Internal anal sphincter: an anatomic study

The anatomy of the internal anal sphincter and surrounding structures was investigated in 24 cadavers using a surgical microscope (6-25 x magnification). An understanding of the anatomy of the internal anal sphincter is helpful in avoiding complications during surgical procedures in the anorectal region. The external anal sphincter was composed of three ellipsoid rings of skeletal muscle (subcutaneous, superficial, and deep) that encircle the anal canal; in contrast, we found that the internal anal sphincter was composed of flat rings of smooth muscle bundles stacked one on top of the other, like the slats of a Venetian blind. In each anal canal, the average number of ring-like slats observed was 26.33 +/- 2.93 (range = 20-30) and each was covered by its own fascia. The smooth muscle fibers and fascia coalesced at three equidistant points around the anal canal to form three columns that extended distally into the lumen and differed in form from the other anal columns. When viewed from an anterior position, the columns were located anteriorly at the observer's right (5 o'clock position), posteriorly at the right (1 o'clock position), and laterally at the left (9 o'clock position). This heretofore unreported anatomy of the internal anal sphincter may play an important role in closing off the lumen of the anal canal and maintaining bowel continence.     

The morphological differentiation of the internal sphincter muscle of the anus in the human embryo and fetus (26.01.1990)

Conclusion The opening of the anal canal appears to be the factor which initiated the differentiation of the sphincter apparatus.The internal sphincter m. of the anus is entirely composed of smooth muscle as distinct from the striated fibers of the m. puborectalis, and the external sphincter which is a mixture of smooth and striated fibers (of skeletal type). It develops in the terminal part of the internal circular layer of the rectal m., outside which are longitudinal fibers which descend early to form the external sphincter (beginning around the third month).This study shows that the internal sphincter is scarcely evident before 12 SA. Thus continence between 10 and 12 SA (after the closure of the anal membrane) is closely related to the other components of the sphincter apparatus. On the other hand, the internal sphincter has become well formed after 28 to 30 SA and then plays a direct role in maintaining continence.     

Membrane properties and the neuro-effector transmission of smooth muscle cells in the canine internal anal sphincter.

The most distal part of the circular muscle layer functions as the internal anal sphincter, which constitutes a high pressure zone at rest, but maintains a relaxed state during defecation. To elucidate such sphincter mechanisms of the smooth muscle cells, the circular muscle layer in the canine anal canal was examined within 2 cm from the anal verge. Both the mechanical and intracellular electrical activities were recorded simultaneously. The examined region could be divided into three different regions according to the pattern of spontaneous activity and innervation and consisted of an upper region (20-15 mm from the anal verge), a transitional region (15-5 mm from the anal verge) and a lower region (within 5 mm from the anal verge), respectively. The spontaneous membrane activity was characterized by ongoing slow potential changes and each potential change was associated with a phasic contraction in the three regions. The mean frequencies of spontaneous electrical activity were 6.8, 15.9, and 24.1 c/min in the upper, transitional and lower regions, respectively. In the transitional and lower region, muscle tone generation was observed. Transmural field stimulation (0.4 msec in pulse duration) evoked membrane depolarization and contractions in the lower region. The application of an alfa adrenergic blocking agent completely suppressed the generation of excitatory responses, leaving a long lasting hyperpolarization associated with relaxation. In the transitional and upper region, stimulation consistently evoked membrane hyperpolarization with relaxation. The characteristics of this hyperpolarization response varied among the three regions. The total duration of hyperpolarization increased distally, while the time to peak hyperpolarization became decreases in a reverse direction. These regional differences in the characteristics of spontaneous membrane activity and innervation indicate that the transitional and lower region might therefore function as the internal anal sphincter.     

