Intestinal Motility PDF

Section 11: Intestinal Motility I. Motility patterns of the intestine (small and large intestine) A. Ileus - Atonic (paralytic) bowel. Pathological Typically, a post-op complication with bowel surgery Fig. 48 Disordered autonomic control (icnreaase SNS tone) + metabolic factors...

Section 11: Intestinal Motility I. Motility patterns of the intestine (small and large intestine) A. Ileus - Atonic (paralytic) bowel. Pathological Typically, a post-op complication with bowel surgery Fig. 48 Disordered autonomic control (icnreaase SNS tone) + metabolic factors open quiescent pipe Increased inhibitory effector neuron activity Inhibitory effects n. - ON Decreased cholinergic effector neuron activity Slow waves dont reach threshold B. Spasm - Unrelenting circular smooth muscle contraction resulting from absence of inhibitory neuron activity Pathological Hirschsprung's disease: absence of ENS in section of colon. Circular musclecontration dominates Due to congenital failure of caudal mesentreric innervation to meet with sacral innervation C. Segmentation - Normal motility pattern that slows passage. Induced by activity of the enkephalineric interneurons. Pattern: alternating segments where inhibitor effectors are OFF (cir.smooth muscle contracts) and cholinergic effectors are ON Slows passage, mixes content but still have aboral progression of digesta D. Peristalsis - Main propulsive pattern for digesta passage. > GVA>CNS> PS action Fed pattern > digesta sensed > local ENS Actio Fasting pattern > CNS directs MMC activity 1. At or aboral to bolus: 2 Contrx. of longitudinal smooth muscle (+ACh) 1 "pulling back the gut wall" Inhibition of circular sm. muscle contrx (+VIP/NO/ATP) opens the lumen 2. Oral to bolus: Relaxation of longitudinal smooth muscle (-ACh) Contraction of circular smooth muscle (- VIP/NO/ATP) inhibitors go off Reduces lumen diameter and impedes back flow 70 71  Pelvic flexture: between ventral/dorsal proximal colon - small colon is (distal) 3. Pacemaker activity: a) Stationary pacemaker activity (driven by Int. Cells of Cajal) - Intiates at same site, fast rate -Duodenum moves content away from pylorus -Colonic in some species (mid-colon, equine pelvic flexure) b) Shifting pacemaker activity (ICC affected by extrinsic nerves) -Large intestine -Initiates at different sites based on content LI sites change to move solid conten aboral II. Specific motility patterns of the large intestine Species diversity A. Functions of the large intestine: 1. Microbial digestion cecum and proximal colon 2. Net reabsorption of electrolytes and water distal colon B. Large intestinal motility patterns that slow digesta passage: allows more time for microbial digestion and/or electrolytes/water absorption Important species differences 1. Segmentation a) Segmentation - Dog, cat, cattle - Same as small intestinal pattern b) Haustral movements - Humans, horse, pig Fig. 48a. Haustra 1) Involves taenia - Longitudinal smooth muscle arranged in bands. 2) Haustrations - Circular muscle contractions at intervals causes the digesta to bulge the bowel wall outward while taenia contract. -Slows digesta passage but aboral progession still occurs. inhibitor neurons OFF at intervals 2. Reverse peristalsis a) Reverse peristalsis originates at midcolon in most spp. b) Reverse peristalsis slows aboral digesta flow. c) Midcolonic pacemaker initiates both reverse peristalsis (oral direction) and forward peristalsis (aboral direction) 72  d) Species differences: a) Horse - Occurs at pelvic flexure (mid-proximal colon) b) Dog - No colonic reverse peristalsis animal with fast transit 3. Cecal retropulsion a) Occurs in species where cecum is major site of large intestinal fermentation -- Ruminants, rodents, rabbits (but not horse) Horse has tight cecocolonic sphincter b) Strong reverse peristalsis in proximal colon propels digesta back into cecum for more fermentation enters via cecocolonic sphincter C. Large intestinal motility patterns that speed digesta passage 1. Cecal motility - Horse a) Movement of fluid/solid digesta train of peristalsis from cecal apex 1) Mass movement from cecal body into base then cupola to base 2) Circular muscle contraction isolates cupola and forces digesta into colon Fig. 49. Horse cecum contraction at base drives content through cecocolonic sphincter Base b) Movement of gas Cupola Fermentation gas cap forms at base of cupula > tension in base sensed > CNS >1) Contraction of base forces gas into cupola 2) Distension of cupola with gas lowers fluid level 3) Contraction of base elevates cecocolonic ostium Base 4) Fluid does not cover ostium of cecocolonic sphincter Apex this is sensed and CNS open cecocolonic sphnicter to pass gas aborally Merck® Manual; Glass Horse® "getting rid of gas" 73  2. Mass movements of the colon all domestic species - Function: Preparation for defecation a. Modified peristaltic movement of colon that attempts to fill rectum CNS directed Peristaltic waves traverse length of distal colon IF RECTUM NOT FILLED - NO DEFECATION b. Repeat every 2 -3 min for 10 - 30 min. If fills rectum, then intiates rectal peristalsis. If not, then disappear for 6 to 12 hrs. c. Mass movements occur few times/day. Initiated by: 1) Gastrocolic reflex 2) Colonic filling GVA > CNS > PNS > Mass Movement 3) Colonic irritation D. Defecation Reflex complex reflex with voluntary and unvoluntary components 1. Stimulus of afferent path: Filling of rectum sensed locally and centrally 2. Efferent paths: Activation of local (ENS) and long (parasympathetic) reflexes local important for defecation in paraplegic animals Action: a. Initiation of peristalsis in rectum Involuntary b. Relaxation of internal anal sphincter (smooth muscle) c. Initiate Valsalva maneuver and contraction of abdominal muscles. Voluntary d. Relaxation of external anal sphincter: > DEFECATION Valsalva: forced expiration against a closed glottis diaphragm moves caudal 74 to increase abdominal pressure  EAS is striated muscle i. Ext. anal sphinter surrounds int. anal sphincter ii. Composed of skeletal muscle and innervated by somatic motor nerve (pudendal) PS nicotinic action at striated muscle iii. Contraction of ext. anal sphincter can be overridden centrally causing bowel incontinence EX: pain, fear, GI tests, epilepsy Objectives: 1. Name the basic patterns of intestinal motility and describe the neural actions that cause each pattern. 2. What colonic motility patterns that function to delay transit? 3. Describe the anatomical relationships necessary for haustral movements. 4. Describe mass movements. 5. How does gas expulsion from the equine cecum occur? 6. Desribe the neuromuscular control involved in defecation. 7. Know how the intestinal segmentation motility pattern is produced by opiate drugs. 8. Know how the intrinsic plexi of the intestine produce the basic peristalsis pattern. Related Questions: 1. What is volvulus? 2. What is the cause of colic in the horse? 3. True or false. Most causes of colic in the horse require surgery. 75