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GI Motility (Hirschsprung's disease (congenital megacolon; Chr10,…

- - - - distention from chyme >> (+) localized concentric contractions
      - chops chyme 2-3x per min. >> progressive mixing
      - frequency:
        
        based on electrical slow-waves
        
        max ~12/min in duo & prox jejunum
        
        max ~8-9/min in terminal ileum
      - ineffective w/o background excitation from myenteric nerve plexus
      - also help propel chyme down intestine ~1cm/min
    - - faster in proximal; slower in terminal
      - normally weak; die out ~3-5cm >> chyme net movement ~1cm/min
      - stimuli:
        
        chyme in duo
        
        gastroenteric reflex: stomach distention >> myenteric plexus >> SI
        
        hormones
        
        (+): gastrin, CCK, insulin, motilin, serotonin
        
        (-): secretin, glucagon
      - fxns:
        
        progression of chyme >> ileocecal valve
        
        spread chyme along mucosa
      - gastroileal reflex: chyme blocked at ileocecal valve until after another meal >> intensified ileal peristalsis >> cecum
      - peristaltic rush
        
        d/t intense irritation of mucosa (infectious diarrhea)
        
        powerful, rapid peristalsis
        
        initiated by extrinsic & intrinsic innervation
        
        travel long distances w/in min >> sweeping intestinal contents into colon
      - coordinated contractions of circular smooth m.
      - relaxation in front of bolus: VIP & NO
      - contraction behind bolus: ACh & substance P
    - - increase surface area >> inc. absorption
      - "milk" villi >> lymph flows from lacteals into lymphatic system
      - initiated manly by submucosal nerve plexus in response to chyme in SI
  - - - thickened circular m. upstream of valve
      - normally remains mildly constricted >> slows emptying & enhances absorption
      - immediately after meal >> gastroileal reflex >> relaxes
      - feedback control
        
        reflexes from cecum
        
        cecum distended, irritated
        
        (+) ileocecal sphincter & (-) ileal peristalsis >> delay emptying
        
        mediated by:
        
        myenteric plexus
        
        extrinsic autonomic nn. (prevertebral sympathetic ganglia)
        
        hormones
- - - - teniae coli
        
        longitudinal strips
        
        antipropulsive movements in proximal colon
        
        long-duration contractions
      - larger circular constrictions
        
        can constrict lumen almost to occlusion
        
        similar to SI mixing
        
        haustrations: bulging unstimulated portions of LI
    - - "mass movements"
      - from cecum to sigmoid
      - only 1-3x per day; ~15min during 1st hr after eating breakfast
      - MOA:
        
        distended/irritated colon (usually trans.) >> (+) constrictive ring
        
        rapidly, 20+ cm distal to ring lose haustrations & contract as unit
        
        propels fecal material "en masse" down colon
        
        inc. contractile force for ~30s >> relaxation next 2-3 min
        
        repeat farther along colon
        
        series usually persists for 10-30 min >> cease >> return ~12 hrs later
        
        when feces reaches rectum >> desire for defecation
      - initiation:
        
        gastrocolic reflex
        
        duodenocolic reflex
        
        distention stimulates >>
        
        most likely transmitted by ANS
    - - rectum usually empty of feces
        d/t weak functional sphincter & sharp angulation at sigmoid jxn
      - mass movement >> feces into rectum >> immediate desire + reflex contraction of rectum & relaxation of anal sphincters
      - tonic constriction of
        
        internal anal sphincter (IAS)
        
        circular smooth m. (involuntary)
        
        majority of tone
        
        external anal sphincter (EAS)
        
        striated sk. m.
        
        surrounds IAS & extends distally
        
        pudendal n. >> voluntary, conscious, subconscious control
      - reflexes:
        
        intrinsic reflex
        
        mediated by local ENS in rectal wall
        
        MOA:
        
        peristaltic wave approaches anus >> (-) IAS
        
        if EAS voluntarily relaxed at same time >> defecation
        
        feces enters >> distention >> afferent signals >> myenteric plexus >> (+) peristaltic waves in desc. & sigmoid colon & rectum >> feces to anus
        
        usually relatively weak alone
        
        parasympathetic defecation reflex
        
        involve sacral segments of spinal cord
        
        MOA:
        
        feces enters >> distention >> spinal cord >> pelvic nn. >> back to desc. colon, sigmoid, rectum & anus
        
        strongly (+) peristaltic waves
        
        reflex relaxation of IAS, rectosphincteric reflex (VIP & NO) & reflex constriction of EAS
        
        postponed defecation: accommodation of rectum (IAS contracts, EAS relaxes)
        
        converts weak effort of intrinsic into powerful process of defecation
        
        intrinsic control w/ extrinsic modulation
        
        initiate other effects:
        
        taking a deep breath
        
        closure of glottis
        
        contraction of abdominal wall mm.
        
        relaxation of pelvic floor & outward pull on anal ring
        
        sympathetic
        
        inhibits motility
        
        post-ganglionics via abdominal sympathetic ganglia