低位直肠癌游离自体肠平滑肌片内括约肌重建术的临床研究

目的:观察低位直肠癌自体肠平滑肌内括约肌重建的功能结果。方法:从1996年10月～1998年12月,31例低位直肠癌患者行自体平滑肌内括约肌重建术,在齿线处行结肛吻合,均没有行预防性结肠或回肠造口,肿瘤距肛缘的距离为4～7cm。结果:术后仅1例吻合口复发,无远处转移,住院期间无患者死亡,术后三个月肛门括约功能明显恢复。优良率达82.1％。结论:本术式在充分根治的基础上,符合肛门排便的三个基本功能:括约、感觉和储存功能,在低位直肠癌的治疗中,是一种较为合理的手术方式。     

Increased anal basal pressure in chronic anal fissures may be caused by overreaction of the anal-external sphincter continence reflex

Chronic anal fissure is a painful disorder caused by linear ulcers in the distal anal mucosa. Even though it counts as one of the most common benign anorectal disorders, its precise etiology and pathophysiology remains unclear. Current thinking is that anal fissures are caused by anal trauma and pain, which leads to internal anal sphincter hypertonia. Increased anal basal pressure leads to diminished anodermal blood flow and local ischemia, which delays healing and leads to chronic anal fissure. The current treatment of choice for chronic anal fissure is either lateral internal sphincterotomy or botulinum toxin injections.In contrast to current thinking, we hypothesize that the external, rather than the internal, anal sphincter is responsible for increased anal basal pressure in patients suffering from chronic anal fissure. We think that damage to the anal mucosa leads to hypersensitivity of the contact receptors of the anal-external sphincter continence reflex, resulting in overreaction of the reflex. Overreaction causes spasm of the external anal sphincter. This in turn leads to increased anal basal pressure, diminished anodermal blood flow, and ischemia. Ischemia, finally, prevents the anal fissure from healing.Our hypothesis is supported by two findings. The first concerned a chronic anal fissure patient with increased anal basal pressure (170 mmHg) who had undergone lateral sphincterotomy. Directly after the operation, while the submucosal anesthetic was still active, basal anal pressure decreased to 80 mmHg. Seven hours after the operation, when the anesthetic had completely worn off, basal anal pressure increased again to 125 mmHg, even though the internal anal sphincter could no longer be responsible for the increase. Second, in contrast to previous studies, recent studies demonstrated that botulinum toxin influences external anal sphincter activity and, because it is a striated muscle relaxant, it seems reasonable to presume that it affects the striated external anal sphincter, rather than the smooth internal anal sphincter.If our hypothesis is proved correct, the treatment option of lateral internal sphincterotomy should be abandoned in patients suffering from chronic anal fissures, since it fails to eliminate the cause of high anal basal pressure. Additionally, lateral internal sphincterotomy may cause damage to the anal-external sphincter continence reflex, resulting in fecal incontinence. Instead, higher doses of botulinum toxin should be administered to those patients suffering from chronic anal fissure who appeared unresponsive to lower doses.     

胎儿肛管微血管分布特点及其临床意义

目的：研究胎儿肛管微血管的分布特点,为探讨肛门缺血性改变提供微血管的形态依据。方法：墨汁灌注,组织揭层透明铺片及组织切片,光镜下测量胎儿肛管肛门内括约肌及肛膜下区毛细血管网眼密度。结果：胎儿肛管在后中线处肛门内括约肌和粘膜下层存在毛细血管低密度区;肛门内括约肌里微动脉网的干动脉—肌束间动脉走行方向与肌纤维走行方向相垂直,括约肌强度收缩易压迫肌束间动脉,致血供不足。结论：肛管后中线处血供不良和肛门内括约肌痉挛也许是原发性肛裂好发于后连合处的主要原因。     

