GI system lesson 4
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Where are the appropriate sites of digestion and absorption?
muscular contractions that mix and move the content of the GI tract
Describe ingestion, mastication, deglutition, and peristalsis
- 1. Ingestion= taking food into mough
- 2. Mastication = chewing food and mixing it with saliva
- 3. Deglutition = swallowing food
- 4. Peristalsis = rhythmic wave- like contractions that move food through GI tract
- 1. voluntary process coordinated by reflex centers to open and close mouth
- 2. Muscles are innervated by motor branch of 5th CN: trigeminal
- 3. Increases surface area of food for digestion
- 4. mixes food with saliva which contains salivary amylase: enzyme that can catalyze partial digestion of starch
What is the chewing reflex?
- 1. food pressed to plate- reflex inhibition of muscles of jaw (jaw drops)
- 2. Stretch reflex causes rebound contraction to raise jaw and close teeth
- 3. Bolus of food to palate initiates another reflex inhibition of jaw muscles
- 4. continues until tongue voluntarily pushes food to pharynx- swallowing
Swallowing: sturcture of reflex
- 1. Tactile receptors mainly in entry to pharynx
- 2. swallowing center in medulla and lower pons
- 3. efferent branch: to pharynx and upper esophagus, esophagus, to repirator center--> the respiratory center of medulla is directly inhibited by swallowing center for very brief time that it takes to swallow
Describe deglutition from beginning 3 phases.
- 1. Begins as voluntary acitivity (oral phase)
- 2. Pharyngeal and esophageal phases are involuntary and cannot be stopped
- 3. Larynx is raised
- 4. Epiglottis covers entrance to respiratory tract: involuntary muscular contractions and relaxations in mouth, pharynx, larynx, and esophagus are coordinated by swallowing centers in medulla
- 5. Esophagus: connects pharynx to stomach: upper 3rd contains skeletal muscle, middle 3rd mixture of skeletal and smooth m., and terminal portion contains only smooth m.
Explain perstalsis in esophagus
- 1. Perstalsis: produced by series of localized refluxes in response to distention of wall by bolus
- 2. wave-like muscular contractions: circular SM contract behing, relaxes in front of bolus. Followed by longitudinal contractions (shortening) of SM
- 3. After food passes into stomach, LES (lower esophageal sphincter) constricts
What is BER?
basal electrical rhythm found in ICC
Swalloing: oral phase
- 1. conscious
- 2. reflex pharynx stimulation by saliva or food
- 3. tongue moves food to upper pharynx by pressing agains hard palate
Swallowing: pharynegeal phase
- 1. reflex, activation by mech. stimulation
- 2. soft palate raises, closes entry to nose
- 3. vocal cords close, larynx raises (epiglottis closure)
- 4. stop breathing
- 5. short relaxation of upper esophageal sphincter (reflex opening after food passage)
- 6. contraction of upper eophagus (skeletal muscle)
- 7. peristaltic wave initiation
Swallowing: esophageal phase
- 1. upper 1/3 of esophagus = skeletal muscle, (longitudinal and circular layer) bottom 2/3 = smooth muscle
- 2. skeletal and smooth: innervated by vagus
- 3. primary peristaltic waves
- 4. if insufficeint, secondary peristaltic wave (reflex from esophagus distension)
Lower esophageal sphincter (resting tone and relaxation during swallowing)
- 1. Resting tone: ACh stimulatory vagal fibers (through stimulation of ENS neurons)
- 2. Relaxation during swallowing: vagal inhibitory neurons (VIP, NO) via activation of ENS inhibitory neurons
- 1. lower sphincter sometimes opens even w/o swallowing (shysiological reflux)
- 2. If too much = esophagitis (burning = heart burn)
- 3. Pessure in esophagus: thoracic < abdominal (used for measuring intrapleural pressure and promotes reflux)
- 4. Esophagus crosses diaphragm @ level of lower sphincter -> diaphragm contraction helps to close sphincter- does not work in diaphragmatic hernia
What is the stomach structure (muscles)?
- circular muscles thickens towards antrum
- longitudinal essentially missing in upper 1/3
- oblique only lower 1/2 (ringing motion to mix food)
What are the stomach's functions?
- 1. reservoir
- 2. Grinding
- 3. Mixing with stomach digestive fluids
- 4. Continuous intestine filling (little bits of food squirted in @ a time so chyme can increase digestive efficiency)
What is the purpose of stomach as a reservoir?
- 1. Mainly fundus and body (weak or no contractions- min. mixing for long time)
- 2. empty volume 50 mL, pressure 5 mmHg
- 3. Volume can increase to 4L: receptive relaxation (vago-vagal reflex: does not contract immediately)
- 4. chyme settles to layers according to density, large pieces leave last
- 5. lipids form film on surface and digested last
- 6. fluids by-pass (go straight through)
*What is purpose of stomach filling the gut?
- 1. continuous processing by duodenum
- 2. prevents injury to duodenum by acid
- 3. strong contractions of antrum (stong muscles, middle oblique layer) against almost closed pylorus (prevents regurgitation- bile could damage stomach wall)
- 4. Stomach empties in 3 hrs
Why does stomach emptying depend on food composition?
intestine digest different nutrientes at varying rates. This dictates rate at which it is filled. That's why fats help prevent drunkeness: fat stays longer in stomach, keeps alcohol there, alcohol resorption from stomach is slower than from gut
What is the electrical activity of the stomach?
- 1. slow waves spread from pacemaker zone (middle corpus)
- 2. BER increases towrads antrum (lower stomach)
- 3. Only in antrum BER amplitude > threshold for AP
- 4. Shape similar to AP in heart but 10x longer
What happens during vomiting?
