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Sphincter

A sphincter is a ring-shaped muscle that surrounds a tube or opening in the body, tightening to close the passage and relaxing to allow the flow of substances such as food, fluids, or waste.^[1] These specialized muscles function as valves, regulating the movement of materials through various bodily systems to maintain proper physiological processes like digestion, urination, and defecation.^[2] Sphincters in the human body number over 60, ranging from large structures in the digestive tract to microscopic precapillary sphincters in blood vessels that control local blood flow.^[3] They are classified into two main types based on muscle composition: involuntary sphincters made of smooth muscle, which operate automatically under autonomic nervous system control, and voluntary sphincters composed of skeletal muscle, which can be consciously regulated.^[4] Involuntary examples include the pyloric sphincter at the stomach's outlet, which governs the release of partially digested food into the small intestine, and the internal urethral sphincter, which helps retain urine in the bladder.^[5]^[4] Voluntary sphincters, often paired with involuntary ones for enhanced control, are crucial for functions requiring conscious effort, such as the external anal sphincter that enables deliberate bowel control and the external urethral sphincter that supports urinary continence.^[6]^[4] Major sphincters in the gastrointestinal tract include the upper and lower esophageal sphincters, which prevent air entry and acid reflux respectively; the ileocecal sphincter, regulating flow between the small and large intestines; and the sphincter of Oddi, which controls bile and pancreatic juice release into the duodenum.^[3]^[7] Dysfunction in these structures can lead to conditions like incontinence, gastroesophageal reflux disease, or biliary obstruction, underscoring their role in overall health.^[2]

Anatomy

Microscopic Structure

Sphincters are primarily composed of circularly arranged smooth or skeletal muscle fibers that form a functional ring around tubular structures or orifices, enabling controlled closure and opening. These muscle fibers are organized in concentric layers, with the circular orientation providing the mechanical basis for constriction. In smooth muscle sphincters, the fibers are fusiform and nonstriated, consisting of elongated cells approximately 15-200 μm in length and 3-10 μm in diameter, each containing a single, centrally located oval nucleus.^[8]^[9] Histologically, smooth muscle sphincters differ markedly from their skeletal counterparts. Smooth muscle lacks the striations characteristic of skeletal muscle, as actin and myosin filaments are arranged obliquely and anchored to dense bodies rather than organized sarcomeres, resulting in a uniform appearance under light microscopy. In contrast, skeletal muscle sphincters feature multinucleated, cylindrical fibers up to several centimeters long, exhibiting cross-striations due to the regular alignment of sarcomeres, with alternating A bands (dark, anisotropic) and I bands (light, isotropic) visible at magnifications of 400x or higher. These differences reflect their involuntary versus voluntary control, respectively.^[8]^[10]^[11] Supporting the muscle fibers are layers of connective tissue that provide structural integrity and facilitate force transmission. The endomysium, a delicate layer of reticular fibers and basal lamina, encases individual muscle cells or small groups, while the perimysium, a denser fibrous sheath rich in collagen type I and elastin, bundles fibers into fascicles and separates circular from longitudinal layers where present. In smooth muscle sphincters, fiber density is often higher in the circular layer compared to adjacent regions, enhancing the sphincter's occlusive capacity, and these bundles are interconnected by gap junctions for coordinated contraction.^[9]^[10]^[12] At the microscopic level, smooth muscle sphincters exhibit variations in innervation patterns, featuring dense autonomic nerve plexuses embedded within the connective tissue framework. These plexuses, comprising unmyelinated postganglionic fibers from sympathetic and parasympathetic divisions, form a rich network around muscle bundles, with varicosities releasing neurotransmitters directly onto the cell membranes rather than at discrete endplates. Skeletal muscle sphincters, by comparison, show more sparse somatic innervation focused on motor endplates, though connective tissue septa may integrate minor autonomic components for vascular regulation. Such histological adaptations underscore the sphincter's role in maintaining tonic closure.^[8]^[12]^[13]

Locations in the Body

Sphincters are distributed throughout the human body, with over 60 identified types primarily situated in tubular structures such as the digestive and urinary tracts, as well as in vascular networks and orifices like the pupil.^[3] Most sphincters encircle or guard passageways between organs or compartments, facilitating compartmentalization, while others regulate access at microscopic levels. Their prevalence is highest in the gastrointestinal system, where multiple sphincters align along the length of the digestive tube to demarcate regions.^[14] In the digestive tract, sphincters are positioned at key junctions to separate compartments. The upper esophageal sphincter is located at the pharyngoesophageal junction, posterior to the cricoid cartilage at the level of the C5-C6 vertebrae.^[15] The lower esophageal sphincter resides at the gastroesophageal junction, where the esophagus meets the stomach.^[16] Further along, the pyloric sphincter lies between the stomach's pylorus and the duodenum, while the ileocecal sphincter is situated at the junction of the ileum and cecum in the large intestine.^[2] At the distal end, the internal anal sphincter forms the superior portion of the anal canal, adjacent to the rectum, and the external anal sphincter encircles the anal canal externally.^[17] Additionally, the sphincter of Oddi is positioned at the major duodenal papilla, where the common bile duct and pancreatic duct converge to enter the duodenum.^[14] Within the urinary system, sphincters are concentrated around the bladder outlet and urethra. The internal urethral sphincter is located at the bladder neck, encircling the proximal urethra just below the bladder base.^[18] The external urethral sphincter surrounds the membranous urethra in males, positioned between the prostatic and penile urethra segments, or the mid-urethra in females.^[4] Vascular sphincters, such as precapillary sphincters, are found at the microscopic level in the microcirculation, located at the junction between metarterioles and capillary beds throughout systemic tissues to control capillary perfusion.^[19] In the ocular system, the pupillary sphincter is embedded within the iris, forming a circular band in the pupillary zone of the stromal layer that encircles the pupillary margin, anterior to the pigmented epithelium.^[20]

