Functional morphology of the sphincter apparatus of the rectum

Galina N. Suvorova; Суворова Г. Н.; Sergei N. Chemidronov; Чемидронов Сергей Николаевич; Antonina S. Pronina; Пронина А. С.; Yuliya V. Grigoryeva; Григорьева Ю. В.; Galina A. Sevryugina; Севрюгина Г. А.

doi:10.35693/SIM701831

Functional morphology of the sphincter apparatus of the rectum

Authors: Suvorova G.N.¹, Chemidronov S.N.¹, Pronina A.S.¹, Grigoryeva Y.V.¹, Sevryugina G.A.¹
Affiliations:
1. Samara State Medical University
Issue: Vol 11, No 2 (2026)
Pages: 98-104
Section: Human Anatomy
Published: 21.03.2026
URL: https://innoscience.ru/2500-1388/article/view/701831
DOI: https://doi.org/10.35693/SIM701831
ID: 701831

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Abstract

Pelvic floor dysfunction has recently become one of the most common problems. Typical clinical manifestations of pelvic dysfunction include urinary and fecal incontinence and pelvic organ prolapse. The musculo-fascial structure of the pelvic floor plays a major role in these risks. Recognized potential factors for the development of pelvic dysfunction include age, obesity, and childbirth.

An analysis of over 50 literature sources demonstrates the importance of the muscular, mucosal, submucosal, and vascular-nerve components of the rectum in sphincter function. This knowledge will enable clinicians to perform diagnostic and therapeutic procedures with minimal adverse effects on the patient, taking into account the morpho-functional characteristics of each sphincter compartment.

Keywords

external anal sphincter, internal anal sphincter, levator ani muscle, muscle fiber

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INTRODUCTION

The study of sphincter mechanisms of the human body is important not only from a purely theoretical perspective but also has great significance for practical medicine. L.L. Kolesnikov, who paid special attention to various sphincters in his scientific research, identified “sphincterology” as a distinct, relevant and promising direction in medicine [1]. Among the closing devices, the anal sphincter complex occupies a special place [2]. This is due to the high prevalence of incontinence, affecting up to 11–15% of adults, more frequently in postpartum women and the elderly, which significantly impacts quality of life and work capacity [3–16].

Detailed knowledge of the anatomically complex architecture of the sphincter system of the rectum is vital for the understanding of anorectal and urogenital dysfunctions. This knowledge is especially important for surgeons: the choice of surgical tactics, decision-making on the possibility of performing sphincter-preserving operations require understanding of the anatomic and histological composition of the sphincter apparatus of the rectum [13–18].

To conduct this review, a literature search was performed in the PubMed, Scopus, and RSCI (Russian Science Citation Index) databases. The search period spanned 50 years (1975–2025). Such a broad timeframe is justified by the objective of this work: to present a comprehensive picture of the morphology of the sphincteric apparatus, based both upon fundamental classical studies that laid the foundations for understanding the anatomy and histology of this area (the works of L.L. Kolesnikov, E.P. Melman, A. Shafik), and upon current data obtained using the latest imaging techniques (MRI, 3D reconstruction, electron microscopy), reflecting an evidence-based perspective on the issue.

Monographs were included due to a necessity to describe systematic anatomy and established concepts, which are rarely presented in their entirety in individual articles. Preference was given to sources with a high citation index and to works published in peer-reviewed journals. This approach allowed us to trace the evolution of views on the structure of the sphincteric complex and to synthesize data from different methodological levels, from macroscopic anatomy to tissue ultrastructure.

SPHINCTER APPARATUS OF THE RECTUM

Relief and valve organs

Morphologists define the sphincter as a thickening of the circumference of the digestive tract; however, the sphincter apparatus is not limited to muscular sphincters as executive mechanisms but includes relief structures and a system of blood vessels capable of regulating the lumen of the organ [13–19].

The relief valve devices that provide relative anatomical and functional autonomy of the rectum include the curvatures of its wall, which are formed in the frontal and sagittal planes [1, 13–18]. In the sagittal plane, two curvatures are distinguished: the sacral and the perineal. The first curvature is located on the anterior surface of the sacrum and coccyx, approx. 2 cm in front of the coccygeal tip, with its concave side facing the pubic symphysis. The second curvature appears as the rectum passes through the pelvic diaphragm, where the rectum forms a short arch, its concave side directed posteriorly and superiorly. In the frontal plane, three curvatures are distinguished. The upper curvature is located at the transition of the sigmoid colon into the ampulla; the middle curvature lies in the middle of the ampulla and is oriented to the right; the lower curvature is situated on the left, immediately above the pelvic floor.

