colon for storage length → The coordinated actions of the inner circular and outer longitudinal smooth muscles produce the motor patterns of gastrointestinal motility → Peristalsis – The result of contractions and relaxation of the inner circular and outer longitudinal muscles propelling food through the GI tract → Myenteric or Auerbach’s Plexus ▪ Lies between the circular and longitudinal muscle layers ▪ Forms a continuous network extending from the upper esophagus to the internal anal sphincter ▪ Primarily influences motor control through its effects on these smooth muscles Serosa → or adventitia in the case of bowel segments located in the retroperitoneal compartment → Outermost layer → Formed by an epithelial layer and connective tissue → Primarily provides a barrier between the blood and the GI tract Figure 5. Large Intestine[Lecturer’s PPT] ILEOCECAL JUNCTION Junction where small bowel transitions into the large intestine PRIMARY FUNCTIONS AND LAYERS OF THE LARGE Area where the chyme passes from the ileum to the cecum while INTESTINE preventing backflow into the small bowel. Absorption of water and electrolytes from chyme, more specifically the undigested food material, to form solid feces → Storage of fecal matter until it is excreted from the body PHYSIOLOGY Lower GIT with Accessory Organs PAGE 2 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD → External anal sphincter ▪ Striated muscle ▪ Partly overlies the internal sphincter ▪ Has a high resting tone but it can be influenced by voluntary efforts to help maintain continence Figure 6. Surface Anatomy of Large Intestine[Lecturer’s PPT] Similar to the small bowel, the muscular layers of the colon are composed of both longitudinally and circularly arranged fibers → Taenia coli ▪ Found at the surface Figure 8. Rectum[Lecturer’s PPT] ▪ Longitudinal fibers are concentrated into three flat bands ▪ Spaced evenly around the circumference of the colon NEURAL REGULATION OF THE INTESTINES ▪ Between the Taenia coli, the longitudinal muscle is much thinner which allows the wall to bulge Figure 7. Taenia Coli (pointer on the band)[Lecturer’s PPT] → Haustra ▪ Irregularly spaced circular constrictions that pinch the colon into a series of pockets Figure 9. Neural Control of the Gut Wall[Lecturer’s PPT] ▪ Gives the colon a sacculated appearance for much of its length The neural control of intestinal function is via the autonomic ▪ Has a large number of Crypts of Lieberkuhn which are nervous system and enteric nervous system. responsible for the secretion of mucus in the lumen Figure 9 shows how the neural connections to the gut are − Functions of Mucus: organized o Contain moderate amounts of bicarbonate which → Intrinsic control serves to neutralize the acids formed in the feces ▪ Within the walls of the gut: the myenteric and submucosal o Protects the colonic wall against excoriation plexuses o Provides an adherent medium for holding fecal → Extrinsic control material together ▪ Through the autonomic nervous system: the sympathetic RECTUM and parasympathetic nervous system. Rectosigmoid Junction These components work together alongside myogenic and → Where the colon and rectum are joined at an acute angle hormonal systems to achieve proper functioning not only of the → Although the rectum is in direct continuity with the colon, it intestines but also of the entire digestive system. lacks a circular muscle layer hence the rectum is surrounded EXTRINSIC REGULATION only by longitudinal muscle fibers Parasympathetic innervation → The rectum functions as a reservoir where the feces can be → Extrinsic control stored before expulsion → Via the vagus and pelvic nerves → Longitudinal contractions in the rectum also form functional ▪ Vagus Nerves valves which retard or delay the movement of feces until it is − In addition to the upper GI tract and accessory organs, convenient the vagus nerve provides extensive innervation to the ANAL CANAL small bowel and the proximal half of the colon Sphincters ▪ Pelvic Nerves → Internal anal sphincter − Caudally, the pelvic nerves originating from the second ▪ Thickened band of circular smooth muscle which has a through fourth sacral segments innervate the distal 1⁄2 relatively high spontaneous tone. of the large intestine in the anorectal region PHYSIOLOGY Lower GIT with Accessory Organs PAGE 3 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD → Sigmoid colon, rectum, and anus are considerably better → Myenteric plexus with interstitial cells of Cajal supplied with parasympathetic fibers than the other parts of ▪ Contribute to the contraction and relaxation of both circular the intestine and longitudinal smooth muscles → In general, the effect of parasympathetic to the GIT is → Submucosal plexus excitation ▪ Controls mainly GI secretions and local blood flow → For the GI smooth muscles, the effect is contraction ▪ Neurons in the ENS are functionally similar to the neurons in the extrinsic part of the ANS Figure 12. Enteric Nervous System[Lecturer’s PPT] Figure 10. Parasympathetic innervation of the GIT[Lecturer’s PPT] CONCEPT CHECKPOINT 1. Which structure of the colon is responsible for its Sympathetic innervation sacculated appearance? Explain its function. → Supplied by the cell bodies in the thoracolumbar region (T5 ○ Ans: Haustra. It has a large number of Crypts to L2) of the spinal cord of Lieberkuhn which secrete mucus in the → Its fibers terminate in the prevertebral ganglia to synapse with lumen the postganglionic neurons before reaching the entire GI Tract ○ The mucus contains bicarbonate which ▪ Recall: Parasympathetic and sympathetic nervous system neutralizes the acids