Physiology

Contraction and Relaxation Mechanisms

Sphincters primarily consist of circularly oriented muscle fibers that contract to constrict the lumen of a tubular structure, thereby closing the passageway, while relaxation of these fibers allows the lumen to dilate and permit the flow of contents.^[4] This biomechanical process relies on the intrinsic properties of the muscle tissue to generate and release tension without requiring external structural support.^[21] In smooth muscle sphincters, which predominate in visceral applications, contraction arises from interactions between actin and myosin filaments organized in a less structured lattice than in striated muscle. Calcium influx into the cytosol binds to calmodulin, forming a complex that activates myosin light chain kinase (MLCK); this enzyme phosphorylates the regulatory light chain of myosin II, promoting cross-bridge formation and cycling between actin and myosin heads to produce shortening and force generation.^[22] Relaxation occurs when calcium levels decrease, leading to dephosphorylation of myosin by myosin light chain phosphatase, which inhibits further cross-bridge activity and allows filament disengagement.^[23] In skeletal muscle sphincters, found in voluntary structures such as the external urethral and anal sphincters, contraction is triggered by somatic nerve impulses generating action potentials along the muscle fiber membrane. This leads to calcium release from the sarcoplasmic reticulum, where calcium ions bind to troponin, causing a conformational change that exposes myosin-binding sites on actin filaments, enabling cross-bridge cycling and muscle shortening. Relaxation follows the removal of calcium by active pumping back into the sarcoplasmic reticulum, allowing troponin to block the binding sites and cease contraction.^[21] Sphincters display distinct contraction patterns: tonic activity involves sustained tension at a relatively constant length to maintain baseline closure, as seen in structures requiring ongoing barrier function, whereas phasic patterns feature intermittent, rhythmic contractions superimposed on lower tone for episodic regulation.^[24] Tonic sphincters inherently develop myogenic tone through spontaneous calcium oscillations and latch-state mechanisms that prolong cross-bridge attachment without continuous energy expenditure.^[25] The baseline tone in sphincters establishes a radial pressure gradient across the lumen, where intraluminal pressure must exceed sphincter pressure to initiate flow, thereby providing a passive barrier against unintended leakage.^[26] This gradient is amplified during contraction, enhancing closure efficiency by increasing the force opposing distal propulsion.^[27]

Regulation and Control

The regulation of sphincter activity is primarily governed by extrinsic neural, hormonal, and local feedback mechanisms that modulate tone and responsiveness across various body systems. The autonomic nervous system plays a central role, with the sympathetic division typically promoting contraction in smooth muscle sphincters through alpha-adrenergic receptors, which enhance basal tone and prevent inappropriate relaxation. For instance, norepinephrine release from sympathetic nerves activates these receptors on sphincter smooth muscle, increasing intracellular calcium and contractile force. In contrast, the parasympathetic division facilitates relaxation of these sphincters, often via cholinergic pathways that inhibit tone, as seen in the coordination of bladder emptying where parasympathetic stimulation relaxes the internal urethral sphincter while contracting the detrusor muscle.^[28]^[29] Voluntary sphincters, composed of skeletal muscle, are regulated by the somatic nervous system, allowing conscious control over contraction and relaxation. For example, the external urethral sphincter is innervated by pudendal nerve fibers originating from sacral spinal segments S2–S4, enabling voluntary constriction to maintain urinary continence; relaxation occurs through inhibition of these motor neurons during micturition. Similarly, the external anal sphincter receives somatic innervation via the inferior rectal branch of the pudendal nerve, supporting deliberate control of defecation. These somatic pathways often integrate with autonomic reflexes for coordinated function, such as in the guarding reflex that enhances sphincter closure in response to urgency.^[29] Hormonal influences further fine-tune sphincter function, integrating systemic signals with local activity. Gastrin, released from gastric G cells in response to meals, directly enhances the tone of the lower esophageal sphincter by stimulating smooth muscle contraction, thereby reducing the risk of gastroesophageal reflux. Similarly, estrogen modulates urethral sphincter tone, particularly in females, where it promotes increased closure pressure through effects on smooth muscle and connective tissue integrity, contributing to urinary continence. These hormonal effects often interact with neural inputs, amplifying or dampening autonomic responses based on physiological needs such as digestion or reproduction.^[30]^[31] In the gastrointestinal tract, the enteric nervous system provides intrinsic regulation through coordinated reflexes mediated by its plexuses, enabling semi-autonomous control of sphincter activity. The myenteric (Auerbach's) plexus, located between the longitudinal and circular muscle layers, orchestrates peristaltic reflexes that relax sphincters like the pyloric or anal during propulsion while maintaining tone elsewhere via inhibitory and excitatory neurons. Short intrinsic reflexes, triggered by local distension or chemical stimuli, propagate through these plexuses to coordinate sequential sphincter opening and closing, independent of central input but modifiable by vagal or sympathetic modulation.^[32] Feedback mechanisms ensure adaptive control, particularly in vascular sphincters such as precapillary sphincters in the microcirculation. Baroreceptors in the carotid sinus and aortic arch detect changes in arterial pressure, initiating reflexes that adjust sympathetic outflow to alter vascular tone; elevated pressure leads to parasympathetic activation and sympathetic inhibition, promoting vasodilation and sphincter relaxation to lower resistance, while hypotension has the opposite effect to conserve blood flow. This baroreflex maintains systemic homeostasis by dynamically regulating capillary perfusion without direct hormonal involvement.^[33]

