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External Anal Sphincter Innervation
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The external anal sphincter is a short tube of skeletal muscle surrounding the inferior portion of the anal canal. It is one of the three structures in the anal sphincter complex which controls defecation. While the internal anal sphincter and the conjoint longitudinal muscle are considered parts of the anal canal wall, the external anal sphincter is actually a part of the pelvic floor (perineal) muscles.
The external anal sphincter is largely under voluntary control by the somatic nervous system, which allows it to stay in a contracted state. The muscle is voluntarily relaxed during defecation to allow the passage of feces.
Some fibres from the deep part of the external anal sphincter blend with the puboanalis muscle. In the anterior part of the same region, some fibres cross over the midline to blend with the superficial transverse perineal muscle of the opposite side. Posteriorly, this part inserts into the anococcygeal ligament.
The external anal sphincter is located at the intersection of many intricate muscle fibers and many anatomical spaces are related to it. The deep part forms a tube around the higher portion of the internal anal sphincter, while the superficial part surrounds the lower portion of the internal anal sphincter. The subcutaneous part surrounds the anal verge, with its most inferior fibres extending past the internal anal sphincter. Its position results in the typical creases seen externally around the anus.
The external anal sphincter is contained in the anal triangle, together with the anococcygeal ligament and the inferior anal nerve. It forms part of the posteromedial wall of the ischioanal fossa.
As mentioned before, the external anal sphincter relates to some muscles of the perineum. Its fibers partially blend with the puboanalis muscle, which is part of the pelvic diaphragm, and the superficial transverse perineal muscle, which is a perineal muscle. The layer of the pelvic diaphragm covering the inferior aspect of the levator ani also extends over the external anal sphincter.
The external anal sphincter is also related to the conjoint longitudinal muscle, the latter lying between the former and the internal anal sphincter. The conjoint longitudinal muscle sends fibres through the external anal sphincter. The inferior fibres go through the lower subcutaneous part of the lower subcutaneous part of the external anal sphincter, while the outward facing fibres go through the deep part.
There is a small potential space between the external anal sphincter and the conjoint longitudinal muscle known as the intersphincteric space. This space contains anal glands, and may need to be surgically accessed if the ducts of these glands become blocked.
The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (S2-S4). As a result, this muscle is under voluntary control.
Inferior rectal arteries follow the inferior rectal nerves and carry blood to the external anal sphincter. They are terminal branches of the internal pudendal arteries, which stem from the internal iliac artery.
The external anal sphincter provides voluntary control for defecation. As the rectum gradually fills with feces at rest and after meals, the continuous tonic contraction of the external anal sphincter helps to prevent defecation. When the urge to defecate passes a certain threshold and the situation is appropriate, the external anal sphincter is voluntarily relaxed, permitting the feces to exit through the anal canal. The function of the external anal sphincter is aided by the sling-like puboanalis muscle, which is one of the levator ani muscles. Together with the perineal muscles, the external anal sphincter helps to support the pelvic floor.
The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.
The study was performed using 45 pelvic half section specimens (41 fetal ones and four adults). The macroscopic dissection followed the nerve branches from their spinal roots up to the external anal sphincter. Three nerve branches were found: the anterior ramus arising from the external perineal nerve, the inferior rectal nerve and an independent posterior branch. The anterior and the inferior rectal nerve branches always emerged from the pudendal plexus. The posterior branch arising either from S4 or from the inferior rectal nerve was only found in (31%) of our cases. Five anatomical distributions are described, percentages of every type notified. The fibre content of these nerve bundle branches was evaluated through histological sections using Heidenhain's azan stain and Luxol fast blue. The branches consisted of 2,896 to 2,137 fibres, 20% of them being unmyelinated and 80% containing various myelinated fibres. The nomenclature of these nerve branches has to be debated. The terms of anterior, middle and posterior anal nerves seem more suitable.