The next system involved in the valve function is the longitudinal and the transverse folds of the mucosal lining of the rectum formed due to the presence of o bundles of elastic fibers and smooth myocytes in the submucous layer [18–21]. Among the transversely oriented folds, three are most prominent: the superior, middle and inferior, located in the ampullary part of the rectum. The superior and the inferior folds are located on the left and the middle, on the right semi-circumference of the rectum. The middle fold, having more smooth myocytes than the other flaps, is most developed [13]. The folds merge into one another in a spiral fashion, providing a certain degree of support for fecal matter. During defecation, these relief structures convey a rotational movement to the intestinal contents, which prevents too rapid passage of feces toward the anal canal, facilitates water absorption, and promotes the formation of formed stool [17–20].

Longitudinal folds of the rectal mucosa located closer to the anal canal, known as anal columns, also contribute to the valve function of the rectum. Their number varies from 4 to 12, and in the distal part the anal columns are connected with a mucosal lining fold called the anal valve. As a result, longitudinally oriented anal (Morgani) crypts are formed, or sinuses bounded from the inferior by semilunar anal valves, that E.P. Melman and I.G. Datsun refer to as valves [23]. These valves form a ridge, the free edge of which is designated as the dentate (pectinate) line, which serves as the border between the columnar and the intermediate zones of the anal canal. In addition to the permanent (anatomical) transverse folds, numerous temporary (physiological) longitudinal folds can also be observed [24].

Muscle component. Internal anal sphincter

The muscle closure devices themselves are formed by two sphincters, the internal and the external. The internal anal sphincter, composed of smooth muscle, is essentially a thickening of the inner circular muscle layer of the rectal wall. This sphincter is directly responsible for preventing the involuntary passage of gas and liquid feces [25–27]. It is considered to contribute 60–80% of pressure at rest. Bundles of myocytes are located both circularly and along a low-angle spiral. The muscle tissue of this sphincter is relatively monomorphic, the dominating population being the compact-positioned dark myocytes. Among them are the individually positioned minor myocytes and cells of Cajal. In the distal part of the internal sphincter, between the compartments of myocytes, are the layers of longitudinally positioned bundles of myocytes of the external layer of the muscle coat which likely contributes to its fixation and raises the sphincter during defecation [25, 26].

The study of the muscle compartments of the internal sphincter reveals that the light and dark myocytes forming it are located without a specific concentration, being integrated in a single system [27]. The myocytes are mononuclear, and most often the nuclei are of rod or oval in shape; depending on the functional condition of the cell, the nucleus configuration may change, in which case the nuclear membrane forms invaginations. The cisterns of the endoplasm network and the free ribosomes are mainly localized in the perinuclear areas, while the mitochondria are mainly situated between the myofilaments and under the cytolemma (Fig. 1). The main volume of the myocytes is the contractive apparatus organized by tightly packed actine and myosin myofilaments. The dense corpuscles are numerous and are located both between the myofilaments and on the internal surface of the cytolemma.

Figure 1. Ultrastructure of a fragment of a smooth muscle cell of the internal sphincter of the rectum: 1 – nucleus; 2 – nucleolus; 3 – mitochondria; 4 – desmosome. TEM. ×6000 magnification (photo by A.S. Pronina, 2025).

Рисунок 1. Ультраструктура фрагмента гладкого миоцита внутреннего сфинктера прямой кишки: 1 – ядро; 2 – ядрышко; 3 – митохондрии; 4 – десмосома. ТЭМ. Увеличение х6000 (фото А.С. Прониной, 2025).

The light myocytes comprise approx. 17% of the population of the muscle cells and manifest a less dense localization of contractile filaments. They often contain a sub-cytolemma area free of myofilaments which, in the opinion of V.A. Govyrin and T.E. Korneeva (1994), is an indication of relaxation of the muscle cells [28].

Myocytes have a well-developed system of vesicles represented by caveolae located under the cytolemma. On the whole, myocytes form a single functional system supported by different types of interactions. On the lateral surfaces and terminal regions, interdigitations are often observed. The most common are simple, non-specialized contacts, spanning on average 2–3 µm. Among the simple contacts, especially in areas of interdigitation, specialized contacts are also found: desmosomes and nexuses [29]. In desmosomal contact zones, vesicles and elements of the smooth endoplasmic reticulum, which is known to be a site for storing unbound calcium, are often concentrated beneath the cytolemma [30, 31].

The muscle compartments of the internal sphincter are divided from one another by layers of interstitium represented by collagen and elastic fibers located in an amorphous matrix. The collagenic fibrils and fibers are located along the longitudinal axis of myocytes intimately weaving into their sarcolemma. Besides, in the opinion of A.S. Ilyasov, F.S. Turaev (2021), the neighboring compartments of myocytes exchange elastic fibers. These fibers then pass through the sphincter and come to the surface facing the lumen of the rectum. There they are fixed on the muscle base of the anal columns thus forming the elastic carcass of the ridge zone [30]. These elastic fibers are partially going over the lower edge of the internal anal sphincter in the lateral direction from the bottom to the top and end in the intersphincteric zone.