Classification

By Muscle Type

Sphincters are primarily classified by their muscle type, which determines their mode of control and functional characteristics. Smooth muscle sphincters, composed of non-striated smooth muscle fibers, operate involuntarily and maintain a tonic baseline contraction that can be modulated for opening or tightening. These sphincters are typically found in visceral organs, such as those in the gastrointestinal and vascular systems, where they regulate continuous physiological processes like peristalsis and blood flow. Their activity is governed by the autonomic nervous system, involving both sympathetic and parasympathetic inputs that mediate relaxation and contraction without conscious effort.^[16]^[34]^[35] In contrast, skeletal muscle sphincters consist of striated muscle fibers similar to those in voluntary skeletal muscles elsewhere in the body. These sphincters enable voluntary control, allowing conscious regulation of closure and relaxation, and are often positioned at external body orifices to provide on-demand containment. Examples include structures at bodily exits, where somatic motor neurons from the spinal cord transmit signals for precise, willful activation. This voluntary mechanism relies on the somatic nervous system, distinguishing these sphincters from their smooth muscle counterparts in terms of neural pathway and responsiveness.^[4]^[36]^[37] Many sphincters exhibit mixed or hybrid compositions, combining smooth and skeletal muscle layers to integrate involuntary tone with voluntary override. For instance, the internal component often features smooth muscle for basal, autonomic-maintained closure, while the external layer incorporates skeletal muscle for somatic, volitional fine-tuning. This dual structure enhances overall efficiency, providing a default barrier that can be consciously reinforced or released as needed. Such arrangements are common at transitional zones between visceral and external environments.^[3]^[4] From an evolutionary perspective, the distinction between smooth and striated (skeletal) muscle types in sphincters traces back to the bilaterian lineage, where smooth myocytes likely represent the ancestral form for visceral regulation, while striated variants evolved for enhanced contractility in more complex motor control systems. Developmentally, both types originate from mesodermal precursors, but smooth muscle sphincters form through differentiation influenced by autonomic neural crest derivatives, whereas skeletal ones arise from somites under somatic innervation cues. This distribution reflects adaptations for involuntary internal homeostasis versus voluntary external guardianship across vertebrate evolution.^[38]^[39]^[40]

By Functional Role

Sphincters can also be classified as anatomical or functional based on their structure and how they achieve closure. Anatomical sphincters consist of localized, often circular arrangements of muscle fibers that form a distinct ring to open and close passages. Examples include the pyloric sphincter and the external anal sphincter.^[3] Functional sphincters, in contrast, arise from the distributed arrangement and coordinated activity of muscle fibers over a longer segment, creating a high-pressure zone that acts as a sphincter without a discrete muscular ring. The lower esophageal sphincter is a classic example, where clasp and sling fibers of the distal esophagus generate tonic pressure to prevent reflux.^[34]^[41] This distinction highlights how sphincters regulate flow through structural specialization, with both types contributing to preventing backflow, controlling transit, and modulating pressure in bodily conduits. Precapillary sphincters in vascular networks, for instance, function to adjust local blood flow by constricting to protect capillaries from high pressure.^[42]

Specific Examples

Gastrointestinal Sphincters

Gastrointestinal sphincters are specialized muscular structures within the digestive tract that regulate the flow of contents, prevent reflux, and coordinate digestion by controlling the passage between adjacent segments. These sphincters, primarily composed of smooth muscle, maintain tonic contraction to act as barriers while relaxing in response to neural and hormonal signals to allow controlled transit of food, chyme, or waste. Key examples include those at the esophageal, gastric, small intestinal, and colonic junctions, each adapted to specific digestive phases such as swallowing, gastric emptying, nutrient absorption, and defecation.^[43] The upper esophageal sphincter (UES), located at the pharyngoesophageal junction, is primarily formed by the cricopharyngeus muscle, a component of the inferior pharyngeal constrictor. This skeletal muscle maintains a resting pressure of about 30-100 mmHg to prevent entry of air into the esophagus during respiration and reflux of esophageal contents into the pharynx or airways. During swallowing, the UES relaxes via inhibition of tonic contraction and traction from suprahyoid muscles, allowing bolus passage; it then rapidly contracts to clear residue and protect the airway.^[44]^[45]^[46] The lower esophageal sphincter (LES), situated at the gastroesophageal junction, consists of a 3-4 cm segment of tonically contracted smooth muscle that generates a resting pressure of 15-30 mmHg to inhibit reflux of acidic gastric contents into the esophagus. Unlike anatomic sphincters, the LES is a physiologic high-pressure zone formed by intrinsic esophageal circular muscle and gastric sling fibers, with transient relaxations (TLESRs) triggered by vagal reflexes during swallowing, belching, or vomiting to permit gas or liquid passage. These TLESRs, lasting 5-30 seconds, are mediated by nitric oxide and vasoactive intestinal peptide, and their dysfunction contributes to gastroesophageal reflux disease.^[34]^[47]^[48] The pyloric sphincter, a true anatomic ring of smooth muscle at the gastroduodenal junction, regulates the intermittent release of partially digested chyme from the stomach antrum into the duodenum, typically in small aliquots of 2-3 mL every 10-20 seconds during the digestive phase. This control prevents overwhelming the duodenum with hyperosmolar or acidic content, allowing time for neutralization and enzyme action; it operates via coordinated antral peristalsis and pyloric contraction, influenced by hormones such as gastrin, which enhances pyloric tone and gastric motility, and motilin, which promotes interdigestive emptying by relaxing the sphincter during migrating motor complexes.^[49]^[50]^[51] The sphincter of Oddi, a complex of smooth muscle fibers surrounding the distal common bile duct, pancreatic duct, and their junction at the ampulla of Vater within the duodenal wall, regulates the flow of bile from the gallbladder and pancreatic juice into the duodenum while preventing duodenobiliary reflux. It maintains a basal pressure of 10-35 mmHg, relaxing in response to cholecystokinin (CCK) and secretin to allow coordinated release during digestion, typically opening for 5-10 seconds several times per meal; neural control via the vagus and sympathetic nerves modulates its tone, and dysfunction can lead to biliary colic or pancreatitis.^[34]^[52] The ileocecal sphincter, formed by thickened circular smooth muscle at the ileocolonic junction, maintains a baseline tone to separate the ileum from the cecum, preventing reflux of bacterially rich colonic contents into the small intestine and thereby limiting bacterial overgrowth. It relaxes in response to ileal distension or gastrin to permit gradual passage of chyme (about 1-2 L daily) into the colon, facilitating segmentation and mixing in the ileum while controlling the rate to match colonic absorption capacity. This sphincter contributes to the ileal brake mechanism, where duodenal feedback hormones like cholecystokinin slow transit.^[53]^[54]^[55] The internal anal sphincter, a direct continuation of the rectal smooth muscle, provides involuntary tonic contraction accounting for 70-85% of resting anal pressure (50-100 mmHg) to maintain fecal continence by closing the anal canal. Composed of circular smooth muscle layers, it relaxes via rectoanal inhibitory reflex during defecation, triggered by rectal distension, allowing coordinated expulsion while the external sphincter provides voluntary override. Dysfunction here can lead to incontinence if tone is reduced.^[56]^[26]^[57] The external anal sphincter, an oval tube of skeletal muscle fibers surrounding the anal canal, provides voluntary control over defecation, contributing 15-30% to resting anal pressure and enabling conscious contraction to maintain continence during social situations or stress. It is divided into subcutaneous, superficial, and deep parts, innervated by the pudendal nerve, and coordinates with the internal sphincter and puborectalis muscle during defecation to relax in response to voluntary effort and rectal filling.^[58]