formed in the feces. It tend to work in opposition, hence: The sympathetic effect is also serves as an adherent medium of the generally inhibitory (excitatory to sphincters) fecal material. → For the GI smooth muscles, sympathetics will cause 2. How does the ileocecal valve function like a flutter relaxation wave? → Exception to this rule: In GI sphincter, such as the anal ○ It allows antegrade flow when a peristaltic sphincters, sympathetic activation tends to induce contraction wave is strong enough to overcome its rather than relaxation resistance, while preventing retrograde flow of colonic contents into the small intestine 3. What supplies the extrinsic regulation of the intestines? ○ Parasympathetic innervation is supplied by the vagus and pelvic nerves ○ Sympathetic innervation is supplied by the cell bodies in the thoracolumbar region of the spinal cord. B. ELECTROPHYSIOLOGY OF THE GI SMOOTH MUSCLE The smooth muscles in the gut are structurally similar to the other smooth muscles in the body Morphology of GI smooth muscle fibers → Fusiform in shape → Packed together in bundles → Presence of gap junctions - connects each fiber to one another Figure 11. Sympathetic innervation of the GIT[Lecturer’s PPT] Table 1. Summary of Effects of SNS and PNS Sympathetic Parasympathetic Stimulation Stimulation GI Smooth Relaxation Contraction Muscles GI Sphincters Contraction Relaxation INTRINSIC REGULATION Intrinsic regulation is served by the enteric nervous system Figure 13. Interstitial cells of Cajal network in the smooth muscle which is still part of the ANS wall of the GI tract[Lecturer’s PPT] PHYSIOLOGY Lower GIT with Accessory Organs PAGE 4 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD Interstitial cells of Cajal (ICC) → Specialized group of cells (Interstitial Cell Network) → Form a network with each other and communicate with the fibers of both longitudinal and circular smooth muscle layers → Receive inputs from neurons of the enteric nervous system → Electrical pacemaker cells of the smooth muscle ▪ Generates pacemaker currents that contribute to peristaltic movement The membranes of the fibers are excited by an almost continual slow intrinsic electrical activity This activity in the GI smooth muscle has two basic types of electrical waves or membrane potentials: → Slow waves → Spike potentials SLOW WAVES Basic Electrical Rhythm (BER) or Pacesetter potential Slow, undulating changes in the resting membrane potential of a GI smooth muscle → Represents the slow depolarization and repolarization of the muscle cells that occur somewhat cyclically Figure 15. Spike potentials[Lecturer’s PPT] The precise cause of slow waves is not completely understood → Appear to be caused by complex interactions among the Spikes result when a slow wave passes over an area of smooth Interstitial Cells of Cajal and smooth muscle cells muscle that has been primed by exposure to neurotransmitters released by enteric neurons Calcium influx to the interior muscle fiber = activate the myosin-actin interaction = contraction of intestinal muscle fibers Figure 14. Slow Waves[Lecturer’s PPT] Under normal conditions, the basic electrical rhythm of the GI smooth muscles is around -50 to -60 millivolts → Reflects the entry of sodium ions into the smooth muscle fiber but not the calcium ions Figure 16. Relationship between spikes and muscle → In the bowels, the slow waves themselves do not cause contraction[Lecturer’s PPT] smooth muscle contractions Frequency of slow waves is region-specific The spikes are the true action potentials which spread to the → 12 per minute in the duodenum surrounding muscle fibers in smooth muscle contraction → 8-9 per minute in the terminal ileum The higher the slow wave potential, the greater the frequency of Despite slow waves not directly resulting in intestinal the spike potentials contractions, it determines the timing and maximum frequency of Strength of smooth muscle contraction is determined by the contractions number of action potential spikes During the peak of a slow wave, when the resting membrane Between the trains of action potentials, the tension developed by potential exceeds a threshold (more positive than about -40 mV), the GI smooth muscle falls, but not to zero a train of action potentials called spikes are triggered Tone → Non-zero resting or baseline tension of smooth muscle SPIKE POTENTIALS Considered the true action potentials in the GI smooth muscles In contrast to slow waves, it is during the spike potentials that SUMMARY OF THE ELECTROPHYSIOLOGY OF GI MUSCLE significant quantities of calcium ions enter the fibers and cause Two basic types of electrical waves in the GI smooth muscle: most of the contraction Recall: In smooth muscle cells, the calcium ions enter the cell through L-type calcium channels → These allow many calcium ions to enter along with a smaller number of sodium ions → Relatively slow to open and close as compared with the sodium channels in their fibers Figure 17. Relationship between spikes and muscle contraction[Lecturer’s PPT] PHYSIOLOGY Lower GIT with Accessory Organs PAGE 5 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD → Slow waves ▪ Cyclical fluctuations in the membrane potential ▪ Excite the appearance of spike potentials, while the spike potential excite muscle contraction → Spike potentials ▪ Amplification of slow waves beyond threshold evokes action potentials in smooth muscle contraction Figure 19. Phases of Migrating Myoelectric Complex → Note: The GI smooth muscles are continually partially contracted (basal tone), but the force of contraction Table 2. Phases of Migrating Myoelectric Complex