Anal canal anatomy

Overview

• Last 4cm of the alimentary tract
  • Usually shorter in females
• Tube of muscle but all fibres are circular consisting of the Internal and External Anal Sphincters
  • Internal Anal Sphincter – Visceral muscle
  • External Anal Sphincter – Skeletal muscle
• Junction of Rectum and Anal Canal:
  • At the Pelvic Floor
  • At the level where the Puborectalis part of the Levator Ani clasps the gut and angles it forwards

• Located 2.5cm anterior to the tip of the Coccyx

• Muscles of Anal Canal

  • ‘A tube within a funnel’
  • Sides of the upper part of the funnel – Levator Ani muscles
  • Stem of the funnel – External Anal Sphincter continuous with the Levator Ani muscle
  • Tube within the funnel – Internal Anal Sphincter – thickened continuation of the inner circular layer of the rectal muscle
  • Internally lies the Submucosa and Mucous membrane

• Corrugator Cutis Ani muscle – separate smooth muscle fibres in the peri-anal skin causing puckering of this area

External Anal Sphincter

• Deep, superficial and subcutaneous parts
• Upper (Rectal) end
  • Circular skeletal muscle fibres blend with the Puborectalis part of the Levator Ani except in the midline at the front (where there are no Levator Ani fibres)
  • Region where Puborectalis fuses with the External Sphincter is termed the Anorectal Ring
    • Also the level of the upper end of the Internal Sphincter
    • Palpable on rectal exam
• Middle part
  • Fibromuscular strands pass backwards to the posterior surface of the Coccyx contributing to the Anococcygeal Ligament
  • Retrosphincteric space occupied by fibrofatty tissue lies between these fibres and the muscular Raphe formed by the Iliococcygeal part of the Levator Ani
  • Post-Anal Plate = Multi-layered fibromuscular Anococcygeal Ligament (with External Sphincter, Iliococcygeus and Pubococcygeus components) + Overlying Superior Fascia of the Pelvic Diaphragm
    • Rectum lies on Post-Anal Plate
  • Anteriorly: Intermingling of External Sphincter muscle fibres with the Transverse Perinei and Bulbospongiosus muscles at the Perineal Body
    • Less evident in males – surgical plane of cleavage can be established between the External Sphincter and the Perineal Body
    • In female, External Sphincter shorter and deep fibres are deficient anteriorly
• Lower Part:
  • Curves inwards to lie below lower end of Internal Sphincter
  • This submucosal apposition of the two sphincters – at site of palpable Intersphincteric Groove in the lower part of the Anal Canal
    • In the anaesthetized patient – Internal Sphincter often extend to anal orifice

Posterior anatomy. AC: Anal canal; ACL: Anococcygeal ligament; Cx: Coccyx; CM: Circular muscle of the anal canal; EAS: External anal sphincter; HL: Hiatal ligament (sometimes referred as anococcygeal ligament by colorectal surgeons); IAS: Internal anal sphincter; LM: Longitudinal muscle of the anal canal; PL: Parks’ ligament; RIP: Raphe of iliococcygeus and pubococcygeus muscle; Red line: Dissection plane during intersphincteric resection.

Internal Anal Sphincter

• Thickened downward continuation of the inner circular muscle of the Rectum
• At anorectal junction, the outer longitudinal layer of the rectal muscle becomes fibroelastic and, together with some striated muscle fibres of Puborectalis, forms the Conjoint Longitudinal Coat
  • Runs down between the two sphincters
  • Strands from this sheet penetrate the Internal Sphincter and lower part of the External Sphincter
  • Some reach the fat of the Ischioanal Fossa and peri-anal skin
  • Some pass through the Internal Sphincter to the mucosa of the Anal Canal, esp. at the Pectinate Line
  • Strands tethering the mucous membrane named the Mucosal Suspensory Ligament
• NB: Internal Anal Sphincter has NO bony attachments

Mucous Membrane

Anal Columns

• Located in the upper third of Anal Canal
• 6-10 longitudinal ridges
• Prominent in children

Anal Valves

• At lower ends of adjacent Anal Columns joined together by small horizontal folds
• NB: These are NOT palpable on rectal examination
• NB: Lie above a smooth surfaced area of the anal canal

Anal Sinuses

• Pockets formed above the Anal Valves
• Anal glands secret mucus into these sinuses
• Half of the Anal glands are submucosal; the rest penetrate the Internal Sphincter
  • Infections in these glands cause anal abscesses and fistulae