Additionally, the internal sphincter is reinforced in certain areas by bundles of longitudinally oriented myocytes, which curve and cross the circularly arranged bundles of muscle cells in an arch-like manner. The internal anal sphincter is in a state of constant tonic contraction; it responds delicately to increases in intra-abdominal pressure and is innervated by both sympathetic and parasympathetic fibers [20–23]. Relaxation of the internal sphincter is not triggered by the peristaltic wave reaching the anal canal, but rather by distension of the rectal ampulla, which precedes defecation.

Muscle component. External anal sphincter and the elevator muscle of anus

The internal anal sphincter is surrounded by the external anal sphincter. Some authors assign the leading role in rectal sealing to the external sphincter [30, 31], while others suggest it contributes only 20–25% of the resting anal canal pressure [32]. The external sphincter extends distally beyond the internal sphincter; its fibers envelop the lower margin of the internal sphincter so that the latter appears to be “incorporated” into the former [33]. The muscular elements of the external sphincter weave into the dermis of the perianal skin, the perineal body, and, in males, connect with the smooth muscle of the membranous urethra. Studies also show that the muscle loops of this sphincter merge with one another, attaching to the puborectalis muscle, as well as to the pubic bone and the coccyx.

The attachment of the external and the internal sphincters is ensured by the elevator muscle of the anus, the majority of whose fibers are securely interwoven in the lower part of the rectum [34–43]. Moreover, the bundles of m. levator ani interweave with the bundles of the external layer of the muscle coat, to which some authors refer to as the conjoint longitudinal muscle. This conjoint muscle penetrates the layer between the external and the internal sphincter [39, 44]. In the distal direction, the conjoint longitudinal muscle forms the central tendon that divides the external anal sphincter into three parts: deep, superficial, and external subcutaneous.

The most superficial is the external subcutaneous part anchored in the dermis of the perianal skin. More deeply lies the superficial layer of the external anal sphincter, which attaches to the perineal body. The deep layer encircles the internal anal sphincter in a ring-like fashion.

The muscle fibers of all parts of the external sphincter have a structure typical of striated skeletal muscle tissue (Fig. 2): cell symplastic structures, coated on the outside with the basal membrane [21, 29].

Figure 2. Fragment of a muscle fiber of striated muscle tissue of the external sphincter of the rectum. Control group. TEM. ×5000 magnification (photo by G.N. Suvorova).

Рисунок 2. Фрагмент мышечного волокна поперечнополосатой мышечной ткани наружного сфинктера прямой кишки. Контрольная группа. ТЭМ. Увеличение х5000 (фото Г.Н. Суворовой).

The central part of the fibers contains densely packed myofibrils, between which there are the individual mitochondria and cisterns of the sarcoplasmic rough. The satellite cells are somewhat rare and are positioned between the cytolemma o fthe symplast and the basal membrane of the muscle fiber. The endomysium contains mainly the collagenic fibers and, in a minor quantity, elastic fibers.

Many authors are of the opinion that the strongest attachment of the sphincter apparatus comes from the m. levator ani [26–29, 36–38]. This muscle is formed from striated skeletal muscle tissue, whose muscle fibers consist of myosymplast and satellite cells. Notably, the ultrastructure of the muscle tissue does not depend on the sex. The myosymplasts are covered with cytolemma and basal membrane (Fig. 3).

Figure 3. Fragment of a muscle fiber of the levator ani muscle: A – myosymplast nucleus, B – myofibrils, C – accumulation of mitochondria around the nucleus and under the sarcolemma. ×10000 magnification (photo by S.N. Chemidronov).

Рисунок 3. Фрагмент мышечного волокна мышцы, поднимающей задний проход: А – ядро миосимпласта, Б – миофибриллы, В – скопление митохондрий вокруг ядра и под сарколеммой. Увеличение х10 000 (фото С.Н. Чемидронова).

The nuclei of the myosymplasts have different electronic densities. In most cases, finely dispersed chromatin predominates in them; heterochromatin is localized beneath the karyolemma and around the nuclei. Mitochondria are arranged in clusters beneath the sarcolemma (Fig. 4). Satellite cells are extremely rare; consistent with the general characterization of skeletal muscle tissue, they are separated from the myosymplast by their own cytolemma and are externally ensheathed by a basal membrane.

