What are anal coulmns?
Longitudinal ridges of mucosa that encircle the anal canal. They are connected at their inferior ends by anal valves
Describe the structure of the rectum (7 points)
Made up of three transverse folds (valves of Houston)
Dilated portion called ampulla
Meets sigmoid colon at S3
No taenia coli
Follows curvature of the sacrum
Pierces pelvic floor to join anal canal
Lined with simple columnar epithelium
Structure of the anal canal
3-4cm long muscular tube that connects the rectum to the anus.
Internal anal sphincter formed by a continuation of the inner circular layer of muscularis externa from the rectum. Innervated by autonomic nerves (involuntary)
External anal sphincters formed from skeletal muscle. Controlled by somatic nerves (voluntary)
Describe the histological features of the anal canal
What is the anorectal angle?
The anorectal angle is a change in the direction at the anorectal junction from anterior to posterior, forming a 90 degree angle.
It is formed by the action of the puborectalis element of the levator ani muscle mulling anteriorly. This forms a pinch valve mechanism which aids in the maintenaance of faecal contidence.
Describe the changes that take place at the pectinate line (5 points)
The pectinite line marks the point where the embryonic hindgut meets the embryonic ectoderm. There is a change in:
Epithelium: from simple columnar to keratinised stratified squamous
Arterial suppy: superior rectal artery above and middle and inferior rectal arteries below the pectinate line
Venuous drainage: superior rectal vein (portal system) above, middle and inferior rectal veins (systemic system)
Nerve supply: Above the pectinate line autonomic and visercal afferents are from the inferior hypogastric plexus (visceral part, stretch). Below the pectinate line is innervated by the inferior rectal nerve which is a branch of the pudendal nerve (somatic part, pain)
Lymphatic drainage: going to Inferior mesenteric nodes if above the pectinate line and superficial inguinal nodes if below.
A volvulus is a condition where a portion of bowel twists on its mesentery
Sigmoid colon is the most common site of volvulus due to its long mesentery which is attached to a narrow part of the bowel.
This twisting can result in obstruction of the bowel lumen and may lead to occlusion of the blood supply
Condition in which part of the rectum descends towards the pelvic floor during straining and often emerges out of the anus.
How are haemorrhoids caused?
Haemorrhoids are caused by disrupted and dilated anal cushions and are usually asymptomatic.
Classified as internal or external. Relationship to the pectinate like determines the response to painful stmuli because of the differences in innervation
Symptoms of rectal bleeding, pain or itch may be due to enlargement, inflammarion, prolapse or thrombosis
List three common anorectal malformations
Imperforate anus: results from a failure of the anal membrane to rupture. Associated with further developmental anomalies
Rectovesical fistula: abnormal connection between the rectum and urinary bladder due to incomplete separation of the urogenital hiatus and the hindgut by the urorectal septum
Describe the stages of the defaecation reflex (5)
Activation of pressure/stretch receptors in rectum by faecal material excites afferent fibres which travel to the sacral spinal cord
This leads to increased activity in parasympathetic pelvic splanchnic nerves resulting in increased peristalsis & relaxation of the internal anal sphincter. An initial reflex contraction of the external anal sphincter prevents defaecation occurring
Sensory signals travel to the cortex so that the desire to defaecate is consciously perceived and a decision taken as to whether to proceed.
If yes, Relaxation of the external sphincter & pelvic floor (increasing anorectalangle) and faeces are expelled. he Valsalva manouvere may be used to increase intraabdominalpressure to aid defaecation
If no, There is conscious further contraction of the external anal sphincter and faecal material is moved back into the rectum. The rectum accommodates this material and sensory afferent signals decrease. The urge to defaecate passes
What is the effect of increased activity in parasympathetic pelvic splanchnic nerves?
Increased peristalsis and relaxation of the internal anal sphincter
Define faecal incontinence
Inability to control the process of defaecation
Give 5 causes of faecal incontinence
Normal structure and function disrupted: e.g. cholera, gastroenteritis
Pelvic floor injury: damage to puborectalis muscle leading to a descending perineum and increased anorectal angle
Pudendal nerve injury: loss of conscious control of external anal sphincter
Visceral nerve injury: results in loss of sensation and poor coordination of reflexes e.g. diabetes, MS
Anal sphincter injury: unable to generate sufficient anal tone to maintain continence e.g. following a tear during parturition
Anatomical anomalies: Congenital or aquired abnormalities that bypass normal continence mechanisms
Higher cortical dysfuntion
Name 5 features of the sigmoid colon
Meets the descending colon at the pelvic brim
Has simple columnar epithelium
Has haustrations, taenia coli and appendices epiploicae
Intraperitoneal, has a mesentery (sigmoid mesocolon)
Is mobile, and can have variable positions
What are anal cushions?
Dilates subepithelial vascular tissue at the anal columns which protrude into the lumen and help maintain faecal continence.
What is defaecation?
The process which elimates faecal material from the body via the rectum and anal canal.
Reflex process initiated by the presence of faeces in the rectum but can be consciously controlled.