Histo-topographic study of the longitudinal anal muscle

The longitudinal anal muscle (LAM) has been described as a vertical layer of muscular tissue interposed between the circular layers of the internal (IAS) and external (EAS) anal sphincters. There is, however, no general agreement in the literature on its composition and attachments. The aim of this study was to investigate the histological structure, attachments, and topography of the LAM in order to evaluate its role in continence and defecation, thus enhancing knowledge of the surgical anatomy of this region. After in situ formalin fixation, the pelvic viscera were removed from eight male and eight female cadavers (age range: 52-72 years). Serial macrosections of the bladder base, lower rectum and anal canal, cervix and pelvic floor complex, cut in the transverse (six specimens) and coronal (six specimens) planes, underwent histological and immunohistochemical studies. Four specimens were studied using the E12 sheet plastination technique. The LAM was identified in 10/12 specimens (83%). Transverse and coronal sections made clear that it is a longitudinal layer of muscular tissue, marking the boundary between the internal and external anal sphincters. From the anorectal junction it extends along the anal canal, receives fibers from the innermost part of the puborectalis and the puboanalis muscles, and terminates with seven to nine fibro-elastic septa, which traverse the subcutaneous part of the external anal sphincter, reaching the perianal dermis. In the transverse plane, the mean thickness of the LAM was 1.68 +/- 0.27 mm. Immunohistochemical staining showed that the LAM consists of predominantly outer striated muscle fibers and smaller numbers of inner smooth muscle fibers, respectively coming from the levator ani muscle and from the longitudinal muscular layer of the rectum. The oblique fibers suggest that the LAM may represent the intermediate longitudinal course of small bridging muscle bundles going reciprocally from the striated EAS to the smooth IAS and vice versa. The spatial result is the helical course of striated and smooth muscle fibers between the EAS and IAS, which contribute not only to the narrowing but also to some shortening of the anal canal during sphincter contraction. Thus, rather than being a boundary, the LAM gives anatomical evidence of a functional connection between two muscle systems with different structures and topography.     

Region-related modular nerve-dependent motor activity in anorectum--cholinergic and nitrergic contribution to rat model

Disturbances of enteric nerve-mediated anorectal evacuation mechanisms have medical and social impact. The study aimed at further eliciting the contribution of cholinergic and nitrergic neurotransmission systems to modular nerve networks in different regions of Wistar rat anorectum. Electrical field stimulation (EFS, 0.8 ms, 40 V, 2, 5 or 10 Hz, 20 s), computerized mechanographic on-line setup and drugs were used to evaluate the motor responses of isolated rings from circular muscle of rectum (proximal, middle, and distal part), internal anal sphincter, and anal canal. Twitch-like frequency-dependent contractions, more pronounced in rectal preparations, characterized the modular motor responses of rectal circular muscle rings and anal canal. Depending on the frequency of stimulation, the motor activity of internal anal sphincter varied from deep long-lasting relaxation to initial short-lasting relaxation, followed by a contraction. Electrically-evoked responses of anorectal preparations were tetrodotoxin (0.1 microM)-sensitive. In the presence of atropine (0.3 microM) the contractions of rectal rings decreased, relaxation of internal anal sphincter increased and inhibition of the contractions of the anal canal occurred, followed by relaxation. During atropine treatment, NG-nitro-L-arginine (0.5 microM) increased the contractile responses and suppressed internal anal sphincter relaxations. L-arginine (0.5 microM) decreased the contractions and extended the relaxations of internal anal sphincter and anal canal. Our results suggest that cholinergic and nitrergic systems are not equally involved in modular nerve networks of various regions of anorectum. Cholinergic transmission is more expressed in distal rectum, underlying its contractile potency, while nitric oxide-dependent transmission(s) control the relaxation ability of the internal anal sphincter and anal canal.     

The role of the longitudinal muscle in the anal sphincter complex

Intersphincteric resection (ISR) enables radical sphincter-preserving surgery in a subset of low rectal tumors impinging on the anal sphincter complex (ASC). Excellent anatomical knowledge is essential for optimal ISR. This study describes the role of the longitudinal muscle (LM) in the ASC and implications for ISR and other low rectal and anal pathologies. Six human adult en bloc cadaveric specimens (three males, three females) were obtained from the University of Leeds GIFT Research Tissue Programme. Paraffin-embedded mega blocks containing the ASC were produced and serially sectioned at 250 μm intervals. Whole mount microscopic sections were histologically stained and digitally scanned. The intersphincteric plane was shown to be potentially very variable. In some places adipose tissue is located between the external anal sphincter (EAS) and internal anal sphincter (IAS), whereas in others the LM interdigitates to obliterate the plane. Elsewhere the LM is (partly) absent with the intersphincteric plane lying on the IAS. The LM gave rise to the formation of the submucosae and corrugator ani muscles by penetrating the IAS and EAS. In four of six specimens, striated muscle fibers from the EAS curled around the distal IAS reaching the anal submucosa. The ASC formed a complex structure, varying between individuals with an inconstant LM affecting the potential location of the intersphincteric plane as well as a high degree of intermingling striated and smooth muscle fibers potentially further disrupting the plane. The complexity of identifying the correct pathological staging of low rectal cancer is also demonstrated. Clin. Anat. 33:567–577, 2020. © 2019 Wiley Periodicals, Inc.     