Figure 4. Fragment of a muscle fiber of the levator ani muscle: A – mitochondria located between myofibrils, B – lipid droplet. ×10000 magnification (photo by S.N. Chemidronov).

Рисунок 4. Фрагмент мышечного волокна мышцы, поднимающей задний проход.: А – митохондрии, расположенные между миофибриллами, Б – липидная капля. Увеличение х10 000 (фото С.Н. Чемидронова).

Myofibrils taking the majority of the myosymplast volume are positioned along its longitudinal axis, and individual mitochondria are on the level of light discs. The endomysium contains collagen fibrils interwoven into the basal membrane. Muscle fibers are sometimes packed so densely that the collagen fibrils are interwoven into the sarcolemma of the neighboring fibers.

It should be noted that in the muscle tissues of the sphincters and the levator ani muscle, the fibrous structures of the endomysium and perimysium, rearranging in various directions, not only surround the bundles of myocytes and muscle fibers but also integrate the sphincters into a single anatomical entity, thereby possibly ensuring synchrony between the internal and external sphincters.

Vascular component as an auxiliary obturator

In addition to the above mentioned relief and muscular structures, the sphincteric apparatus also includes the submucosal veins as auxiliary elements. The latter, located above the sphincters, form large-capacity venous plexuses which, according to several authors, e.g., V.F. Baitinger et al. (1993), act as specialized ‘cushions’ during the opening and closing of the sphincter [39–46].

The rectal venous plexus includes an internal part (the submucosal venous plexus) and an external part (the veins of the external coat). In the submucosa, veins of varying calibers are interconnected by numerous, multidirectional anastomoses. In certain locations, they dilate, forming ampullae and lacunae. At the level of the anal valves, arteriovenous anastomoses may be present. The coordinated filling and drainage of the vascular plexus of the anal canal are ensured by a ‘sphincteric’ mechanism inherent to the vessels [47–52].

Overall, cavernous structures rich in mechanoreceptors and baroreceptors form in this region. These structures not only regulate blood drainage and pooling but also ensure hydraulic sealing of the rectum: after rectal emptying, the cavernous veins fill with blood, the submucosa thickens, and elastic closure of the anal orifice occurs.

CONCLUSIONS

The structural foundation of the sphincter section of the rectum possesses both macro- and microstructural specificity and complexity of architecture. These peculiarities need to be taken into account while performing surgical, radiological or endoscopic examinations.

ADDITIONAL INFORMATION	ДОПОЛНИТЕЛЬНАЯ ИНФОРМАЦИЯ
*Study funding.* The study was the authors’ initiative without external funding.	*Источник финансирования.* Работа выполнена по инициативе авторов без привлечения финансирования.
*Conflict of interest.* The authors declare that there are no obvious or potential conflicts of interest associated with the content of this article.	*Конфликт интересов.* Авторы декларируют отсутствие явных и потенциальных конфликтов интересов, связанных с содержанием настоящей статьи.
*Contribution of individual authors.* Suvorova G.N.: concept development, study design. Chemidronov S.N.: editing of the manuscript. Pronina A.S., Grigoryeva Yu.V., Sevryugina G.A.: selection of scientific literature, its analysis, writing of the manuscript. The authors gave their final approval of the manuscript for submission, and agreed to be accountable for all aspects of the work, implying proper study and resolution of issues related to the accuracy or integrity of any part of the work.	*Участие авторов.* Суворова Г.Н.: разработка концепции, дизайн исследования. Чемидронов С.Н.: редактирование рукописи. Пронина А.С., Григорьева Ю.В., Севрюгина Г.А.. подбор научной литературы, ее анализ, написание текста рукописи. Все авторы одобрили финальную версию статьи перед публикацией, выразили согласие нести ответственность за все аспекты работы, подразумевающую надлежащее изучение и решение вопросов, связанных с точностью или добросовестностью любой части работы.
*Statement of originality.* No previously published material (text, images, or data) was used in this work.	*Оригинальность.* При создании настоящей работы авторы не использовали ранее опубликованные сведения (текст, иллюстрации, данные).
*Data availability statement.* The editorial policy regarding data sharing does not apply to this work.	*Доступ к данным.* Редакционная политика в отношении совместного использования данных к настоящей работе не применима.
*Generative AI.* No generative artificial intelligence technologies were used to prepare this article.	*Генеративный искусственный интеллект.* При создании настоящей статьи технологии генеративного искусственного интеллекта не использовали.
*Provenance and peer review.* This paper was submitted unsolicited and reviewed following the standard procedure. The peer review process involved 2 external reviewers.	*Рассмотрение и рецензирование.* Настоящая работа подана в журнал в инициативном порядке и рассмотрена по обычной процедуре. В рецензировании участвовали 2 внешних рецензента.