Urogenital Sphincters

The urogenital sphincters play essential roles in urinary continence and reproductive isolation within the pelvic region. These structures primarily consist of the urethral sphincters, which regulate urine flow from the bladder, while also contributing to functions such as semen expulsion in males and menstrual regulation in females. The pelvic floor integrates these sphincters, providing coordinated support for both urinary and reproductive tracts. The internal urethral sphincter, located at the bladder neck, is composed of involuntary smooth muscle that maintains basal tone to prevent urine leakage during bladder filling. This sphincter operates autonomously via sympathetic innervation, ensuring continence without conscious effort. In males, smooth muscle extensions into the prostatic region enhance this mechanism, contracting during ejaculation to close the bladder neck and direct semen forward through the urethra while preventing retrograde flow into the bladder.^[4]^[59]^[60] The external urethral sphincter, also known as the rhabdosphincter, is a striated skeletal muscle under voluntary somatic control, allowing deliberate initiation or cessation of urination. It encircles the membranous urethra distal to the prostate in males and surrounds the mid-urethra in females, integrating with the pelvic floor muscles for enhanced closure. This sphincter provides the primary mechanism for stress continence, contracting reflexively during activities like coughing. In the female reproductive system, smooth muscle in the cervix forms a functional sphincter at the internal os, contracting to retain the fetus during pregnancy and relaxing to permit menstrual blood flow.^[4]^[61]^[62] Due to anatomical differences, urogenital sphincter demands vary by gender; the female urethra is shorter (approximately 4 cm) compared to the male (about 20 cm), increasing susceptibility to incontinence as there is less distance for sphincter closure to prevent leakage. The male internal sphincter exhibits greater volume, supporting dual urinary and ejaculatory roles, while female sphincters rely more on pelvic floor synergy for continence. Estrogen modulates female urethral sphincter tone, enhancing closure efficiency.^[63]^[59]^[64]

Vascular and Ocular Sphincters

Vascular sphincters, particularly precapillary sphincters, consist of circular rings of smooth muscle cells located at the junction where metarterioles connect to capillary beds, enabling precise control of blood flow into individual capillaries based on local tissue metabolic demands such as oxygen and nutrient requirements.^[65] These structures, typically measuring around 3-4 micrometers in diameter, function at a microscopic scale to regulate perfusion by constricting or relaxing in response to chemical signals from surrounding tissues, thereby optimizing capillary recruitment and preventing excessive blood flow or stagnation.^[42] A key adaptation in these sphincters is myogenic autoregulation, where the smooth muscle intrinsically contracts in response to increased intravascular pressure to maintain stable blood flow and protect downstream capillaries from pressure fluctuations.^[66] In contrast, the ocular sphincter, known as the sphincter pupillae, is a circular band of smooth muscle embedded within the iris that encircles the pupil and contracts to constrict its diameter in response to bright light, thereby reducing the amount of light entering the eye to protect the retina and enhance visual acuity.^[67] This muscle, approximately 0.7-1.0 mm wide and 100-170 micrometers thick, operates under parasympathetic innervation via short ciliary nerves from the oculomotor nerve, triggering miosis as part of the pupillary light reflex pathway.^[68]^[69] The vascular and ocular sphincters differ markedly in scale and purpose: precapillary sphincters manage microcirculatory perfusion at the cellular level to support tissue oxygenation, while the sphincter pupillae facilitates macroscopic adaptation for vision by modulating light intensity across the entire retinal field.^[42]^[67] This distinction underscores their specialized roles in circulatory homeostasis versus optical regulation, with the former relying heavily on intrinsic myogenic mechanisms and the latter on extrinsic neural inputs for rapid environmental responsiveness.