The ramifications of adrenergic nerve terminals in the rectum, anal sphincter and anal accessory muscles of the guinea-pig

Summary The anatomy and the adrenergic innervation of the rectum, internal anal sphincter and of accessory structures are described for the guinea-pig. The distribution of adrenergic nerves was examined using the fluorescence histochemical technique applied to both sections and whole mount preparations. The longitudinal and circular muscle of the rectum and the muscularis mucosae are all supplied by adrenergic nerve terminals. The density of the adrenergic innervation of the muscularis externa increases towards the anal sphincter. There is a very dense innervation of the internal anal sphincter, of the anal accessory muscles and of the corrugator ani. Non-fluorescent neurons in the ganglia of the myenteric plexus are supplied by adrenergic terminals. The ganglia become smaller and sparser towards the internal anal sphincter and non-ganglionated nerve strands containing adrenergic axons run from the plexus to the sphincter muscle. Adrenergic fibers innervate two interconnected ganglionated plexuses in the submucosa. Very few adrenergic nerve cells were found in the myenteric plexus and they were not found at all in the submucosa. The extrinsic arteries and veins of the pelvic region are heavily innervated by adrenergic nerves. Within the gut wall the arteries are densely innervated but there is little or no innervation of the veins.This work was supported by grants from the Australian Research Grants Committee and the National Health and Medical Research Council. We thank Professor G. Burnstock for his generous support.     

Study on reconstruction of rectal sensation based on wavelet packet analysis and SVM strategy

To control anal incontinence, we have developed an artificial anal sphincter system with sensor feedback. The artificial anal sphincter system is a novel hydraulic-electric muscle which mainly comprises an artificial anal sphincter, a wireless power supply subsystem, and a rectal sensation reconstruction subsystem. To investigate the features of the patients' rectal sensation, we have developed an in vitro experimental platform of artificial anal sphincter. In vitro experiments have been performed, and demonstrate that the traditional threshold method is not suitable for predicting the time for defecation. The traditional threshold method only uses single-dimensional pressure time series which may contain a few interdependent components simultaneously. A wavelet packet analysis algorithm is employed to extract the feature vector of the rectal pressure signal, then the rectal sensation prediction model is constructed based on a support vector machine for defecation pattern recognition. The results show that the proposed method is an effective approach for the reconstruction of patients' rectal sensation.     

A COMPARATIVE ELECTRON MICROSCOPIC STUDY OF LEIOMYOSARCOMA AND LEIOMYOMA OF THE DUODENUM

Comparison of ultrastructural features between leiomyosarcoma and leiomyoma of the duodenum was made. There was no absolute difference between the two, but the following ultrastructural features of leiomyosarcoma were different from leiomyoma: (1) occasional conglomerations of mitochondria surrounded by granular endoplasmic reticulum; (2) quite rare presence of single cilia; (3) frequent occurrence of intranuclear inclusion bodies; (4) variability in nuclear size and shape; and (5) wide interstitial space with scanty collagen fibrils. Although both tumors were considered to be of smooth muscle origin, their ultrastructural features were distinctively different from the normal smooth muscle in some points. In addition to the light microscopic findings, ultrastructural features were considered to be valuable for distinguishing the malignant from the benign smooth muscle tumors in the gastrointestinal tract.     

Surgical or biologic sphincterotomy in the treatment of chronic anal fissure

BACKGROUND: Chronic anal fissure is a lineal ulcer of the lower part of the anal canal. It is a painful condition characterized by postdefecational pain and bleeding. It is associated with internal anal sphincter spasm. The relief of internal anal sphincter spasm is the key for providing fissure healing. Gold standard in the treatment of chronic anal fissure is partial lateral internal anal sphincterotomy. METHODS: Sixty patients with chronic anal fissure were randomly assigned into two groups treated either by surgical sphincterotomy or injections of botulinum toxin into internal anal sphincter. Manometric measurements were performed before and three months after treatment. Follow up period was six months. The aim of the study was to compare results between these two groups. RESULTS: Both methods efficiently reduced resting anal pressure and successfully healed chronic anal fissure. CONCLUSION: Surgical and biologic sphincterotomy are almost equally effective in the treatment of chronic anal fissure. Injecting botulinum toxin into internal anal sphincter is a safe, easy to apply and effective method in the management of anal fissure.     

