The anatomy and physiology of normal anorectum

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  • A normal anorectum is one of the greatest creation of nature and is one of the best examples of structural and physiological coordination which is under voluntary human control. This is best appreciated by the patients who have structural and neurological deficiencies in this regard. This group of patients is represented by the ones having anorectal malformations and spina bifida. Applying the clinical anatomy of normalcy is one of the basic requirements when we treat the mentioned set of pediatric surgical patients. Hence today we will try to understand the basics of the normal structural anatomy with the understanding of various factors responsible for continence.
  • Within this discussion and the alloted time it will be difficult to describe the complex structural and neural organisation of the normal anorectum. However we will try to highlight of the basic structural anatomy and the continence appratus of the anorectum with the physiology of the defecation. I guess the following speakers will be speaking on the abnormal and the ARM part of it, it will be irresistable not to mention the structural and neural abberations in patients of ARM.
  • The rectum as we know is a hindgut derivative which can be differentiated from the proximal sigmoid by the absence of the tanea and the haustration it arbitarily starts opposite the sacral promontary. The upper one third is is covered by the peritoneal reflection whereas the lower 2/3 rd ir devoid of any peritoneum. The middle part of the rectum is capacious and contributes to the major capacity of the rectum called as the ampula. This serves to store the fecal matter and contributes to the system of continence, this storage function is absent to some extent in the pulled through gut (colon/small intestine). The rectum terminates into the anal canal and has an angulation at this level which again contributes to the continence. There are several other anatomical features of the normal rectum which shall be described as an when they find mention while I describe the anatomy of continence later in the presentation.
  • The anal canal is the final exit conduit for what we eat, is an ectodermal derivative. The books have always confused the medical student over the definition of extent of surgical anal canal and anatomical anal canal. What I feel is that it is more than sufficient to understand that it commences at the level where the rectum passes through the levator diaphragm and angulates ending at the anal verge. Still for some understanding the surgical anal canal is at the level at which the internal anal sphincter turns into a less bulky circular muscle coat of rectum and is appreciated on a pr examination. The anatomical canal is till where the anal columns end in their height This is the most important structure with its entire apparatus of surrounding sphincters. This can be appreciated by the fact that all the patients of hirschsprungs who undergo a pull through operation don’t have a rectum but are continent as this anal canal with the sphincter is intact, in contrast to the ARM patients who do not posses an anal canal (except for rectal atresia, even rectal atresia patients are continent by the virture of intact anal canal). The dentate line is the sensing zone capable of unimaginable differention between touch, pain, tenprature, air/solid fecal matter. Again the finer details are described later on.
  • Now the following discussion will intererst the audience more as it is always been fascinating to know how a normal individual is capable of defecating on will, passing flatus only when u had a bad lunch at the office, and how can our anorectal malformation patients be made to achieve all this. The anorectal continence as seen in this slide occupying the central space is a result of several structural and neural organization. I have divided them into 4 groups. Striated muscle complex, the internal sphincter or the smooth muscle complex, the minor anatomical factors as the perineal body, anorectal angle, rectal ampula and hutson valves and last but even more important the neural innervations in the form of somatic and autonomic control. We shall now try to gain knowledge on the structure and the role played by them.
  • The striated muscle complex is a two part muscle. It comprises of the external anal sphincter and the levator ani. By virtue of being striated they are under volunatary control. The levator ani forms the pelvic diaphragm and is a major muscular support to the rectum. These are decscibed in detail in the subsequent text.
  • Levator ani is the 3 part muscle which arises from the inside walls of pelvis converging in midline it forms a funnel shapped portal of exit for the terminal part of the rectum and upper anal canal. The muscle comprises of 3 striated muscles Puborectalis, Pubococcygeous and illiococcygeous. The puborecatlis is the bulkiest and innermost of the three. It orignates from the myotome S1234. It encircles the rectum in a sling like fashion with the two arms inserted onto the inferior ramus of pubes on both sides. This sling is responsible for the acute angle between the rectum and the anal canal. The muscle is capable of shutting the lumen of the anorectum as a voluntary action. The arching fibers of the muscle merge with the deep part of the external sphincter. The muscle is supplied by the pudendal nerve and carries several proprioceptive sensory stimulus generated by rectal distention or increased intrabdominal pressure. Thisis later described.
  • It is said that patients with complete sacral agenesis will have no puborectalis. If S3 is missing there will be thin PR muscle continence will be doubtful if S4 is missing PR will be week. The remaining two muscles PC and IC help to close the urogenital diaphragm they will also help to elevate straighten steady and suspend the rectum which is supported by the perineal body which is another major structure responsible for pelvic visceral stability
  • The second part of the striated muscles are the external sphincter which completely surrounds the anal canal. It forms a color of varied thickness around the anal canal is again under voulntary control. It is structurally composed of slow twitch fibers and have the property of muscle fatigue.
