The sphincter is a circular muscle that constricts a passage or closes a natural orifice. When relaxed, a sphincter allows materials to pass through the opening. When contracted, it closes the opening.
There are four main sphincter muscles along the alimentary canal that aid in digestion: The cardiac sphincter, between the esophagus and the stomach, opens at the approach of food, which is then swept into the stomach by rhythmic peristaltic waves.
The pyloric sphincter controls the opening from the stomach into the duodenum. It is usually closed, opening only for a moment when a peristaltic wave passes over it. Two anal sphincters, internal and external, control the anus, allowing the evacuation of feces.
In addition, there are sphincters in the iris of the eye, the bile duct (sphincter of Oddi), the urinary tract, and elsewhere in the body. .
THE URINARY PROCESS. Urination is a complex process controlled by muscles of the bladder and sphincter mechanism and by modulatory centers in the central and peripheral nervous systems. The detrusor muscle is a complex meshwork of interlaced smooth muscle bundles that contract in a way that squeezes urine from the bladder vesicle.
The sphincter mechanism consists of smooth muscle in the bladder neck and proximal urethra, a striated muscle sphincter consisting of slow twitch muscle fibers in the urethral wall, and periurethral muscles that are a component of the pelvic floor muscles. The detrusor relaxes during bladder filling to accommodate increasing volumes at a low pressure, and the sphincter remains tightly closed. During urination, the detrusor contracts and the sphincter mechanism relaxes, allowing smooth outflow of urine.
As the bladder fills, modulatory centers in the brain prevent the occurrence of contractions. Meanwhile, the sphincter mechanism remains closed under autonomic and somatic nervous system control. During urination, the inhibition of contractions is removed and a reflex originates in the pontine micturition center. That causes the detrusor muscle to contract and the sphincter mechanism to relax.
The location of the final output of central nervous system impulses to and from the bladder and sphincter is the sacral micturition center, located in spinal segments 2, 3, and 4, which must be intact for contraction to occur. Injury to the pontine micturition center will compromise the smooth coordination between sphincter mechanism and detrusor muscle, and injury to the brain will affect the volitional control of urine (bladder stability). See also "incontinence".