Pectinate Line/Dentate Line

• Level of Anal Valves

• Below Pectinate Line:

  • Pale, smooth surfaced area, the Pecten, extends down to the Intersphincteric Groove
  • Below groove, area is truly cutaneous
    • Histologically: Typical skin with keratinized stratified squamous epithelium, hair follicles, sebaceous glands and sweat glands
  • Above the groove, the lining of the Pecten is non-keratinized stratified squamous epithelium, with no hair follicles, sebaceous glands or sweat glands

• Anal column area – typical columnar intestinal cells and tubular glands

  • Immediately above and below the Pectinate line, zone of variable often mixed epithelial structure
  • Thus, no abrupt line of change from single-layered gut type to multi-layered pecten type

• Above Dentate Line:

  • Lymphatics drain to Inferior Mesenteric Lymph Nodes
  • Receives autonomic innervation from the Inferior Hypogastric Plexus
    • This region is sensitive to stretch

• Below Dentate Line:

  • Lymphatics drain to Superficial Inguinal Lymph Nodes
  • Receives somatic innervation from branches of the Pudendal Nerve
    • This region is sensitive to pain, temperature and touch

Anal Cushions

• Small submucous masses
• Comprises fibroelastic connective tissue, smooth muscle, dilated venous spaces and arteriovenous anastomoses
• Located at 3 (left lateral), 7 (right posterior) and 11 (right anterior) o’clock positions in upper anal canal
• Via their apposition, the Anal Cushions help the sphincter maintain water-tight closure of the Anal Canal
• Haemorrhoids – enlargement of Anal Cushions from excessive straining at stool

Development

• Lining of upper part of Anal Canal – derived from the Cloaca, i.e. endoderm
• Lining of lower part of Anal Canal – derived from Proctodeum or Anal Pit, i.e. ectoderm
• Dividing line between these territories considered to be at the Pectinate line

Blood Supply

• Superior Rectal Artery branches – supply upper end of Anal Canal
  • Terminations lie within the Anal Columns
• Median Sacral Arteries – supply a small part of the muscular wall
• Inferior Rectal Arteries – supply lower end of Anal Canal including mucous membrane
  • Vessels cross the Ischioanal Fossae
• Good anastomosis between the vessels within the walls
• Veins correspond to the above arteries
  • Continuous with the Rectal Venous Plexuses
  • Upper part of the canal and plexus drain via Superior Rectal and Inferior Mesenteric Veins to the Portal system
  • Lower end drains to the Internal Iliac Veins through the Inferior and Middle Rectal Veins
  • Anal Canal thus a site of porta-systemic anastomosis – union being in the region of the Anal Columns

Lymph Drainage

• Upper Anal Canal ⇒ Join lymphatics of Rectum
• Lower Anal Canal ⇒ Superficial Inguinal Lymph Nodes

Nerve Supply

• Inferior Rectal branches of the [[Pudendal Nerve

  • Supply the External Anal Sphincter
  • Sensory supply from 1-2cm above the Pectinate Line downwards (where lining of Anal Canal is highly sensitive)
  • Motor fibres originate from Onuf’s Nucleus – located mainly in anterior horn of S2 segment
    • Also innervates the Sphincter Urethrae

• Puborectalis and deep part of the External Anal Sphincter – high proportion of slow twitch fibres

  • Function as tonic muscles
  • Constant electromyographic activity even in sleep and light anaesthesia

• Autonomic Nerves – pass to the Internal Anal Sphincter and Upper Part of the Anal Canal

• Sympathetic fibres from the Pelvic Plexus:

  • Pre-Ganglionic cell bodies in the first 2 lumbar segments of the cord
  • Cause contraction of the Internal Anal Sphincter
  • Pelvic Splanchnic (Parasympathetic) nerves relax it

• Afferent fibres from upper end of the Anal Canal – carried by both sympathetic and parasympathetic nerves