  • This muscle is a three part muscle comprising of subcutaneous, superficial and deep part. The three parts essentially forms a tripple loop system around the anus. This as observed in the diagrams have varying thickness in their coccygeal and perineal parts. the configuration of these loops have some variation in the the two sexes, by virtue of the differences anterior to the anal canal, however they do not differ in the strength.
  • The deep part of the external sphincter is bulkier perineally that coccygeally. It merges above with the fibers of puborectalis above. The anatomy of the parts of the internal sphincters is best revealed when we study them in relation to the sphincteric abscesses which follow the rules of the anatomical boundries when spreading. The resting tone of the pelvic diaphragm muscles as well as the external sphincter although is taken as of relaxation state but researchers have demonstrated a continuous low level tonicity which is accurately modulated by the sensory reflexes generated from the nerve endings present around the dentate line.
  • Internal anal sphincter is the second most important spoke of the wheel of continence described earlier. It is essentially the continuation of the circular muscular coat of the rectum. Below it stays in a tear drop configuration. The intersphincteric fascial plane which seperated the external and the internal sphincter is the derivative of the longitudnal muscle coat of the rectum. The muscle is a non fatigable smooth muscle and the in the basal resting state is that of continous tonic contraction. This tonic state of contraction generates a high pressure zone in the lower rectum keeping it empty. The muscle tends to relax with rectal distension a reflex known a the recto anal inhibitory reflex. This reflex is absent in Hirschsprungs owing to the absence of the ganglion cells. Here actually we can understand the orign of the cause of the tonic contracted state of the distal colon producing functional obstruction. As also described later on the autonomic supply of the rectum is by the nervi errigentis – para sympathetic exitatory (increasing peristalsis) and sacral plexus – sympathetic inhibitory (decreasing peristalsis making the bowel lie in contracted state). The misseners and the aurbachs plexus in the hirschsprungs carry predominantly the parasympathetic components and since they are missing it will result in a sympathetic overdrive resulting into a state of tonic contraction. This presumption is still debatable in the literature.
  • Once we have an understanding of the structural integration of the muscles of continence it will be easy to understand the dynamics involved in the system of continence. In a resting state the intra rectal pressure is approximately 30 mm Hg. Of this almost 2/3 rd is contributed by the puborectalis and the external sphincter when ever the individual wants to hold the defecation. The remaining 1/3 rd is from internal sphincter which is tonically contracted, this preassure is good enough to prevent leakage during resting state or sleep. But during times of raised intra abdominal pressure like coughing and sneezing additional pressure is generated by the striated muscle complex.
  • Apart from the muscles described there are several other anatomical components which help in continence. Perineal body is an important structure which I feel has not been adequately described in relation to the continence structures. The importance of perineal body can be realized from the fact that if an episiotomy incision made during vaginal child birth is not sutured at all or not done properly will result in total anorectal incontinence or stress fecal incontinence. Anatomically perineal body is a central tendinous area with 4 muscles conversing onto it. 2 are paired and 2 are unpaired. Superficial transverse perineii and deep TP are paired coming from lateral origins. The arching fibers of puborectalis along with fibers of bulbospongiosus being unpaired come from the posterior and anterior direction respectively. Thus there will be a null movement of the perineal body if all the four muscles are contracting. Lact of structural integrity of specially the anterio-posterio direction will result in instability and resulting incontinence. Apart from this there are other minor factors like the anorectal angle and the rectal mucosal valves which will act as a speed breakers to the downstream of the fecal matter bolus. Rectal ampula act as a storage area allowing to decrease the bulk of fecal bolus travelling down at one time. There minor components are missing in the ARM pullthrough patients.
  • The neural supply of the anorectum comes from the S2,3 and 4 th nerves. The somatic supply is via the pudendal nerve. the autonomic supply is via the hypogastric plexus and the nervi erigentis for the parasympathetic component. Onuf nucleus which innervates the pelvic floor muscles occupies an intermediate position between the visceral and somatic nuclei it recives suprasegmental afferents which are direct corticospinal fibers.
  • Hence one can see the basic neural gear is the S234 sacral vertebrae nerves which drive both the somatic as well as the parasympathetic part of the neural control. Some of our ARM patients with sacral agenesis are unfortunate to have this important component missing and hence are incontinent.