- 1. preceded with nausea, sometimes anorexia, autonomic rxns (salivation, sweating, cold skin)
- 2. vomiting center in medulla
- 3. mechanical stimuli (distension), injury, pain
- 4. stomach/duodenum, larynx entry, inner ear
- 5. emetics (chemoreceptors in stomach/duodenum or bottom of 4th chamber)
- 6. reverse peristalsis from middle of SI to larynx
- 7. forced inspiration against olosed glottis- decrease intrathoracic pressure, increase abdominal (diaphragm)
- 8. Strong contraction of abdominal muscles and diaphragm
- 9. relaxation, then closure of pylorus, relaxation of LES and finally UES (glottis closure, inhibition of breathing)
- 10. Protective reflex against toxicity; however longer vomiting can cause metabolic alkalosis and dehydration
What makes up the small intestine?
duodenum 1st 5% of length, jejunum next 40%, and ileum is the rest
*How much digestion takes place in the SI?
- 1. most in duodenum and jejunum, not the ileum
- 2. peristalsis simultaneously only in short segments (except MMC)
- 3. slow waves set BER which occurs @ 12/min
- 4. AP occurs on top of slow waves
- 5. Parasymp increases contractions and symp decreases contractions
- 1. most frequent type of motility: mixing intestinal contents
- 2. closely spaced contraction of circular muscle layer, dividing the SI into small neighboring segments. In rhythmic segmentation the sites of circular contractions alternate (mixing)
- 3. frequency of segmentations decreases in aboral direction (slow forward transport of food content
- 4. Peristalsis: propels chyme forward, contractions behind bolus and relaxation in front, ENS
*What is intestinal reflexes?
- *1. Local = peristaltic reflex (ENS only)
- 2. Mediated by both ENS and external innervation: intestinointestinal reflex, gastroileal refelx and ileogastric reflex
What is Intestinointestinal reflex?
excessive distension of one part of gut relaxes the rest
***What is Gastroileal reflex?
increase stomach activity. increase chymus movement through ileocecal sphincter (transfer of chyme from SI to LI). If problem is Hirschsprung's disease (no ENS)
decrease stomach motility elicited by distention of ileum
*What happens during emptying of ileum?
- 1. Ileocecal sphincter: normally closed. short range peristalsis in terminal ileum and distension relaxes IC sphincter (small amount of chyme is squirted into ceceum)
- 2. Distension of cecum contracts IC sphincter
- 3. Gastro-ileal reflex enhances ileal emptying after eating
- 4. Hormone gastrin relaxes ileocecal sphincter
What is the main function of the large intestine?
- 1. water absorption and ions
- 2. storage of food remnants that are not needed
- 3. mixing is more difficult b/c increased density
- 4. Haustrations (tide movement), mass movement
*What are haustrations?
- 1. similar to segmentation, but more marked in anatomically predefined locations of circular muscle layer
- 2. Governed by BER from interstitial cells
- 3. Usually no AP
- 4. Contractions by prolonging BER
*What happens during colon motility?
- 1. mass movement: 1-3x/day (usually after meal) wave of contraction moves content to larger distances, colon remains contracted for a while
- 2. Overall movement is slow
- 3. Controlled by ENS, symp. stimulation blunts movements, parasymp. timulates haustrations of proximal parts and expulsive movements of distal parts
What are the reflexes of the large intestine?
- 1. Colono-colic: distension of one part relaxes rest (partly symp.)
- 2. Gastro-colic: filling of stomach increases frequency of mass movements (symp, parasymp, CCK, gastrin)
- 3. Duodeno-colic
*Explain what happens in rectuma nd anal canal
- 1. rectum usually empty (retrograde contractions return content to sigmoid colon, until there is too much of it)
- 2. Just before defecation mass movement in sigmoid colon fills rectum, which increases pressure, then reflex relaxation of inner sphincter (SM) and contraction of outer sphinter (skeletal muscle controlled voluntary via pudendal nerves)
- 3. Stretch receptors in rectal wall can adapt- urge to defecate can temporarily subside if suppressed
*What happens during defecation?
- 1. reflex controlled from sacral spinal cord, modulated from higher levels (conscience, will)
- 2. Efferent branch- ACh parasymp. fibers in pelvic nerves
- 3. Highly propulsive contraction of descending colon and sigmoideum
- 4. Relaxation of both sphincters (outer volutary)
- 5. Inspiration pushes diaphragm downwards
- 6. Contraction of expiratory muscles with full lungs and contraction of abdominal muscles increase abdominal pressure
What stimulates defecation?
stretching of rectum
Inrinsic defecation reflex is innervated by which plexus?
Defecation reflex causes what to happen to the internal sphincter?
Defecation only occurs when?
if exernal anal sphincter is voluntarily relaxed
parasymp. defecation reflex involves what?
Stretching of rectum sendssensory signals to what?
What nerves return signals that intensify parastalsis and what muscles increase abdominal pressure as levator ani leifts anal canal upwards?
splanchnic n. and abdominal muscle contractions
What causes Irritable Bowl Syndrome?
- 1. stress
- 2. alterating constipation and diarrhea
- 3. mediated by ANS
What causes Megacolon (Hirschprung's Disease)
- 1. absense of ENS in colon
- 2. constriction of afferent segments dilation and accumulation of GI contents in segments proximal to contracted/affected segments
- 3. severe constipation
What causes gas (flatus)?
- 1. swallowing air
- 2. foods and beverages. Amount of gas that differ. foods cause varies from person to person
- 3. constipation
- 4. bowel obstruction or Crohn's disease
- 5. changes in hormone levels
- 6. bloating right before periods b/c bodies retain fluid
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