Clinical Aspects

Associated Disorders

Sphincter incompetence refers to the failure of these muscular valves to maintain adequate closure, allowing inappropriate passage of contents and leading to various gastrointestinal and urogenital disorders. A prominent example is gastroesophageal reflux disease (GERD), where weakness or abnormal relaxation of the lower esophageal sphincter permits stomach acid to backflow into the esophagus, causing symptoms such as heartburn, regurgitation, and potential esophageal inflammation. This condition affects approximately 20% of the adult population in Western countries, with risk factors including obesity, smoking, and hiatal hernia that further compromise sphincter integrity.^[70] Hypertonicity disorders involve excessive contraction or failure to relax, resulting in obstructive symptoms and tissue damage. Achalasia of the esophagus exemplifies this, characterized by impaired relaxation of the lower esophageal sphincter due to loss of inhibitory neurons in the myenteric plexus, leading to dysphagia, chest pain, and food retention. Similarly, chronic anal fissures often arise from spasm of the internal anal sphincter, which reduces blood flow to the anal canal, perpetuating the fissure and causing severe pain during defecation. These hypertonic states can stem from local inflammation or neural dysregulation, exacerbating symptoms like constipation and bleeding.^[34]^[71]^[72] Neurological impacts on sphincters frequently manifest as incontinence due to disrupted neural control, particularly in conditions involving denervation. For instance, spinal cord injuries can lead to detrusor-sphincter dyssynergia or external urethral sphincter weakness, resulting in urinary incontinence through uncoordinated bladder and sphincter activity, with symptoms including frequent leakage and urinary tract infections. Such dysfunction affects up to 80% of individuals with spinal cord injuries, highlighting the role of somatic and autonomic nerve damage in sphincter failure. Failures in normal neural regulation, as seen in these cases, underscore the sphincter's reliance on intact innervation for proper function.^[73]^[74] Congenital disorders like Hirschsprung's disease involve aganglionosis of the enteric nervous system, affecting distal bowel sphincters and causing tonic contraction without relaxation, which leads to chronic constipation, abdominal distension, and intestinal obstruction from birth. This absence of ganglion cells in the myenteric and submucosal plexuses disrupts peristalsis and sphincter coordination, often requiring early intervention to prevent enterocolitis. The condition has a prevalence of about 1 in 5,000 live births, with genetic factors such as RET proto-oncogene mutations increasing susceptibility.^[75]^[76] Aging significantly contributes to sphincter disorders through progressive weakening and structural changes, elevating prevalence across multiple systems. Fecal incontinence, often linked to anal sphincter atrophy, affects 9.3% of individuals aged 60 and older compared to 4.9% in younger adults, with women at higher risk due to obstetric trauma and hormonal shifts. Similarly, GERD prevalence reaches 23% in the elderly, compounded by reduced lower esophageal sphincter pressure and delayed gastric emptying. Risk factors include multimorbidity, reduced mobility, and neurodegenerative changes, which collectively impair sphincter tone and increase systemic complications like malnutrition or skin breakdown.^[77]^[78]

Diagnostic and Therapeutic Interventions

Diagnostic techniques for assessing sphincter function primarily involve physiological measurements and imaging modalities to evaluate pressure dynamics, structural integrity, and muscle activity. Manometry is a cornerstone method, utilizing pressure-sensitive catheters to measure intraluminal pressures in sphincters such as the esophageal or anorectal regions, providing insights into motility and coordination during swallowing or defecation.^[79]^[80] For anorectal manometry specifically, it assesses resting and squeeze pressures of the internal and external anal sphincters, helping identify hypotonic or dyssynergic function.^[81] Imaging techniques complement manometry by visualizing structural defects and dynamic function. Endoanal ultrasound serves as the gold standard for evaluating the integrity of the anal sphincter complex, detecting defects in the internal and external sphincters with high resolution via a transrectal probe.^[82] Fluoroscopy-based defecography captures real-time pelvic floor movements during simulated defecation, revealing abnormalities like rectocele or intussusception that affect sphincter performance.^[83] Magnetic resonance imaging (MRI), including dynamic MR defecography, offers non-ionizing assessment of sphincter anatomy and pelvic floor coordination without radiation exposure.^[84] Endoscopy, often integrated with ultrasound (endosonography), allows direct visualization and biopsy if needed, while electromyography (EMG) records electrical activity in sphincter muscles to diagnose nerve damage or denervation, typically using needle electrodes in the anal sphincter.^[85]^[86] Therapeutic interventions for sphincter dysfunction range from conservative to invasive approaches, tailored to the underlying impairment such as hypertonicity or weakness. Pharmacological options include botulinum toxin (Botox) injections into hypertonic sphincters, like the pyloric or anal, to temporarily relax muscles and alleviate spasms without permanent damage, often combined with biofeedback for sustained improvement.^[87]^[88] Biofeedback training uses visual or auditory cues from manometry or EMG to strengthen and coordinate sphincter muscles, particularly effective for fecal incontinence by enhancing pelvic floor control.^[89] Surgical interventions encompass sphincterotomy for relieving chronic anal fissures by incising the internal anal sphincter to reduce pressure, and augmentation procedures like sling placement or artificial sphincter implantation to support weakened urinary or anal sphincters in severe incontinence cases.^[90] Emerging therapies focus on regenerative and neuromodulatory techniques to restore sphincter function long-term. Sacral neuromodulation involves implanting a device to deliver electrical impulses to sacral nerves, modulating bladder and bowel sphincter control for refractory urinary or fecal incontinence, with success rates up to 70% in select patients.^[91] Stem cell therapies, such as autologous muscle-derived cells injected into damaged sphincters, aim to promote tissue regeneration and improve continence, as demonstrated in clinical trials for fecal incontinence where engraftment enhances muscle strength.^[92]^[93] Outcome measures standardize evaluation of intervention efficacy, with scoring systems quantifying symptom severity and quality of life. The Wexner (Cleveland Clinic) score assesses fecal incontinence frequency and type, ranging from 0 (perfect continence) to 20 (severe), guiding treatment decisions and monitoring progress post-therapy.^[94] Similar scales, like the Vaizey score, incorporate lifestyle impact for a comprehensive assessment across sphincter-related disorders.^[95]