  • With all the structural components intact an incredible physiological co ordination is present for the mechanism of continence. This is contributed by the sensory and the motor reflex pathways and some unique properties of the rectal peristalsis. We will now eloborate on this one by one.
  • Two types of sensory reflexes prevail. One is the reflex generated by the rectal distension resulting in the relaxation of the internal sphincter. This results in the initial urge in an individual to defecate, and one an opportunity is present the remaining external sphincters also relax and if not the striated muscle complex will prevent the stool coming down in fact it will be pushed proximally. There is another finer proprioceptive reflex which is generated by the sensitive receptors at the dentate line and the upper anal canal. These receptors sample the content coming in their contact and allow the individual to discriminite between air and solid. Subsequently the external anal sphincter is made to relax via the cortical afferents through the onuf nucleus and the pudendal nerve. So it is again quite unfortunate to have a missing anal canal and dentate line in our ARM patients who are not able to make this discrimination and have soiling accidents.
  • The motor reflex is contributed by the tonic contraction of the internal anal sphincter and the reflex levator with external anal sphincter once the individual decides to hold. This again may be fine tuned by the sensory reflexes for gas discrimination.
  • There are some some unique rectal properties of its contractility which is not present in the proximal colon. Mass isoperistaltic waves are generated by various GI reflexes as the gastrocolic reflex it results in the initiation of the defecation. As described in one of the previous slides the lower part of the rectum is kept empty because of some low amplitude basal electrical rhythm which tend to push the contents proximally. This reverse peristalsis also helps in times of restraint when we decide against defecaton this along with the strong contraction of the levator will push the rectal contents up into the sigmoid. And this is how and urge to defecate will dissapear one the rectum is empty and rectal distension is relived. Rectal adaption is something similar to what happens with our urinary bladder where a larger volume of urine is accomodated without the urge to urinate but this is not by the virtue of the mucosal properties of the rectum unlike the urinary bladder. It is rather by the phenomenas described above.
  • Now the sequence of events in the defecation can be understood easily via this diagram.
  • So far if we have understood the importance of all the structural and neural components taking part in the continence mechanism. It will now be easy to appreciate why ARM patients are incontinent even after reconstructive surgery. The fig explains the missing important structural components.
  • Similarly the there are several missing neural components. These are highlighted in the fig.
  • The anatomy and physiology of normal anorectum

    1. 1. Pediatric Surgery Update April 2o10” MAMC New Delhi
    2. 2. Scope 26
    3. 3. Ano-rectal anatomy <ul><li>Rectum </li></ul><ul><ul><li>Hind gut termination starting opposite the sacral promontory </li></ul></ul><ul><ul><li>With no haustrations and tenaea converging to form longitudinal muscle coat </li></ul></ul><ul><ul><li>The middle dilated part is the ampula with mucosal valves </li></ul></ul><ul><ul><li>Has a normal angle of 120 deg with the anal canal </li></ul></ul>25
    4. 4. Ano-rectal anatomy <ul><li>Anal canal </li></ul><ul><ul><li>Commences at the level where the rectum passes through the pelvic diaphragm and ends at the anal verge. </li></ul></ul><ul><ul><li>Dentate line </li></ul></ul><ul><ul><li>Anal valves </li></ul></ul>24
    5. 5. The anatomy of continence 23 The ability to retain-voluntarily or involuntarily-the intestinal contents until an evacuation is desired”. Holschneider
    6. 6. The anatomy of continence 22
    7. 7. Levator ani <ul><li>Conglomeration of 3 striated muscles </li></ul><ul><ul><li>Puborectalis </li></ul></ul><ul><ul><li>Pubococcygeous </li></ul></ul><ul><ul><li>Illiococcygeous </li></ul></ul><ul><li>Puborectalis is innermost and merges with the external anal sphincter. It causes the 120 deg angulation of anorectum due to the pull against the pubic bones </li></ul><ul><li>By its action it also shuts the lumen by apposing the post and the side walls of rectum </li></ul>21
    8. 8. <ul><li>Puborectalis </li></ul><ul><ul><li>Originates from myotomes S1-S4 </li></ul></ul><ul><li>Pubo coccygeous and illiococcygeous help to close the urogenital diaphragm by voluntary contraction, as well as elevate, straighten steady and suspend the rectum. </li></ul><ul><li>They are helped by the perineal body in the suspending action </li></ul>20 PR IC PC
