Dominic Stone
The external anal sphincter is a group of muscles that surround the anal canal and control the release of stool from the rectum. It is made up of striated, voluntary muscle fibres that extend from the puborectalis and levator ani muscles in the pelvic floor down to the perineal skin at the anal verge. The external anal sphincter is innervated by somatic nerves, specifically the inferior rectal nerves, which are derived from the pudendal nerve. These nerves pass through the ischioanal fossa to reach the muscle and play a crucial role in maintaining faecal continence. The external anal sphincter works in conjunction with the internal anal sphincter to regulate defecation and ensure the proper functioning of the anal canal.
| Characteristics | Values |
|---|---|
| Type of muscle | Skeletal |
| Type of innervation | Somatic |
| Innervated by | Motor branches of the pudendal nerve (S2-S4) |
| Location | Distal to the internal urethral sphincter, in the deep perineal pouch |
| Function | Controls the flow of urine |
| Control | Voluntary |
Explore related products
What You'll Learn
- The external anal sphincter is innervated by the pudendal nerve
- The somatic nervous system controls the external urethral sphincter
- The internal urethral sphincter is supplied by the inferior vesical artery
- The external urethral sphincter receives blood from the internal pudendal artery
- Pelvic floor exercises can treat anismus, a disorder of the external anal sphincter
The external anal sphincter is innervated by the pudendal nerve
The external anal sphincter is a cylindrical sheet of striated, voluntary muscle that extends from the puborectalis and levator ani muscles in the pelvic floor down to the perineal skin at the anal verge. The external anal sphincter is conventionally described as consisting of three parts: a subcutaneous part, a superficial part, and a deep part. It is under voluntary control and receives somatic innervation from the motor branches of the pudendal nerve (S2-S4), also referred to as the inferior rectal nerves. These nerves pass through the ischioanal fossa to reach the muscle.
The internal anal sphincter, on the other hand, is the distal extension of the inner circular muscle layer in the rectal wall. It is approximately 3-4 cm long and 4-5 mm thick in adults. Its motor innervation is derived from the autonomic nervous system, specifically the inferior pelvic plexus. The internal anal sphincter is not innervated by the pudendal nerve, which is responsible for providing motor and sensory innervation to the external anal sphincter.
The external anal sphincter is important for maintaining faecal continence and controlling the release of stool from the rectum. It works in conjunction with the internal anal sphincter to occlude the anal aperture and aid in the expulsion of faeces. The internal anal sphincter is normally in a state of continuous maximal contraction to prevent leakage, while the external anal sphincter relaxes voluntarily to allow defecation.
The pudendal nerve plays a crucial role in maintaining the function of the external anal sphincter. When stimulated, the pudendal nerve increases the resting tone in the sphincter, contracting the skeletal muscle and preventing the release of stool. Inhibition of the pudendal nerve results in relaxation of the external anal sphincter, facilitating defecation.
Hand Muscles: The Intricate Flexors and Extensors
You may want to see also
Explore related products
The somatic nervous system controls the external urethral sphincter
The external urethral sphincter is a complex of muscles that encircle the urethra and control the flow of urine. The urethral sphincter complex receives both somatic and autonomic innervation, which supply its voluntary and involuntary components, respectively. The external urethral sphincter is under voluntary control, which is enabled by its skeletal muscle composition. This is in contrast to the internal urethral sphincter, which is made of smooth muscle and is under involuntary control.
The skeletal muscle fibres of the external urethral sphincter receive somatic motor innervation from the perineal branches of the pudendal nerve (S2-4). These nerves stem from neurons found in an area of the spinal cord called Onuf's nucleus, which in turn receives fibres from the pontine micturition centre of the central nervous system. When stimulated, Onuf's nucleus acts to increase the resting tone in the external urethral sphincter, contracting the skeletal muscle and stopping or slowing urination. Conversely, central nervous system inhibition of Onuf's nucleus results in relaxation of the external urethral sphincter, facilitating voiding.
In females, the external urethral sphincter is more elaborate, consisting of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibres wrap around the vagina and urethra, and their contraction leads to the constriction of both organs. The origin of the compressor urethrae muscle is the right and left inferior pubic ramus, and it wraps anteriorly around the urethra, squeezing it against the vagina when contracted.
The external urethral sphincter is located distal to the internal urethral sphincter, in the deep perineal pouch. Its skeletal muscle fibres arise from the ischial rami, inferior pubic rami, and adjacent fascia, extending to surround the middle-lower, membranous part of the urethra. The external urethral sphincter is a cylindrical sheet of striated, voluntary muscle extending from the puborectalis and levator ani muscles in the pelvic floor down to the perineal skin at the anal verge.
Relaxing Eyebrow Muscles: Self-Massage Techniques for Stress Relief
You may want to see also
Explore related products
The internal urethral sphincter is supplied by the inferior vesical artery
The urethral sphincter is a complex of muscles that encircle the urethra and control the flow of urine. The urethral sphincter can be divided into two parts: an internal urethral sphincter and an external urethral sphincter. The internal urethral sphincter is located at the junction of the urethra and the urinary bladder. It is made of smooth muscle and is under involuntary control. The external urethral sphincter, on the other hand, is located inferiorly and surrounds the membranous or intermediate part of the urethra. It is formed from skeletal muscle and is under voluntary control.