increases dramatically with each burst of action potentials → The more action potentials evoked = the stronger the force Phase Duration Description of contraction → A number of factors can change the voltage level of the Phase 1 ~70 mins Slow waves (BER) resting membrane potential Longest phase ▪ Recall: When the membrane potential becomes less Period of quiescence negative, the muscle fibers become more excitable. In No significant mechanical activity contrast, if it becomes more negative then the fibers become less excitable Phase 2 10-20 Irregular contraction with varied mins amplitude Intermittent appearance of spike potentials superimposed on the slow waves Phase 3 5 mins Stimulated by the hormone, motilin Plasma level of motilin increased during this phase Regular rhythmic contractions with varied amplitude Large contractions causes the bulk of the luminal contents to move to the small bowel in a peristaltic pattern Pylorus and ileocecal valves are fully open so that the large undigested food items can pass Figure 18. Membrane potentials in intestinal smooth muscle[Lecturer’s PPT] The extent of depolarization and the frequency of action The motility of the intestine reverts back to phase 1 with the potentials are enhanced by the following: entire cycle taking about 90-120 mins in adults → Stretching of the muscle → recurring pattern of movement unless a meal is ingested → Stimulation by acetylcholine released from parasympathetic Frequency of slow waves and velocity of the aboral migration fibers, as well as other neurotransmitters released from declines from the proximal to the distal small intestine excitatory nerve endings such as Substance P → progressively slower as it moves along Factors that hyperpolarize the membrane: Main function: → Effect of norepinephrine or epinephrine on the fiber → Propel undigested and liquified chyme towards the ileocecal membrane and stimulation of sympathetic nerves that valve secrete these catecholamines → Other hormones and neural effectors substances include VIDEO: Fluoroscopy of Migrating Myoelectric Complex the vasoactive intestinal polypeptide and nitric oxide - may diminish or abolish action of the GI smooth muscles C. MOTILITY OF SMALL INTESTINE Reflects the action of two smooth muscle layer in the Muscularis Externa → Inner Circular Layer → Outer Longitudinal Layer Movements are paired with that of the stomach Motor patterns classified according to two independent states: → Interdigestive State (Fasting) → Postprandial State (Fed) Figure 20. Screenshot of video on Fluoroscopy of Migrating Myoelectric Complex MOTILITY DURING FASTING Basal rhythm of the smooth muscle contractions is characterized Seen in the distal ileum, the infused contrast medium is by occasional waves of motor activity called Migrating propelled aborally by the peristaltic waves of the MMC Myoelectric Complexes → As it reaches the ileocecal region, the peristaltic waves will → Also known as Migrating Motor Complexes propel the bolus into the cecum, thus sweeping the contents → This type of motility usually originates in the stomach but can of the small bowel into the colon also initiate in the duodenum or jejunum → Along the length of the small bowel, several peristaltic → Distinct pattern of electromechanical activity observed in the waves representing different MMC occurs simultaneously gastrointestinal (G.I.) smooth muscle → It divides the remaining luminal contents, thus producing a → Spreads from the antrum to the ileum pattern similar to a string of pearls → Migrates aborally or away from the mouth (towards the anus) at a rate of ~5 cm/min → Dependent on the enteric nervous system and its 3 phases PHYSIOLOGY Lower GIT with Accessory Organs PAGE 6 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD PROPULSIVE CONTRACTION (Peristaltic Wave) Figure 22. Propulsive Contraction[Lecturer’s PPT] Mainly responsible for the propulsion of chyme to the colon Adjacent segments of bowel alternately contract and relax. Law of the Intestine → states that stimulation (through distention or irritation) of the gut (in the presence of food) will result in two responses by the segment in an orderly sequence ▪ contraction or excitation = above the bolus ▪ relaxation = below the bolus Figure 21. Propagation of Migrating Motor Complex along the → resulting in peristalsis or a peristaltic wave which permits the GIT[Lecturer’s PPT] contents to be conveyed over considerable distance → In contrast to segmentation, this involves adjacent segments During fasting, the pattern of electrical and motor activity in the of the bowel to alternately contract and relax GIT smooth muscles is modified so that the cycle of motor Actions of the smooth muscle in the Muscularis Externa activity can migrate from stomach to distal ileum → Outer longitudinal muscle → Horizontal lines indicates sequential recording points along ▪ Contraction = ↓ length the length of the bowel ▪ Opens the lumen in front of it → Green Box represents 1 cycle ▪ Relaxed = returns to original length → Arrows denotes the downward propagation → Inner circular muscle → MMC starts proximally and as it reaches the distal ileum, it ▪ Contraction = ↓ diameter eventually fades ▪ Acts as a sphincter ▪ A new complex is generated in the antrum as a cycle of ▪ Relaxed = returns to original diameter MMC fades ▪ As the complex migrates down the ileum, there is Table 3. Effect of Bolus on Muscularis Externa decreasing velocity of propagation and increasing duration ▪ This pattern of motility occurs only during the interdigestive Muscularis Behind the bolus In front of the bolus period Externa (Orad) (Caudad) Circular Contract