Purpose: Fecal incontinence is a common problem after anal sphincter-preserving operations. The intersphincteric autonomic nerves supplying the internal anal sphincter (IAS) are formed by the union of: (1) nerve fibers from Auerbach's nerve plexus of the most distal part of the rectum and (2) the inferior rectal branches of the pelvic plexus (IRB-PX) running along the conjoint longitudinal muscle coat. The aim of the present study is to identify the detailed morphology of nerves to the IAS.
Inferior to the pectinate line, the anal canal is lined by non-keratinised stratified squamous epithelium (known as the anal pecten). It is a pale and smooth surface, which transitions at the level of the intersphincteric groove to true skin (keratinised stratified squamous).
At the junction of the rectum and the anal canal, there is a muscular ring - known as the anorectal ring. It is formed by the fusion of the internal anal sphincter, external anal sphincter and puborectalis muscle, and is palpable on digital rectal examination.
Neurovascular Supply and LymphaticsAs discussed above, the pectinate line divides the anal canal into two parts - which have a different arterial supply, venous drainage, innervation and lymphatic drainage.
processing.... Drugs & Diseases > Clinical Procedures Anal Sphincter Electromyography and Sphincter Function Profiles Updated: Aug 06, 2019 Author: Jasvinder Chawla, MD, MBA; Chief Editor: David C Spencer, MD more... Share Email Print Feedback Close Facebook Twitter LinkedIn WhatsApp webmd.ads2.defineAd({id: 'ads-pos-421-sfp',pos: 421}); Sections Anal Sphincter Electromyography and Sphincter Function Profiles Sections Anal Sphincter Electromyography and Sphincter Function Profiles Overview Background Indications Contraindications Technical Considerations Anatomy Show All Periprocedural Care Patient Education & Consent Equipment Patient Preparation Monitoring & Follow-up Show All Technique Approach Considerations Anal Sphincter Electromyography Procedure Show All Questions & Answers Media Gallery References Overview Background Anal sphincter electromyography may be clinically useful in the evaluation of patients with urinary, bowel, and sexual dysfunction. It should be considered for all patients with a history and clinical examination suggestive of a central or peripheral sacral neuropathic lesion. The external anal sphincter (EAS), innervated by the pudendal nerve, is the best muscle for detection of neuropathic lesions in lower sacral myotomes. [1, 2] Its circular superficial location and muscle bulk allows for easier access and less painful needle insertion in order to diagnose particularly proximal sacral nervous system disorders. [1, 3]
Surface recordings from the sphincter have shown increased activity with body actions and decreased activity in sleep. Although surface EMG has been studied, needle EMG is clearly superior. [4] This article describes the clinical utility, commonly used technique, and role of EMG in various neurological disorders associated with anal sphincter abnormalities.
Quantitative EMG of the external anal sphincter (EAS) is highly useful in the diagnosis of patients with suspected neuropathic sacral lesions. [7] It is useful for the confirmation or exclusion of cauda equina or conus medullaris lesion in the context of appropriate clinical and other laboratory findings. [7] However, no single diagnostic criterion has both satisfactory sensitivity and satisfactory specificity. [6] Combined with neurophysiologic measurement of sacral reflexes, it is highly sensitive (94-96%) for diagnosing chronic cauda equina or conus medullaris lesions.
Quantitative anal sphincter EMG is likely of greater value in women, for whom sacral reflex testing is less useful. [8] EAS EMG has also been shown to be abnormal, with evidence of denervation or reinnervation, in postpartum women with fecal incontinence. EMG could possibly be used to identify those at risk for pelvic floor disorders. [9]
Anal sphincter EMG has also proven useful in earlier detection of pudendal neuropathies and even possibly for preclinical markers for future development of pelvic floor disorders. Clouds analysis may be particularly helpful for evaluating the pelvic floor, as it can be used irrespective of the force of muscle contraction. This is particularly important for tonically contracting pelvic floor muscles which, like most facial muscles, do not move bones through a measurable range of motion. [10] 153554b96e
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