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In smooth muscle, by contrast, calcium binds to CaM, which then interacts with myosin light-chain kinase (MLCK), causing it to phosphorylate the myosin light- ...
[25]
Is the muscular tone of the internal anal sphincter a property of ... - NIH
Contractions are often classified as phasic or tonic depending on the type of mechanical activity displayed by the tissue. Tonic activity is defined as the ...
[26]
Role of rho kinase in the functional and dysfunctional tonic smooth ...
Tonic smooth muscles differ from phasic smooth muscles in the ability to spontaneously develop myogenic tone.
[27]
Control of Motility in the Internal Anal Sphincter - PMC
Between these extremes is a range of Em where tone, phasic contractions and SWs are all present. Thus motility in the IAS is highly dependent upon Em. Since ...
[28]
Determinant of Anal Resting Pressure Gradient in Association With ...
Resting pressure gradient of the anal canal depends more on pressure factor than length factor in association with continence function.
[29]
SYMPATHETIC (ADRENERGIC) INNERVATION MODULATES BUT ...
The smooth muscle of the internal anal sphincter (IAS) performs 2 important functions: It maintains tone in the basal state and relaxes in response to the ...
[30]
Autonomic Regulation of the Bladder - Neuroscience - NCBI Bookshelf
When the bladder is full, afferent activity conveying this information centrally increases parasympathetic tone and decreases sympathetic activity, causing the ...
[31]
Action of gastrin on the lower oesophageal sphincter in man - Gut
Abstract. Both hog gastrin and synthetic gastrin stimulate the cardiac sphincter to increase tone and augment the resistance to reflux.
[32]
Estradiol increases urethral tone through the local inhibition of ...
We show here that long-term treatment with high doses of E2 increases the urethral tone and reduces nNOS expression in the urethra. This mechanism potentially ...
[33]
Physiology, Gastrointestinal Nervous Control - StatPearls - NCBI - NIH
The enteric nervous system includes the myenteric plexus, which exhibits control over the longitudinal and circular muscle layers.
[34]
Arterial Baroreceptors - CV Physiology
These receptors respond to stretching of the arterial wall so that if arterial pressure suddenly rises, the walls of these vessels passively expand, which ...
[35]
Physiology, Lower Esophageal Sphincter - StatPearls - NCBI - NIH
Excitatory cholinergic nerves release ACh to promote smooth muscle contraction, which enhances the tonic, myogenic property of the LES and favors contraction.
[36]
Sphincter mechanisms at the lower end of the esophagus - Nature
May 16, 2006 · The LES is composed of smooth muscles, and it maintains tonic contraction owing to myogenic as well as neurogenic factors. It relaxes due to ...
[37]
Urethral sphincters: Attachments, innervation, action | Kenhub
The skeletal muscle fibers of the external urethral sphincter receive somatic motor innervation from the perineal branches of the pudendal nerve (S2-4).
[38]
Sphincter Ani Muscles - Physiopedia
They include the internal anal sphincter and the external anal sphincter muscle, and they both play important roles in maintaining faecal continence. Between ...Introduction · Origin · Action · Assessment<|control11|><|separator|>
[39]
The evolutionary origin of bilaterian smooth and striated myocytes
Dec 1, 2016 · The dichotomy between smooth and striated myocytes is fundamental for bilaterian musculature, but its evolutionary origin is unsolved.
[40]
The evolutionary origin of bilaterian smooth and striated myocytes
The dichotomy between smooth and striated myocytes is fundamental for bilaterian musculature, but its evolutionary origin is unsolved.
[41]
10.8 Development and Regeneration of Muscle Tissue
Satellite cells help to repair skeletal muscle cells. Smooth muscle tissue can regenerate from stem cells called pericytes, whereas dead cardiac muscle tissue ...
[42]
Precapillary sphincters maintain perfusion in the cerebral cortex
Jan 20, 2020 · Here, we reveal the structure and function of brain precapillary sphincters, which may serve to protect capillaries from high blood pressure ...
[43]
Physiology, Gastrointestinal - StatPearls - NCBI Bookshelf - NIH
Apr 8, 2023 · The ileum is the last segment of the small intestine and contains the ileocecal valve, a sphincter that controls the flow of material from the ...
[44]
Functional anatomy and physiology of the upper esophageal sphincter
The physiological role of this sphincter is to protect against reflux of food into the airways as well as prevent entry of air into the digestive tract.
[45]
Anatomy and physiology of the upper esophageal sphincter - PubMed
All 3 muscles may at times function to maintain tone in the UES, but only the CP contracts and relaxes in all physiologic states consistent with the UES. The CP ...
[46]
Upper Esophageal Sphincter - Motor Function of the Pharynx ... - NCBI
It has two major functions: (1) to prevent air from entering into the esophagus during breathing and (2) to prevent reflux of esophageal contents into the ...Missing: anatomy | Show results with:anatomy
[47]
Gastroesophageal reflux disease: From pathophysiology to treatment
Physiologically, the LES is a 3-4-cm-long segment of tonically contracted smooth muscle at the distal end of the esophagus[11]. It is intuitive that the LES ...Missing: tonic | Show results with:tonic
[48]
Regulation of basal tone, relaxation and contraction of the lower ...