Relax MOTILITY DURING FED STATE During active digestion, the MMC are inhibited Two contractile mechanisms are utilized Longitudinal Relax Contract → Mixing contractions → Propulsive contractions Peristalsis → occur in any part of the small intestine MIXING (SEGMENTATION) CONTRACTION → moves towards anus = velocity of 0.5-2 cm/sec Primary pattern of motility during the fed state ▪ faster in proximal bowel than in the distal part of the Localized concentric contractions are spaced at intervals along intestine the length of the intestine ▪ contraction are weak and die out after traveling only 3-5cm → elicited by the stretching of the wall that occurs when a portion ▪ Slow net movement of chyme of the small bowel is distended by chyme ▪ about 3-5 hours are required for the passage of chyme Stationary, oscillatory, alternating contractions and relaxation of from the pylorus to the ileocecal valve non-adjacent segments of the alimentary tract give the intestine → the preparation of mixing and propulsive contraction is the appearance of a chain of sausages artificial because essentially, all movements of the small → Back and forth action that breaks apart chunks of food with bowel cause at least some degree of mixing and propulsion the secretions of the small bowel Peristaltic Rush → Produce ring-like contraction occurring at intervals wherein the previously contracted regions relax, then adjacent regions → powerful and rapid peristalsis due to intense irritation of the contract intestinal mucosa → “Chop” the chyme 2-3 times per minute ▪ occurs in some cases of infectious diarrhea → Travels 1cm or so in the aboral direction → commonly initiated by Involve the mixing of food and slow propulsion ▪ nervous reflexes (ANS and brain stem) → occurs at the middle of the bolus, spreading the contents in ▪ enhancement of myenteric plexus reflexes within the gut each segment both proximally and distally for the effective wall mixing of the chyme → travel long distances along the bowel within minutes, → The mixing function exposes nutrient to the surface of the sweeping contents of the small intestine into the colon and mucosa so it can be easily absorbed by the villi thereby relieving it of irritative chyme and excessive Maximum frequency of segmentation contraction: distension → Duodenum and proximal jejunum = ~12/min → Terminal ileum = 8-9/min ENTERIC NEURAL CIRCUITRY OF PERISTALSIS → Determined by the frequency of electrical slow waves Primary afferent neuron fires unto the intermediary ascending The frequency of the segmentation decreases as the food travels and descending neurons from the duodenum to the ileum, thus there is a slow net → Ascending Fibers: head towards the mouth movement of chyme from segmentation contractions alone → Descending Fibers: directed aborally or away from the mouth PHYSIOLOGY Lower GIT with Accessory Organs PAGE 7 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD Stretching or distortion of the gut wall by food bolus is the → Longitudinal muscle layer contraction primary initiator of a neural reflex arc which propagates ▪ Longitudinal muscle will contract because of the same peristalsis excitatory neurotransmitter that causes the contraction of circular smooth muscle behind the bolus This stimulate sensory neurons in the gut wall either by: → Direct activation of mechanoreceptive endings of primary Table 5. Summary of Intestinal Mobility during Fasting and Fed afferent neurons states → Indirect activation by serotonin released from the Interdigestive (Fasting) Postprandial (Fed) enterochromaffin cells. Migrating Motor Complex Segmentation Peristalsis clears small intestines of ensures transit of small residue bowel content at a rate prevents net orad consistent with normal migration of colonic digestion and absorption bacteria ILEOCECAL JUNCTION: FASTING VS FED Figure 23. Enteric Neural Circuitry[Lecturer’s PPT] Table 4. Neurotransmitters released Muscularis Behind the bolus In front of the bolus Externa (Orad) (Caudad) Circular Excitatory nT: Inhibitory nT: Acetylcholine, Nitric oxide, VIP Substance P, Neuropeptide Y Longitudinal Inhibitory nT: Excitatory nT: Nitric oxide, VIP Acetylcholine, Figure 24. Ileocecal Junction [Lecturer’s PPT] Substance P, Specialized segment of the gut which regulates passage of Neuropeptide Y chyme from the ileum to the cecum Function: → regulates filling of the colon Behind the bolus → prevents colo-ileal reflux → Circular muscle layer Ileocecal Valve ▪ interneurons synapse with cholinergic nerves → prevents retrograde flow across ileocecal junction ▪ The goal is to propel food towards the anus, the circular Ileocecal Sphincter muscle must contract so the axons release specific → Slow emptying the cecum During fasting, the filling of the cecum is slow and erratic chemicals that will stimulate muscle contraction → chyme is retained in the distal ileum for prolonged periods − Acetylcholine is the most powerful stimulator of the If there is resistance to emptying at the ileocecal valve, the smooth muscle contraction chyme is further retained in the ileum, and thus facilitates − Other excitatory neurotransmitters includes Substance absorption P and Neuropeptide Y When excess pressure or irritation builds up in the cecum and − These chemicals will stimulate the contraction of the tries to push cecum contents backwards against the valve; inner circular muscle layer by increasing the influx of → The Ileocecal valve closes, thus contracting of the Ileocecal sphincter increases cations into the smooth muscle membrane thus, ▪ Peristalsis in the ileum is inhibited producing spike proteins ▪ This delays the emptying of additional