Study on reconstruction of rectal sensation based on wavelet packet analysis and SVM strategy

To control anal incontinence, we have developed an artificial anal sphincter system with sensor feedback. The artificial anal sphincter system is a novel hydraulic-electric muscle which mainly comprises an artificial anal sphincter, a wireless power supply subsystem, and a rectal sensation reconstruction subsystem. To investigate the features of the patients’ rectal sensation, we have developed an in vitro experimental platform of artificial anal sphincter. In vitro experiments have been performed, and demonstrate that the traditional threshold method is not suitable for predicting the time for defecation. The traditional threshold method only uses single-dimensional pressure time series which may contain a few interdependent components simultaneously. A wavelet packet analysis algorithm is employed to extract the feature vector of the rectal pressure signal, then the rectal sensation prediction model is constructed based on a support vector machine for defecation pattern recognition. The results show that the proposed method is an effective approach for the reconstruction of patients’ rectal sensation.     

Dynamic intersection of the longitudinal muscle and external anal sphincter in the layered structure of the anal canal posterior wall

Purpose

The minute details of the structure of the anal canal are still not well understood. The complex structural configuration of the muscles, ligaments and raphes remains unclarified. This study was undertaken to determine the precise structure of the posterior part of the anal canal and to facilitate an understanding of previous studies.

Methods

For macroscopic examination, 14 right pelvic halves from 14 Japanese cadavers were used. Observation and dissection were performed from the median plane. In the histological examination, six left pelvic halves were used. The sections of the posterior parts of the anal canal were stained with hematoxylin and eosin, Elastic van Gieson, anti-smooth actin antibody and anti-skeletal myosin antibody.

Results

We identified the following muscles arranged from the internal side to the external side: internal anal sphincter, longitudinal muscle (LM), external anal sphincter (EAS) and levator ani muscle (LAM). Two different types of conformation of the posterior part of the anal canal were found, each bearing a different shape of EAS. In both types, LM penetrated the inferior part of EAS. After penetrating EAS, some fibers of LM ran posterosuperiorly and attached to the “the posterior fibers” which reach the dorsal side of the coccyx.

Conclusions

We defined and labeled the connective tissues between the anal canal and coccyx on the basis of their relative position to LAM. Based on a comparison of the two types of the posterior part of the anal canal, we propose that there are two phases due to constriction and relaxation of LM.     

Fecal incontinence. Studies on physiology,pathophysiology and surgical treatment

The thesis consists of ten previously published studies and a review. The physiological and pathophysiological mechanisms in fecal incontinence has been studied by anal manometry, both by standard static anal manometry and by a new method, dynamic anal manometry, where anal sphincter pressure can be measured during simultaneous opening and closing of the anal canal. Patients with fecal incontinence showed abnormal sphincter pressures more frequently when dynamic anal manometry was used compared to standard anal manometry. The physiology and pathophysiology of the rectum was studied using rectal compliance measurements. Patients with normal anorectal function had a large variation in rectal compliance. Patients with fecal incontinence had as a group, lower rectal compliance than continent patients. This may lead to increased frequency of incontinence episodes in patients with fecal incontinence. The relationship between idiopathic fecal incontinence and pudendal nerve terminal latency was studied in 178 patients. The far majority of patients had normal latencies, and there was no correlation between latency and anal manometry. In contrast to previous suggestions, idiopathic fecal incontinence does not seem to be caused by pudendal nerve damage. Reconstruction of the external anal sphincter in patients with fecal incontinence due to obstetric sphincter lesion showed a poorer functional result among patients older than forty years compared to younger. This indicates that the general muscular weakening with age contribute to the incontinence in these patients. The treatment of more complicated forms of fecal incontinence consists of, apart from conservative treatment or colostomi, mainly in muscle transpositions or artificial anal sphincter. Transposition of the distal part of the gluteus maximus muscle to encircle the anal canal, did not lead to acceptable continence in any of the patients studied. Transposition of the gracilis muscle lead to acceptable continence in half the patients. Patients where the transposed muscle were stimulated by a neurostimulator had satisfactory continence in most cases. However, with this method several re-operations were necessary in some patients. In addition, some patients developed severe evacuation difficulties. Implantation of an artificial sphincter resulted in long-term improvement of continence in that half of patients in whom the artificial sphincter remained implanted. The other half of the patients had the artificial sphincter explanted due to various reasons, most frequently due to infection around the device. In selected patients with more complicated fecal incontinence, stimulated gracilis transposition or implantation of an artificial anal sphincter may be offered as an alternative to colostomy. Sacral nerve stimulation is a new method which seems to provide the best results among the more advanced procedures. Its minimally invasive character also contribute to the increasing use of this method in the last few years. Evaluation and treatment of fecal incontinence is presently in a state of rapid change with focus on more elaborate investigative methods and more diversified treatment.     