Transient lower oesophageal sphincter relaxations are responsible for the postprandial increase in gastro–oesophageal reflux and may play an important role in ...
[49]
Physiology, Stomach - StatPearls - NCBI Bookshelf - NIH
17 de jul. de 2023 · The stomach is a hollow organ that is part of the gastrointestinal system, and it is responsible for functions including the formation of chyme, synthesis of ...
[50]
Advances in the physiology of gastric emptying - PMC
The distal corpus and proximal antrum constitute the peristaltic pump that primarily serves as a mixer. The terminal antrum and the pyloric sphincter form the ...
[51]
Gastrointestinal Hormones and Regulation of Gastric Emptying - PMC
The major effects on gastric emptying result from actions of incretins, particularly GIP, GLP-1 and PYY, and the gastric orexigenic hormone, ghrelin.Missing: sphincter | Show results with:sphincter
[52]
The ileocecal (ileocolonic) sphincter - PubMed - NIH
By preventing reflux of colonic contents the sphincter may serve to minimize colonization of the small bowel by large bowel bacterial flora. The response of the ...
[53]
Anatomy, Abdomen and Pelvis, Small Intestine - StatPearls - NCBI
Feb 18, 2025 · The small intestine is the digestive segment spanning from the gastric pylorus' distal tip to the ileocecal valve, the region connecting the ...
[54]
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Jan 31, 2024 · The ileocecal valve separates the ileum from the large intestine (colon). Hormones and nerves signal the valve to open and close, allowing food ...
[55]
Physiology, Defecation - StatPearls - NCBI Bookshelf
Nov 13, 2023 · Two sphincters control defecation. The internal anal sphincter consists of smooth muscle cells and is under involuntary control. The ...
[56]
Anal sphincter structure and function relationships in aging and fecal ...
Fecal continence is maintained by a complex sphincter system involving three anatomical elements: the smooth muscle internal anal sphincter (IAS), the striated ...
[57]
Achalasia - StatPearls - NCBI Bookshelf - NIH
Jul 31, 2023 · Achalasia is an esophageal smooth muscle motility disorder that occurs due to a failure of relaxation of the lower esophageal sphincter.
[58]
Esophageal Achalasia: Pathophysiology, Clinical Presentation, and ...
Apr 1, 2018 · ... esophageal peristalsis and failure of the lower esophageal sphincter to relax in response to swallowing. These abnormalities lead to ...
[59]
New insights into the pathophysiology of achalasia and implications ...
The pathophysiological underpinnings of this condition are loss of esophageal peristalsis and insufficient relaxation of the lower esophageal sphincter (LES).
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Clinical and Functional Anatomy of the Urethral Sphincter - PMC - NIH
Within the deep perineal pouch, a group of muscle fibers surround the urethra and form the urethral sphincter [5]. Two muscles as urethral sphincters are used ...
[61]
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Sep 15, 2022 · The muscles of the prostate also ensure that the semen is forcefully pressed into the urethra and then expelled outwards during ejaculation. The ...
[62]
[PDF] The Urethral Sphincter Muscle in the Male
ABSTRACT. The male urethral sphincter is a striated muscle in contact with the urethra from the base of the bladder to the perineal membrane. The.
[63]
Extracellular matrix rigidity modulates human cervical smooth ... - NIH
... sphincter whose function is to hold the fetus in the uterus in pregnancy. In addition, this functional smooth muscle body in the upper aspect of the cervix ...
[64]
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Oct 22, 2018 · Females and males differ significantly in gross anatomy and physiology of the lower urinary tract, and these differences are commonly ...
[65]
Anatomy, Abdomen and Pelvis, Pelvic Floor - StatPearls - NCBI - NIH
The pelvic floor is a unique anatomical location where the balance of the different pressures, either visceral, muscular, or liquid play a fundamental role
[66]
The Cardiovascular System: Blood Vessels and Circulation - OERTX
The precapillary sphincters, circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood ...
[67]
Regulation of Vascular Tone and Oxygenation - NCBI - NIH
In addition to the regulation of resistance vessels, the myogenic response may play a role in regulating precapillary sphincter tone and thereby capillary ...
[68]
Anatomy, Head and Neck: Eye Iris Sphincter Muscle - NCBI - NIH
The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction (miosis) and accommodation.[3] ...
[69]
Sphincter Pupillae Muscle - an overview | ScienceDirect Topics
In a cross section of the iris, the sphincter pupillae can be seen as an annular band of smooth muscle (100–170 μm thick; 0.7–1.0 mm wide) encircling the pupil ...
[70]
Pupillary Light Reflex - StatPearls - NCBI Bookshelf - NIH
The pupillary light reflex constricts the pupil in response to light, and pupillary constriction is achieved through the innervation of the iris sphincter ...
[71]
Gastroesophageal reflux disease (GERD) - Symptoms and causes
If the sphincter does not relax as is typical or it weakens, stomach acid can flow back into the esophagus. This constant backwash of acid irritates the lining ...Diagnosis and treatment · GERD surgery · Can certain medications make...
[72]
The Pathogenesis and Management of Achalasia - PubMed Central
Achalasia is a primary esophageal motor disorder of unknown etiology characterized manometrically by insufficient relaxation of the lower esophageal sphincter ...
[73]