chyme into the → Longitudinal muscle layer cecum form the ileum ▪ Longitudinal contraction causes the lumen behind the ▪ Any irritant in the cecum delays emptying bolus to open up After a meal, a gastroileal reflex intensifies peristalsis in the ileum − will not aid propel food forward followed by emptying of the contents into the cecum If there is resistance to emptying at the ileocecal valve, the ▪ Therefore, axon release inhibitory substances like nitric chyme is further retained in the ileum and thus facilitates oxide and vasoactive intestinal polypeptide (VIP) absorption. − These chemicals will cause the efflux of potassium ions leading hyperpolarization, then muscle relaxation. D. MOTILITY OF LARGE INTESTINE Unabsorbed or undigested food materia (chyme) from the small In front of the bolus → Circular muscle layer relaxation bowel is passed on to the large intestine (cecum) at the ▪ circular smooth muscle respectively opens or relaxes due ileocecal junction to the release of inhibitory substance such as nitric oxide → About 1.5-2 liters each day and VIP By the time these indigestible materials have reached the large bowel, most nutrients and water have already been absorbed PHYSIOLOGY Lower GIT with Accessory Organs PAGE 8 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD Function: FORMATION OF STOOL → absorb the remaining water and nutrients in the undigested material → solidify to form the feces TYPES OF MOTILITY IN THE COLON Figure 26. Consistency of stool from respective segments [Lecturer’s PPT] Consistency of the stool → varies throughout the length of the large intestine → dependent on how much water is being reabsorbed → most of the absorption take place in the proximal half of the Figure 25. Types of Motility in the Colon [Lecturer’s PPT] colon Ascending and transverse colon MIXING MOVEMENTS → in the ascending and transverse colon, the semi-fluid stool Also known as Haustration becomes mushy in consistency. Predominant pattern of motility in the colon. Distal colon → caused by the contraction of circular muscle and teniae coli → feces transforms into semi solid states ▪ about 2.5 centimeters of the circular layer contracts, thus Rectum narrowing the lumen almost to occlusion → stool is formed in a solid state ▪ Teniae coli, a thick band of longitudinal muscle also → stool is pushed towards the rectum by a series of mass contract movement occuring 1-3 times a day ▪ Combined contraction of circular muscle and teniae coli Transit time cause the unstimulated portion of the large bowel to bulge → refers to the time it takes the food to pass through the outwards into bag-like sacs digestive tract Each haustrations reaches peak intensity in 30 seconds and then disappears during the next 60 seconds E. DEFECATION Functions: mixing and propulsion of colonic contents. → As colonic contents shuttle back and forth between haustra, Involves a complex process of expulsion of indigestible residues there will be mixing of digesta through the anus with the influence of an appropriate stimulus → slow down the passage of the undigested material and When a mass of feces enter the rectum, the desire to defecate maximizing contact time with the epithelium thus further occurs almost immediately absorption of fluid and dissolved substance Rectal filling can be appreciated by as little as 10 mL volume of → haustrations moves slowly towards the anus during feces, the desire for defecation is elicited by an increase in contraction and contribute a very small amount of forward intrarectal pressure to 20-25 cm of water propulsion Rectum The purpose of this type of motility is to mix chyme and fecal → can tolerate more than 300 mL before a feeling of fullness material while providing slow forward movement develops then the desire to defecate becomes urgent Requires a series of coordinated actions of the colon, rectum, PROPULSIVE MOVEMENTS muscles of the pelvic floor, and anal sphincter muscle. Also known as Mass movements Rectosphincteric of Defecation Reflex Multi-haustral contraction from the cecum to the sigmoid colon → triggers defecation via the distention of the rectal walls. Modified type of peristalsis consisting of powerful propulsion that moves the feces from the colon to the anus. TYPES OF DEFECATION REFLEX → occurs 1-3 times per day in humans → series of mass movements persist for 10-30 minutes. INTRINSIC DEFECATION REFLEX When a ring of constriction (in the transverse segment) occurs at Mediated by the local enteric nervous system in the rectum a distended or irritated point in the colon, the area of the colon When the feces enters the rectum, stretching of the wall is distal to the constriction loses haustration sensed by the local mechanoreceptors → instead contract as a unit pushing fecal matter in towards the → Rectal wall distension initiate peristaltic waves (via myenteric rectum plexus) in the descending colon, sigmoid colon, and rectum Estimated transit time through the colon → As peristaltic waves approach the anus, the myenteric plexus → about 2-3 days prior to ingestion sends inhibitory signals so that the Internal Anal sphincter → there are considerable variations among individuals, even on (IAS) is relaxed. the same person, with respect to the rate ▪ Done through the release of vasoactive intestinal ▪ composition of the meal significantly affects the rate polypeptide and generation of nitric oxide required for the material to move through the digestive tube ▪ Reflex relaxation of the IAS permits the anal sampling ▪ Hormones, gender, anatomic, or psychological factors may mechanism, which allows the upper anal canal to influence the transit time