Morphologic analysis of the neuromuscular development of the anorectal unit in fetal rats with retinoic acid induced myelomeningocele

To investigate whether myelomeningocele (MMC) is associated with a global neuromuscular maldevelopment of the lower gastrointestinal (GI) tract and anorectum, the distribution and staining intensity of non-neuronal (alpha-smooth-muscle-actin), neural crest cell (NCC, [Hoxb5]), and neuronal markers (PGP-9.5, synaptophysin, neurotubulin-beta-III) within the distal colon, rectum, and anal sphincters were analyzed by immunohistochemistry and Western blot in rat fetuses with retinoic acid (RA) induced MMC. At term (E22), no gross-morphological differences of the anorectal unit of OIL (n=21) MMC (n=31), and RA-exposed-non MMC (RA, n=19) fetuses were found. Smooth muscle cells were evenly distributed within the muscle layers of the rectum and the internal anal sphincter in OIL, MMC, and RA fetuses. Density and staining intensity of NCC and mature enteric neurons within the myenteric plexus of the distal colon and rectum and innervation pattern within anal sphincters in MMC fetuses were analogous to RA and OIL controls. Normal smooth muscle and myenteric plexus development of the rectum and normal innervation of the anal sphincters and pelvic floor suggests that MMC is not associated with a global neuromuscular maldevelopment of lower GI structures in this short-gestational model.     

Excitability of the motor cortical representation of the external anal sphincter

This study was designed to assess the excitability of the motor cortical representation of the external anal sphincter by using transcranial magnetic stimulation (TMS). In six healthy volunteers, the rest motor threshold and the duration of the cortical silent period were determined with single TMS pulses, and the intracortical inhibition and facilitation were measured with paired TMS pulses. Values obtained from the anal sphincter were compared with those obtained from a muscle in the right hand. All subjects completed the study. Rest motor threshold and intracortical facilitation were similar in both muscles. In contrast, cortical silent period duration and intra-cortical inhibition were less for the anal sphincter than for hand muscle. This study has opened new perspectives for the investigation of anal sphincter cortical control in humans.     

Efferent sympathetic nervous control of rectal motility in the cat

The sympathetic nervous control of rectal motility was studied in anesthetized cats. Division of the sympathetic nerves, i.e. the hypogastric nerves and the lumbar colonic nerves and alpha-adrenergic blockade reduced rectal tone indicating that these nerves are tonically active. Efferent electrical stimulation of the nerves at high intensities caused an immediate and sustained contraction which was inhibited after phentolamine but unaffected by hexamethonium suggesting a direct alpha-adrenergic effect on the rectal smooth muscle. However when prevailing rectal tone was high beta-adrenergic inhibitory responses unaffected by hexamethonium were observed. In addition the hypogastric nerves seem to convey cholinergic excitatory fibres to the rectum. The results imply that the sympathetic nerves are integrated in the nervous regulation of rectal motility in a fashion similar to the nervous control of the internal anal sphincter.