Defecation

• Factors contributing to normal anal continence:
  • Contraction of Puborectalis and External Anal Sphincter
  • Maintenance of the angle between the Rectum and Anal Canal with abdominal pressure flattening the lower anterior rectal wall over the upper end of the canal
  • Presence of Mucosal Cushions in the Anal Canal
• Internal Anal Sphincter
  • Can only maintain continence if there is no distension (which causes relaxation of the sphincter)
• Rectum able to receive a certain amount of colonic content without any significant increase in pressure
  • No specialized receptors in rectal wall
  • Specialized receptors present in anal canal – gas, fluid and solid can be distinguished by the Cerebral Cortex
  • Stretch receptors present in the Levator Ani and Peri-rectal tissue
• Increasing rectal pressure ⇒ Faeces enters upper anal canal ⇒ External Anal Sphincter contracts ⇒ Forces contents back into the Rectum
• If only gas enters, its presence can be tested by a slight conscious increase of abdominal pressure which will let it escape
• Defecation allowed to occur via release of cortical inhibition that developed during childhood training
• Abdominal pressure increases ⇒ Puborectalis relaxes ⇒ Anorectal angle straightens + Relaxation of External Anal Sphincter + Contraction of the lower Colon and Rectum (via parasympathetic supply)
• Incontinence:
  • May follow damage to the External Anal Sphincter or Pudendal Nerve
  • May occur in cerebral or spinal cord lesions with loss of cortical control

Ischioanal Fossa

• Wedge-shaped space filled with fat lateral to the Anal Canal
• Base: Skin over the anal region of the Perineum
• Medial wall: External Anal Sphincter of Anal Canal and sloping Levator Ani muscles
• Lateral wall: Ischial Tuberosity below with Obturator Internus (covered by its fascia) above
• Apex of wedge = Where the medial and lateral walls meet – where the Levator Ani is attached to its tendinous origin over the Obturator Fascia
• Base of anterior boundary – posterior border of the Perineal Body and muscles of the Urogenital Diaphragm
• Posterior boundary – Sacrotuberous Ligament overlapped by the lower border of Gluteus Maximus
• Fat in lower part of the fossa adjacent to the skin – small lobules
• Fat in upper reaches of the fossa – large lobules

• Each fossa has an anterior recess that passes forwards above the Perineal Membrane
  • Potentially as far as the posterior surface of the Body of the Pubis
  • Recesses of two sides do not communicate across the midline
  • NB: Thus, Ischiorectal Fossa extends anteriorly above the Urogenital Diaphragm
    • Because the Ischiorectal Fossa extends forwards below the Levator Ani muscles which are attached to the Body of the Pubic Bone above the level of the Urogenital Diaphragm
• Posteriorly, the two fossae communicate with one another, low down through the fibrofatty tissue of the Retrosphincteric Space within the Anococcygeal Ligament
  • Provides a horseshoe-shaped path for infection spread

[[Pudendal Canal

Of Alcock- Connective tissue tunnel in the lower lateral wall of the fossa

• Overlies Obturator Internus and medial side of the Ischial Tuberosity

• Canal formed by a splitting of the Obturator Fascia above the Falciform Process of the [[Sacrotuberous Ligament]]

• Contains:

  • Pudendal Nerve
  • Internal Pudendal Vessels

• The canal conducts from the Lesser Sciatic Notch to the Deep Perineal Pouch above the Perineal Membrane

• Pudendal Nerve and Internal Pudendal Vessels leave the pelvis through the Greater Sciatic Foramen

  • Passes beneath the lower border of Piriformis to reach the buttock
  • Then turn and enters the Lesser Sciatic Foramen
  • Vessels pass over the tip of the Ischial Spine and the nerve pass more medially over the Sacrospinous Ligament

• Inferior Rectal Branches of the Pudendal Nerve:

  • Run transversely across the Ischioanal Fossa from the Pudendal Canal towards the Anal Canal
  • Course arches convexly upwards through fat towards apex and then down towards the canal
  • Incisions to drain Ischiorectal abscesses do not usually interfere with them
  • Supplies the External Anal Sphincter, mucous membrane of the lower Anal Canal and peri-anal skin NB: Lateral wall of the Left Ischioanal Fossa from behind

• At the front of the Ischioanal Fossa, the Posterior Scrotal (Labial) Nerves and Vessels (From the Pudendals) pass superficially into the urogenital region

• At the back of the Ischioanal Fossa, the Perineal Branch of S4 Nerve and Perforating Cutaneous Nerve traverse the fossa

Perineal Body

• A.k.a. Central Tendon of the Perineum
• Midline fibromuscular mass
• Attached to the posterior border of the Perineal Membrane
• Lies between Anal Canal and Vagina or Bulb of the Penis
• Rectovaginal Septum blends into it above
• Muscles running into the Perineal Body include:
  • External Anal Sphincter
  • Pubovaginalis (Puboprostaticus) part of the Levator Ani
  • Bulbospongiosus
  • Superficial and Deep Transverse Perineal Muscles