Anatomy and Physiology of the Gastrointestinal Tract

4th week

Midgut

Formula feeding

Reduces diversity of gut bacteria

Dorsal part of the yolk sac

Endoderm

Coeliac trunk; Superior mesenteric artery; Inferior mesenteric artery

Epigastric region

Umbilical region

Hypogastric region

Upper part of the anal canal

Vagus nerve for foregut and midgut; Pelvic splanchnic nerves for hindgut

Foregut = T5-T9 via the coeliac ganglion; Midgut = T9-T12 via the superior mesenteric ganglion; Hindgut = L1-L2 via the inferior mesenteric ganglion

Oesophageal atresia; Tracheo-oesophageal fistula; Meckel’s diverticulum; Malrotation; Duodenal atresia; Imperforate anus

Birth defect where the oesophagus develops a gap so food cannot pass into the stomach

An abnormal connection between the oesophagus and the trachea allowing feeds to enter the lungs

A remnant of the vitelline duct persists, causing a pouch to form in the ileum

The midgut fails to complete its normal rotation before returning to the abdomen

The duodenal lumen is narrowed or closed, preventing food from passing through

Where enteric ganglion cells are missing from the colon and rectum; Peristalsis is therefore impaired in the affected section

Rectoanal inhibitory reflex (RAIR)

Neural crest cells

Failure to pass meconium within 48 hours; Abdominal distention; Green vomiting (bilious)

The ability to control the passage of faeces and gas until it is appropriate to go to the toilet

Internal and external anal sphincter

Allows you to voluntarily squeeze and delay defaecation

Voluntary skeletal muscle

Continuation of the inner circular muscle layer of the rectum

Encircles the internal anal sphincter and extends further down towards the skin of the anal opening

Maintains resting anal pressure and helps prevent involuntary leaking; Essentially helps keep you clean at rest

Involuntary smooth muscle

Puborectalis; Pubococcygeus; Iliococcygeus

Levator ani muscles - Puborectalis, pubococcygeus, iliococcygeus

Skeletal voluntary muscle

Maintains the anorectal angle which helps prevent stool from passing out involuntarily

Incontinence

Provides sensory and motor innervation to the perineum and external anal and urethral sphincters

S2-S4

Perineal nerve; Inferior rectal nerve; Dorsal nerve of the penis or clitoris

Perineal muscles, including the external urethral sphincter and muscles of the superficial and deep perineal pouches

Supplies the external anal sphincter

Perineal skin and lower third of the anal canal

Skin over the perineum and posterior scrotal skin; Posterior aspect of the labia majora, labia minora and vaginal vestibule

Anatomical landmark where nerve supply and type of sensation differ above and below it

Above - autonomic innervation; Below - somatic via pudendal nerve

Above the line is less sensitive to pain; Below the line is highly sensitive to pain

Above the line is sensitive to stretch and distention; Below the line is sensitive to pain, temperature and touch

Sensory supply to the skin of the penis or clitoris

Relaxes the internal anal sphincter and stimulates peristaltic activity, helping defaecation

Maintain internal anal sphincter tone and help preserve continence

Sympathetic fibres from the lumbar sympathetic pathways via the superior and inferior hypogastric plexuses; Parasympathetic fibres from pelvic splanchnic nerves (S2-S4) via the inferior hypogastric plexus

Anorectal angle; Stool consistency and colonic transit; Rectal compliance; Rectal filling and RAIR; Clinical problems such as constipation and faecal incontinence

Puborectalis

Angle formed at the junction between the central axis of the anal canal and a line parallel to the posterior wall of the rectum

Increases it

Acts like a flap valve by pulling the rectum forwards, which prevents stool from entering the anal canal; Helps prevent accidental passage of faeces

Rectum holding stool until defaecation is appropriate

Valves that act as mechanical barriers which slow the progression of stool

The ability of the rectum to stretch and accommodate increasing amounts of stool without a large rise in pressure

Automatic involuntary relaxation of the internal anal sphincter that happens when the rectum is full and stretches; Allows the rectum to distinguish between gas and stool

Symptom-based disorder defined by symptoms such as: Fewer than three bowel movements per week; Excessive straining during at least 25% of bowel movements; Hard or pellet-like stools during at least 25% of bowel movements; Feeling that the bowel has not completely emptied

Abdominal pain; Infrequent bowel movements; Nausea and vomiting; Bloating and abdominal distention; Faecaloma

Primary constipation - no identifiable underlying cause; Secondary constipation - occurs due to an identifiable underlying condition or medication

The inability to coordinate the abdominal, rectoanal and pelvic floor muscles during defaecation

Part of or all of the rectum or its lining protrudes through the anus

Weakening of the tissue between the rectum and vagina which causes the rectum to bulge into the vagina

The rectal wall folds or telescopes into itself, into the anal canal, or in some cases externally during evacuation

Visible faecal soiling; Scars or visible defects; Sphincter defects; Pelvic floor dyssynergia

Failure of the mechanisms that maintain continence, such as weak or damaged anal sphincters, impaired rectal sensation or compliance, abnormal stool consistency, or pelvic floor dysfunction

Test that assesses the pressures and function of the anal sphincters and rectum; Measures RAIR, endurance squeeze, squeeze pressure and rectal sensation

Urge faecal incontinence

Constipation and evacuation issues

Allows clinicians to observe the process of simulated defaecation; Provides information about rate of rectal emptying, how completely the rectum empties, the process of evacuation and the anatomy of the rectum and anal canal