The internal urethral sphincter is a bundle of smooth muscle fibres situated around the proximal end of the urethra, at the neck of the bladder. These muscle fibres are continuous with the detrusor vesicae muscle of the bladder. The smooth muscle fibres of the internal urethral sphincter receive both sympathetic and parasympathetic innervation. Sympathetic supply arises from the lower thoracic and upper lumbar (T11 - L2) segments of the spinal cord, which maintain tonic contraction of the sphincter and prevent urine outflow from the bladder into the urethra. Parasympathetic supply, on the other hand, arises from sacral levels S2 - S4 (the spinal micturition centre). It acts to inhibit the internal sphincter muscle, causing it to relax and allow urine to pass from the bladder into the urethra.
The external urethral sphincter, in contrast, receives blood supply from the internal pudendal artery in females and the artery of the bulb of the penis (bulbourethral artery) in males. The skeletal muscle fibres of the external urethral sphincter receive somatic motor innervation from the perineal branches of the pudendal nerve (S2-4). These nerves originate from neurons in an area of the spinal cord called Onuf's nucleus, which receives fibres from the pontine micturition centre of the central nervous system. Stimulation of Onuf's nucleus increases the resting tone in the external urethral sphincter, contracting the skeletal muscle and stopping or slowing urination. Inhibition of Onuf's nucleus by the central nervous system results in relaxation of the external urethral sphincter, facilitating voiding.
Walking: Muscles Required and How They Work Together
You may want to see also
Explore related products
The external urethral sphincter receives blood from the internal pudendal artery
The urethral sphincter is a complex of muscles that encircle the urethra and control the flow of urine. It is divided into two parts: the internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter is located at the junction of the urethra and the urinary bladder and is made of smooth muscle. The external urethral sphincter, on the other hand, is located inferiorly and surrounds the membranous or intermediate part of the urethra. It is primarily composed of striated skeletal muscle and is under voluntary control.
The external urethral sphincter receives its blood supply from the internal pudendal artery, which is a branch of the anterior division of the internal iliac artery. The internal pudendal artery is the major supplier of blood to the perineal region, providing blood to the perineum, skin of the anal region, skin of the urogenital triangle, and muscles in the anal and urogenital regions. It is also involved in supplying blood to the erectile bodies of the external genitals.
In females, the internal pudendal artery supplies blood to the external urethral sphincter, while in males, the artery of the bulb of the penis (bulbourethral artery) is responsible for blood supply. The internal pudendal artery is larger in men compared to women, and it plays a crucial role in pelvic health and function.
The external urethral sphincter is innervated by the perineal branches of the pudendal nerve (S2-4). These nerves originate from neurons in an area of the spinal cord called Onuf's nucleus, which is part of the central nervous system. When stimulated, Onuf's nucleus increases the resting tone in the external urethral sphincter, leading to skeletal muscle contraction and the inhibition of urination. Conversely, when Onuf's nucleus is inhibited by the central nervous system, the external urethral sphincter relaxes, facilitating the voiding process.
Massaging Your SCM Muscle: Techniques for Relief
You may want to see also
Explore related products
Pelvic floor exercises can treat anismus, a disorder of the external anal sphincter
The external anal sphincter is a ring-like muscle that surrounds the anal canal and regulates defecation. It is under voluntary control, receiving somatic innervation from the motor branches of the pudendal nerve (S2-S4). Pelvic floor dysfunction, such as anismus, can occur when the pelvic floor muscles and nerves fail to coordinate correctly during bowel movements. Anismus, also known as dyssynergic defecation, is characterised by the failure of the pelvic floor muscles to relax, resulting in obstructed defecation and constipation.
Pelvic floor exercises, also known as pelvic floor muscle training, can be an effective treatment for anismus. These exercises aim to strengthen the contraction force of the pelvic floor muscles, including the external anal sphincter, and improve rectal sensation and bowel function. Pelvic floor muscle training is a component of pelvic floor physical therapy (PFPT), which is a non-invasive treatment approach for various pelvic floor dysfunctions. PFPT may also include biofeedback, bowel habit guidance, electrical stimulation, and lifestyle modifications.
Biofeedback therapy has been highlighted as the most effective treatment for anismus. This involves the use of surface electromyography (EMG) to provide visual or auditory feedback on the activity of the pelvic floor muscles. By monitoring the severity of muscle spasms or hypertonicity, therapists can guide patients in learning to relax the pelvic floor muscles, including the external anal sphincter, during defecation.
In addition to conservative management with pelvic floor exercises, other treatment options for anismus may be considered. Manual techniques, soft tissue mobilisation, stretching exercises, visceral manipulation, neuromuscular re-education, and therapeutic exercises can be incorporated into a comprehensive treatment program. In certain cases, surgical intervention may be recommended if conservative measures are ineffective.
Overall, pelvic floor exercises targeting the external anal sphincter can be a valuable component of treatment for anismus. These exercises aim to improve the coordination and function of the pelvic floor muscles, promoting normal bowel movements and relieving the symptoms associated with anismus.
Sharks' Muscles: Shivering or Static?
You may want to see also
Frequently asked questions
The external sphincter muscle is a group of muscles that surround the anus and control the release of stool from the rectum. It is also known as the anal sphincter muscle.
The external anal sphincter muscle receives somatic innervation from the motor branches of the pudendal nerve (S2-S4).
The external sphincter muscle, along with the internal sphincter muscle, plays an important role in maintaining faecal continence.
Damage to the external sphincter muscle can lead to faecal incontinence, which is the involuntary release of stool from the rectum. This condition can be distressing and may require surgery to fix the sphincter. This can also be treated with pelvic floor exercises and electrical stimulation. In the case of the urethral sphincter, damage can lead to urinary incontinence.