determine whether rectal contents are solid, liquid, or gaseous in nature If at the same time, the External Anal Sphincter (EAS) is also consciously and voluntarily relaxed, defecation occurs When the intrinsic defecation reflex is functioning by itself, it is relatively weak. PHYSIOLOGY Lower GIT with Accessory Organs PAGE 9 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD → In order to be effective in causing defecation, it is usually When defecation is desired, the adoption of sitting or squatting enhanced by another type of defecation reflex called position alters the relative orientation of the intestine and the parasympathetic defecation reflex surrounding muscular structures → thus widening of the anorectal angle straightening the pathway for the exit of feces PARASYMPATHETIC DEFECATION REFLEX → Relaxation of the puborectalis muscle will increase the Stimulated stretch receptors in the rectum transmit signals along anorectal angle afferent fibers to the sacral segments of the spinal cord. Defecation can stimulate or initiate other effects Parasympathetic impulse travels along the pelvic splanchnic nerves back to the colon, rectum, and anus. → This impulse intensifies the peristaltic waves in the distal DEFECATION PROCESS colon and rectum, and relaxes the IAS to enhance the weak Requires coordinated actions of both skeletal and smooth intrinsic defecation reflex muscle in the rectum, anus, and pelvic floor → This impulse stimulates the longitudinal contraction of the 1. When rectum is filled with feces, defecation reflex is rectum forcing the feces into the anal canal activated and a signal is sent to the brain about time to find External Anal Sphincter a toilet. → Controlled by somatic nerve fibers which passes through the 2. At the defecation proper: pudendal nerve a. Anorectal angle is straightened either by squatting or → Voluntary input from the spinal cord regulates the contraction sitting and relaxation of the EAS and the muscle of the pelvic floor b. The rectum contracts → If it is convenient to defecate, the motor neurons are inhibited c. Puborectalis and anal sphincters are relaxed by the impulses from the cerebral cortex, allowing EAS to d. Contracting abdominal muscle aid in pushing fecal relax so that stool is passed contents further downward 3. Evacuation of Stools 4. After fecal expulsion, closing reflex occurs VOLUNTARY EFFORTS ON DEFECATION a. External anus sphincter regains its tone to maintain Defecation is also influenced by posturing and abdominal muscle continence at rest contraction. Sometimes defecation is followed by additional mass movements Muscle of the pelvic floor of feces along the length of the large bowel. → Play a role in the evacuation of stools and maintaining anal Defecation can be postponed continence → By voluntary contraction of the external anal sphincter → Puborectalis muscle maintaining a high pressure zone in the distal anal canal ▪ Most important structure among pelvic floor muscle → By voluntary effort such as squeezing the buttocks that aids ▪ Sling wrapped around the rectum in a U-shaped or in the delay the passage of stool and prevents leakage. horseshoe fashion between each attachment to the pubic → If defecation is deferred for a longer period of time, the EAS symphysis may fatigue thus fecal evacuation is inevitable ▪ In a constant tone that serves to pull the rectum anteriorly → Accommodation in the rectum and elevate it ▪ a retroperistaltic contraction or reverse peristalsis takes ▪ Anorectal Angle place in the rectum to propel stool back to more proximal − angle where the rectum meeting the anal canal parts of the colon − prevents the entry of feces into the upper anal canal ▪ IAS constricts and the EAS relaxes until another mass − if muscle contracts, the angle decreases movement occurs. RECTAL MOTOR COMPLEX Also known as Periodic Rectal Motor Activity Occurs in the sigmoid colon and the rectum Composed of short-extent retrograde propagating events Acts functionally as a retrosigmoid brake → braking mechanism to keep rectum empty Restrain flow of colonic contents into the rectum. Figure 27. Anorectal Angle in a sitting position [Lecturer’s PPT] Figure 29. Rector Motor Complexes (through high resolution manometry)[Lecturer’s PPT] Exhibits a high-resolution manometric recording at the rectosigmoid junction showing bursts of regular pressure waves plotted along the lines Red arrows denote the retrograde cyclic motor patterns in the Figure 28. Puborectalis muscle in anatomic position State[Lecturer’s PPT] distal colon and rectum known as the RMC Studies have shown that these cyclical motor patterns are more In the anatomic position or standing, the anorectal angle is abundant at night, suggesting that the distal colon and rectum maintained at approximately 80° undergo these tonic contractions to prevent defecation while we → this angle kinks the rectum, which shut offs the downward flow sleep of feces into the anus PHYSIOLOGY Lower GIT with Accessory Organs PAGE 10 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD FECES ENTERO-ENTERIC REFLEX Composition: Table 6. Entero-Enteric Reflexes → ¾ water → ¼ solid matter CATEGORIES DESCRIPTION ◼ 30% dead bacteria ◼ 30% undigested roughage (from food and dried Intrinsic reflex aka “Local reflex” constituents of digestive juices) Integrated entirely by the enteric ◼ 10-20% fat nervous system (ENS) ◼ 10-20% inorganic matter ENS acts on the fast internal ◼ 2-3% protein response to any stimulus in the Odor G.I. lumen → Caused by chemicals such as indole, skatole, hydrogen sulfide, and mercaptans produced by bacterial action Enteroenteric Comes from one part of