    9. 9. The External Sphincter <ul><li>Three part striated muscle (slow twitch)forming a collar around the middle and the lower part of anal canal </li></ul>19
    10. 10. The External Sphincter <ul><li>Triple loop system </li></ul>18 Female
    11. 11. <ul><li>The superficial external sphincter </li></ul><ul><li>Deep part of ext sphincter </li></ul><ul><ul><li>Continuous with the puborectalis </li></ul></ul><ul><li>Subcutaneous part with the corrugate cutis ani </li></ul>17
    12. 12. <ul><li>Circular layer of rectum be-comes internal anal sphincter </li></ul><ul><li>Longitudinal layer of rectum becomes intersphincteric fascial plane </li></ul><ul><li>Muscle responsible for the recto-anal inhibitory response (RAIR) on manometery. </li></ul>Internal anal sphincter
    13. 13. Continence dynamics 15 <ul><li>Continence during sleep and rest is by the continuous tonic contraction of the Internal sphincter </li></ul><ul><li>Continence during raised intra abdominal pressure is by the levator ani and the external sphincter </li></ul>
    14. 14. Supportive structures for continence <ul><li>Perineal body </li></ul><ul><ul><li>4 muscles 2 paired 2 unpaired </li></ul></ul><ul><ul><li>Superficial transverse perinei, deep transverse perinei (paired) </li></ul></ul><ul><ul><li>Arching fibers of puborectalis around the rectum+fibers of bulbospongiosus (unpaired) </li></ul></ul><ul><li>Anorectal angle </li></ul><ul><li>Rectal ampula </li></ul><ul><li>Mucosal valves </li></ul>14
    15. 15. 13 Spinal control by Onuf Nucleus
    16. 16. <ul><li>Pudendal nerve supplies to </li></ul><ul><ul><li>All 3 muscles of levator ani </li></ul></ul><ul><li>nervierigentis </li></ul><ul><ul><li>Distal most nerve fibers relay on to the ganglia of aurbachs plexus </li></ul></ul><ul><ul><li>The parasympathetic nerves follow the path of major blood vessels to rectum </li></ul></ul><ul><ul><li>Sensory reflex of rectal distention </li></ul></ul><ul><li>Presacral sympathetic nerves </li></ul><ul><ul><li>Motor to involuntary internal sphincter </li></ul></ul><ul><ul><li>Inhibitors of bowel wall motion </li></ul></ul>12
    17. 17. Physiological coordination responsible for continence 11
    18. 18. 10
    19. 19. Fine control over defecation reflex <ul><li>Mucosa over pecten contains </li></ul><ul><ul><li>Nerve endings - pain </li></ul></ul><ul><ul><li>Meisseners corpuscules – touch </li></ul></ul><ul><ul><li>Krause end organs – pressure </li></ul></ul><ul><ul><li>Pacini corpuscules - friction </li></ul></ul><ul><li>The relaxation of the internal sphincter allows the contents to come in contact with the above sensory receptors. </li></ul><ul><li>They are able to discriminate air v/s solid fecal matter </li></ul><ul><li>The relaxation of external sphincter will occur if air is leading the flow – if one wants. </li></ul>9
    20. 20. 8
    21. 21. 7
    22. 22. Defecation reflex 6 Initiation by voluntary abdominal contraction, gastrocolic reflex, colonic reflex, direct rectal mucosal stimulation, rectal distention Filling and distension of lower rectum Increase rectal pressure Stimulation of receptors in the pelvic floor – desire to defecate- relaxation of the internal sphincter If not voluntary inhibited The external sphincter also relaxes No social opportunity, contraction of external anal sphincter and puborectalis- Rectal adaptation occurs Defecation
    23. 23. 5
    24. 24. Aberrations in ARM- structural 4
    25. 25. Anatomy as seen during PSARP <ul><li>During posterior saggital approach we see two groups of muscle fibres </li></ul><ul><ul><li>One group running in vertical direction (on table PSARP position) </li></ul></ul><ul><ul><li>2 nd group more bulky running in transverse direction merging with levator within the depth of incision </li></ul></ul>3 The vertical group also know as the para saggital fibers are a part of the external sphincter and are presumably its superficial part
    26. 26. Anatomy as seen during PSARP <ul><li>The lower transverse fibers are the muscle complex fibers (as termed by Pena et al) is also the external sphincter part which converge with the levator ani in deeper planes at almost right angles. </li></ul><ul><li>This muscle complex is perpendicular to para saggital fibers. </li></ul><ul><li>The crossing of the muscle complex creates the anterior and the posterior limit of the neo anus </li></ul><ul><li>The levator muscle lies deeper in the incision and higher the malformation </li></ul><ul><li>deeper is the levator </li></ul>
    27. 27. Abberations in ARM- neural 1
    28. 28. 0 Thank you

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