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This is the primary muscle for maintaining continence of urine, a function shared with the external urethral sphincter which is under voluntary control. It prevents urine leakage as the muscle is tonically contracted via sympathetic fibers traveling through the inferior hypogastric plexus and vesical nervous plexus. [4]
It is a secondary sphincter to control the flow of urine through the urethra. Unlike the internal sphincter muscle, the external sphincter is made of skeletal muscle, therefore it is under voluntary control of the somatic nervous system. [1]
The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
At the anus, there are two anal sphincters which control the exit of feces from the body, the internal anal sphincter and external anal sphincter. The inner sphincter is involuntary and the outer is voluntary. The microscopic precapillary sphincters function to control the blood flow into each capillary in response to local metabolic activity. [1]
The muscular systems in vertebrates are controlled through the nervous system although some muscles (such as the cardiac muscle) can be completely autonomous. Together with the skeletal system in the human, it forms the musculoskeletal system , which is responsible for the movement of the body .
The somatic nervous system (SNS), also known as voluntary nervous system, is a part of the peripheral nervous system (PNS) that links brain and spinal cord to skeletal muscles under conscious control, as well as to sensory receptors in the skin. [1] [2] The other part complementary to the somatic nervous system is the autonomic nervous system ...
Smooth muscle is grouped into two types: single-unit smooth muscle, also known as visceral smooth muscle, and multiunit smooth muscle. Most smooth muscle is of the single-unit type, and is found in the walls of most internal organs (viscera); and lines blood vessels (except large elastic arteries), the urinary tract , and the digestive tract .
During the storage phase, the internal urethral sphincter remains tense and the detrusor muscle relaxed by sympathetic stimulation. During micturition, parasympathetic stimulation causes the detrusor muscle to contract and the internal urethral sphincter to relax. The external urethral sphincter (sphincter urethrae) is under somatic control and ...