the → Smell varies depending on the person’s colonic bacterial flora reflex digestive system to affect another and the type of food eaten part of the G.I. tract Brown color → Caused by stercobilin and urobilin, which are derivatives of Central reflex aka “Long reflex” bilirubin Originates and/or processed Bristol Stool Chart through the CNS → Classifies the stools based on appearance and consistency Involves a sensory neuron within → Ranges from Type 1 to Type 7 the G.I. tract or outside of the body ▪ Type 1: separate hard lumps like nuts, hard to pass e.g. reaction to food and other ▪ Type 2: sausage with many lumps emotional responses. ▪ Type 3: sausage with cracks on the surface These types of reflex include: ▪ Type 4: sausage with smooth texture, snake-like response to food or emotional ▪ Type 5: soft blobs with clear cut edges, easier to pass from responses here onwards ▪ Type 6: fluffy with ragged edges, mushy ▪ Type 7: watery without solid pieces GASTROENTERIC REFLEX → Normal stool would fall under Bristol Types 3 or 4, but Type 4 Distention of the stomach stimulates small intestinal activity is the ideal Prepares the intestine for the incoming chyme Impulses travel from the stomach via the myenteric plexus The Ileocecal valve is normally closed preventing chyme from readily entering the large intestine → As food enters and builds up, this distends the stomach → This distension is the stimulus for the Gastroileal reflex GASTROILEAL REFLEX Stimulus: Stomach distention Response: peristalsis of the Ileum Forces chyme into cecum via the Ileocecal valve GASTROCOLIC REFLEX Stimulus: Stomach distention Figure 30. Bristol Stool Chart[Lecturer’s PPT] Response: Increased Colon (Large intestine) motility F. GASTROINTESTINAL REFLEX Responsible in eliciting the urge to defecate along with the Form of neurally mediated behavior of effector systems that Gastroileal Reflex occur in response to stimulation of sensory neurons → Stimulated when 90-120 ml of stool is pushed into the rectal Involved in regulating functions of the alimentary tract, including bulb as result of mass movement the sphincters and the accessory digestive organs DUODENOCOLIC REFLEX Stimulus: Duodenal distension (through chyme) Response: Increased Colon motility Facilitates the appearance of mass movements postprandially (after meals) together with the Gastrocolic reflex ENTEROGASTRIC REFLEX Stimulus: Duodenal Acid pH of 3 - 4, Stomach Acid pH of 1.5 Response: Inhibition of gastrin release (in-turn affects secretion of HCl), and decreases gastric emptying Occurs when excessive amount of chyme leaves the stomach thus stimulating more receptors in the duodenum Inhibition of gastrin protects the duodenal cells from eroding Decreased gastric emptying increases the duration of chyme digestion and limits the chyme that passes through the duodenum ILEOGASTRIC REFLEX Stimulus: Distended/Full Ileum Response: Inhibition of gastric motility When Ileum is filled with chyme, the Ileogastric reflex inhibits gastric emptying thus preventing more chyme from reaching the ileum Figure 30. Gastrointestinal Reflexes[Lecturer’s PPT] PHYSIOLOGY Lower GIT with Accessory Organs PAGE 11 of 24 PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD 2. OTHER REFLEXES Dynamic ileus → Impaired relaxation of smooth muscles. OTHER REFLEXES STIMULUS RESPONSE → (+) spastic contraction of the circular musculature → Cause of Ileus is multifactorial and complex Intestinointestinal Severe Inhibition of → Most common cause is post-operative state Inhibitory Reflex intestinal intestinal motility → No mechanical bowel obstruction however (+) paralysis of distention or Contractile smooth mm’s. Irritation of the intestines or the peritoneum irritation activity of the during abdominal surgery directly inhibits the smooth mm’s other parts of the and increases sympathetic discharge. bowel will be → (+) diffused decrease in peristaltic activity inhibited so long as the mucosa is excessively irritated or remains overdistended Anointestinal Anal distention Inhibition of Inhibitory Reflex (via fecal intestinal motility matter) Colocolonic Distal colonic Proximal colonic Inhibitory Reflex distention motility Orthocolic Reflex Rising from Urge to defecate Figure 31. Supine view of the abdomen in a patient with ileus[Lecturer’s the bed in the Mechanism PPT] morning unclear, In Fig. 31: Note the massive dilatation of the bowels, caused by postulated that obstruction gross motor activity upon standing from a CONSTIPATION recumbent Slow movement of feces through the large intestine. position Absence or lack of mass movement could lead to excessive stimulates mass absorption of fluid in the feces movements The American College of Gastroenterology defines constipation as having: unsatisfactory defecation characterized by infrequent Peritoneointestinal Irritation of Inhibition of stools, difficult stool passage, or both. Reflex peritoneum intestinal motility Causes of constipation are broad. It may arise anywhere in the patients with Strongly inhibits large intestine, may be due to external factors, or may be from an peritonitis the excitatory inorganic cause (functional constipation) enteric nerves, Functional Constipation will cause → Any two of the following symptoms within the past three intestinal months: paralysis ▪ straining especially in ▪ lumpy hard stools patients with ▪ sensation of incomplete evacuation peritonitis ▪ use of digital maneuvers ▪ sensation of anorectal obstruction blockage with 25% of bowel movements Renointestinal Kidney Inhibition of ▪ decrease in stool frequency (200 grams/day on low-fiber content diet Adynamic ileus Types: → Impaired