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
• 15% 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)

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

9. A ‘normal’ ultrasound
• Cone
• Internal Anal
Sphincter
• External Anal
Sphincter
• Longitudinal
layer
• Subepithelial
tissues
VAGIN
A

10. 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 -> 1st sensory perception (40-
50mls)

11. Sampling mechanism
Image courtesy of St Mark’s

12. 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

13. Resisting the urge
Image courtesy of St Mark’s

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

15. 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.

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

17. 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

18. Puborectalis and the anorectal
angle

20. 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.

21. 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.