The Large Intestine. The Large Intestine.

The Large Intestine.

1. The Large Intestine Image courtesy of St Mark’s Hospital

2. Colon Smooth muscle Inner circular (mixing) Outer longitudinal (peristaltic) Haustra Taenia Coli Background motor activity (washing machine!) 20-30 mins segmental propulsive contraction / massive migrating motor complexes (toothpaste!)

3. 3 Colonic Layers

4. Nerve Supply to Colon Extrinsic autonomic nerves (lumbar sympathetic and sacral parasympathetic innervation) Local enteric neurones via submucosal and myenteric plexus within the layers of gut wall (capable of reflex activity without CNS activity -> peristalsis

5. Anal Sphincter Complex External Anal Sphincter (EAS) Striated 3 portions Deep - puborectalis / puboanalis Middle – coccyx posteriorly/ perineal body anteriorly Subcutaneous – no bony attachments 1 5 % resting pressure Phasic Pudendal innervation (sensory/motor/autonomic)

6. Anal Sphincter Complex contd . Internal Anal Sphincter (IAS) Smooth muscle 3cm long/2-3mm wide Ends 10mm above anal verge Long periods tonic activity 85% resting pressure No conscious control (autonomic)

7. © ACPWH Images courtesy of St Mark’s Hospital

8. A ‘normal’ ultrasound Cone Internal Anal Sphincter External Anal Sphincter Longitudinal layer Subepithelial tissues VAGINA

9. Sampling Rectum fills IAS relaxes exposing contents to rectal mucosa Every 8-10 mins 10s duration EAS contract RAIR (Recto-anal inhibitory reflex) Critical volume -> 1 st sensory perception (40-50mls)

10. Sampling mechanism Image courtesy of St Mark’s Hospital

11. Resisting the urge/deferring EAS squeeze Generates enough pressure to prevent expulsion and return stool to rectum Endurance required 10-15 seconds Urge diminishes if deferment successful

12. Resisting the urge Image courtesy of St Mark’s Hospital

13. Levator Ani Pubo - visceral/ Pubo-coccygeus muscle Pubo -vaginalis Pubo-perinealis Pubo-analis Puborectalis Ileococcygeus Coccygeus

14. Muscle fibre types Type I Fibres approx 70% Slow oxidative - endurance Type II Fibres approx 30% Fast glycolytic – speed and pressure Variations from anterior to posterior less type II posterior 1989 Gilpin et al.

15. Puborectalis , Puboanalis and the Anorectal Angle www.anatomy.tv

16. Puborectalis Stabilises ano -rectum Maintains the ano -rectal angle Must be able to “pay out” /lengthen to allow for defaecation Pubo -rectal continence reflex – involuntary contraction of pubo-rectalis during rectal filling

17. Puborectalis and the anorectal angle

18. Endopelvic Fascia Support Level 1- Upper 1/3 vaginal wall to pelvic sidewalls Level 2 – Middle 1/3 via pubocervical and endopelvic fascia Level 3 – Lower 1/3 via perineal body, posterior EAS and transverse perineal muscle.

19. Defaecation Cycle Rectum fills with faeces/stool 1st sensory perception-> urge Voluntary defaecation initiated Sphincters relax Pubo-rectalis lengthens and ano -rectal angle straightens IAP raises ( valsalva ) Gut contracts pressure funnels into pelvis Faeces expelled into anal canal and out! Closing anal reflex (final curtain down!) Resting tone resumes .

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