contraction of smooth muscles. → Secretory (e.g., infectious) → Active slow wave, no spike wave activity ▪ decreased absorption or increased secretion of electrolytes → Recall: slow waves do not directly cause contractions except in water in the stomach ▪ rapid and powerful peristalsis leading to an increased → In the absence of spikes there is continuous inhibitory neural number of bowel movements activity, hence the motility of the bowel is impaired ▪ lack of time for absorption and water excretion ▪ In this condition, food stasis may have occurred causing ▪ most cases in adults are of infectious etiology accumulation in the site of occlusion and potential → Osmotic (e.g., malabsorption) peritoneal irritation ▪ water thrown into the bowel down concentration gradient PHYSIOLOGY Lower GIT with Accessory Organs PAGE 12 of 24  PHYSIOLOGY | LE1 Lower GIT with Accessory Organs | Ralph Louie Bautista, MD → Inflammatory (e.g., ulcerative colitis) ▪ diffused from the blood ▪ ulcerative colitis: walls of the large intestine are ulcerated ▪ formed within the gut from bacteria, especially CO2, CH4, and inflamed and H2 → Functional (e.g., IBS, psychogenic) → Odorous Constituents ▪ Psychogenic diarrhea: occurs due to emotional distress; ▪ Include low molecular weight fatty acids (e.g. butyric acid) caused by excessive stimulation of the parasympathetic ▪ Include reduced sulfur compounds nervous system which excites both motility and secretions → Volume of the colon. ▪ Gas expelled through the anus: 500 mL to 1.5 L per day ▪ Gas formed in the large bowel: 7-10 L BRISTOL STOOL CHART → Frequency: 10-20 times per day Refer to Figure 30. Flatulence involves a similar sequence of events as described for A medical aide designed to classify stools based on shape and defecation, except that no relaxation occurs in the Puborectalis consistency muscle Table 7. Bristol Stool Chart Summary OTHER DISORDERS RELATED TO DEFECATION Factors by which continence is maintained: STOOL TYPE DESCRIPTION → Intact anal sphincters → Pelvic floor muscles Types 1-2 Indicative of Constipation → Anorectal sensation → Anorectal reflexes Types 5-7 Maybe indicative of Diarrhea Structural or functional impairments in the anatomic barriers (i.e., anal sphincters and puborectalis muscle) can lead to either fecal or anal incontinence IRRITABLE BOWEL SYNDROME (IBS) Incontinence One of the most common G.I. disorders for which medical advice → Distressing social condition associated with loss of is sought self-esteem and poor personal hygiene Heterogeneous collection of functional disorders whose sufferers complain of diarrhea, constipation, or alternating patterns of both. FECAL INCONTINENCE Etiology is not completely understood Involuntary loss of solid or liquid feces → One theory suggests that altered motility may reflect the conditioning of autonomic responses from repeated exposure to stressful situations ANAL INCONTINENCE Involuntary loss of solid or liquid feces or flatus MEGACOLON (HIRSCHSPRUNG’S DISEASE). Caused by congenital absence or deficiency of ganglion cells in ANISMUS the myenteric plexus in a segment of the sigmoid colon. Paradoxical contraction of the anal sphincter rather than relaxing RECALL during attempted defecation. → Myenteric plexus controls motility, thus deficiency affects Inappropriate contraction or inadequate relaxation of the muscles peristalsis. Neither defecation reflex nor strong peristaltic which may cause symptoms of IBS or obstruction motility can occur in the segment of the bowel. Fecal matter cannot pass thus accumulates proximally, causing megacolon SUMMARY The esophagus acts as a conduit between the pharynx and the stomach → (+) has a unique system of propelling food from the proximal to distal end The stomach serves as an initial reservoir of food and can accommodate large changes in the volume → Breaks the food into smaller particles and mixes them with gastric juices → Slow waves determine the frequency of contractions and the rate of gastric emptying; Reflexes also contribute to the rate of gastric emptying or the activity of the bowels Duodenum is the first part of the intestine → Receives the chyme from the stomach as well as the secretions from the pancreas and the liver Small intestines function primarily for food absorption (mainly in the Jejunum and the Ileum) Figure 32. Hirschsprung’s disease[Lecturer’s PPT] → Small bowel displays an unsynchronized pattern of contractions → Segmentation moves the food back and forth to allow SMALL INTESTINAL BACTERIAL OVERGROWTH (SIBO) mixing of the chyme and expose nutrients to the intestinal Characterized by an increased number of bacteria and presence mucosa for the absorption by the villi of abdominal flora in the small bowel. → Peristaltic contractions serve to propel the unabsorbed May be due to impaired GI motility (e.g., lack or disordered chyme in the aboral direction pattern of MMC phase III) and ileocecal valve function. → Ileocecal junction is located towards the end of the small bowel which regulates emptying and prevents FLATULENCE contamination of the small bowel with colonic bacteria Refers to the passage of rectal gas through the rectum Water and electrolyte absorption occurs mainly in the proximal Other definition: presence of “excessive” amounts of gas half of the colon Discerning the difference between normal and excessive requires → Goal: produce solid feces from undigested food quantification → mucus produced holds the fecal material together Flatus (Gas) Feces move towards the rectum via mixing movements called → Sources haustrations ▪ swallowed air PHYSIOLOGY Lower GIT with Accessory Organs PAGE 13 of 24