Anal Fissures - StatPearls - NCBI Bookshelf
Sep 15, 2025 · In some cases, tears may extend sufficiently to expose the internal or external sphincter muscle. These lesions, combined with sphincter spasms, ...
[74]
Bladder Sphincter Dyssynergia - StatPearls - NCBI Bookshelf - NIH
Nov 12, 2023 · [9] As the bladder fills with urine, sympathetic activation causes the contraction of the internal sphincter and closes the bladder neck. On the ...
[75]
Neurogenic bladder in spinal cord injury patients - PMC - NIH
SCIs at the sacral level result in parasympathetic decentralization of the bladder and denervation of the sphincter. In cases of complete lesion, conscious ...
[76]
Hirschsprung Disease - StatPearls - NCBI Bookshelf
Jun 3, 2023 · In Hirschsprung disease, there is a disruption of the migration process and differentiation of neural crest cells at the level of the enteric ...
[77]
development and pathophysiology of Hirschsprung disease - PMC
Nov 25, 2024 · Hirschsprung disease (HSCR) is a congenital enteric neuropathy in which the enteric nervous system (ENS) fails to develop along variable lengths of the distal ...
[78]
Examining the prevalence of FI and its relationship with age, sex ...
Oct 22, 2024 · FI prevalence was greater for people age 60 and older (9.3%; 95% CI, 6.6% to 12.0%), compared with younger people (4.9%; 95% CI, 2.9% to 6.9%).
[79]
Gastrointestinal Tract Disorders in Older Age - PMC - PubMed Central
The prevalence of GERD among the elderly is about 23% and it was one of the most commonly found conditions among almost 20,000 nursing home residents in the ...
[80]
Esophageal Manometry | Johns Hopkins Medicine
An esophageal manometry is a diagnostic test that examines the coordinated muscle movement of the esophagus.Missing: electromyography | Show results with:electromyography
[81]
The Clinical Utility of Anorectal Manometry: A Review of Current ...
Anorectal manometry (ARM) is a diagnostic test that utilizes pressure sensors to dynamically measure intraluminal anal and rectal pressures, thus providing ...
[82]
Gastrointestinal Manometry - Medical Clinical Policy Bulletins - Aetna
Anorectal manometry uses a pressure sensitive tube to check the sensitivity and function of the rectum. It also measures the ability of the anal sphincter ...<|control11|><|separator|>
[83]
Ultrasound imaging of the anal sphincter complex: a review - PMC
Endoanal ultrasound is now regarded as the gold standard for evaluating anal sphincter pathology in the investigation of anal incontinence.Missing: fluoroscopy | Show results with:fluoroscopy
[84]
MRI Defecography Test: Purpose, Procedure & What to Expect
Through detailed fluoroscopic X-ray or magnetic resonance imaging (MRI), healthcare providers can see how different muscles and organs are moving when you poop.
[85]
Imaging Pelvic Floor Disorders: Trend Toward Comprehensive MRI
Endoanal ultrasound can be used to evaluate the integrity of the internal anal sphincter (IAS) and the EAS muscles in patients with fecal incontinence. The ...
[86]
Anal endosonography for identifying external sphincter defects ...
Defects of the external anal sphincter have traditionally been diagnosed by palpation, anal manometry and electromyography (EMG), but anal endosonography ...Missing: function endoscopy
[87]
Anal Sphincter Electromyography Testing - Aurora Health Care
Anal sphincter electromyography (EMG) and anorectal manometry are simple outpatient tests to assess whether the anal muscle (sphincter) and nerves are working ...Missing: endoscopy imaging
[88]
Botulinum toxin type A injection combined with biofeedback in the ...
Relevant studies have confirmed that injection of botulinum toxin can reduce the pressure of the anal sphincter while avoiding permanent damage to the anal ...
[89]
Treatment of Gastrointestinal Sphincters Spasms with Botulinum ...
In group A, 30 patients underwent pyloroplasty, and in group B injection of 200 BT units into the pyloric sphincter muscle was used in 30 patients. An isotope- ...
[90]
advances in the diagnosis and management of anorectal motility ...
Jul 25, 2024 · The goals of biofeedback therapy are threefold: to enhance anal sphincter muscle strength, improve coordination among abdominal, gluteal and ...
[91]
Neurogenic bladder and bowel management - Mayo Clinic
Apr 29, 2022 · There are several different surgeries that could be performed, including creating an artificial sphincter, bypassing the bladder, increasing ...
[92]
Sacral Neuromodulation - StatPearls - NCBI Bookshelf - NIH
Apr 18, 2024 · Sacral neuromodulation is a well-established, safe, effective, and minimally invasive advanced therapy for managing various lower urinary tract and bowel ...
[93]
Stem cell therapy combined with controlled release of growth factors ...
Mar 16, 2023 · Cell- or growth factor-based therapy is a promising approach for neuromuscular regeneration and the recovery of sphincter function.
[94]
Evaluating Cell Therapy for Chronic Fecal Incontinence Treatment
May 14, 2025 · For DigniFI study participants, the hope is that these injected cells will engraft into damaged or weakened sphincter tissue and subsequently ...
[95]
Wexner Score - an overview | ScienceDirect Topics
The Wexner score, also termed the Cleveland Clinic Fecal Incontinence Severity Scoring System (CCIS) is a fecal incontinence score from 0-20; where 0 is perfect ...
[96]
New objective scoring system to clinically assess fecal incontinence
Several scoring systems are used to assess fecal incontinence (FI), among which, the most commonly used are Wexner and Vaizey's scoring systems.