Sphincters

SPHINCTERS (from Greek sphingo-I squeeze, draw together), a system of muscular ring fibers located around any opening, closing or narrowing it upon contraction. In the digestive and urogenital systems, they serve as regulators for the passage of various contents into subsequent sections. The sphincter of the pupil regulates the amount of light falling on the retina. By structure, sphincters are divided into voluntary, consisting of striated muscles, and involuntary—of smooth musculature. The latter include: the sphincter of the pupil; the cardial sphincter at the border of the stomach and esophagus; the pyloric sphincter between the duodenum and stomach; s. ileo-caecalis at the transition from the small intestine to the cecum; two internal sphincters of the rectum and the sphincter of Oddi, surrounding the mouth of the common bile duct. Voluntary sphincters include: s. ani externus- of the anal opening and s. urethrae - of the urethra. The question of sphincter innervation proves to be more complex than previously thought. Clinical practice does not always confirm the strict division of sphincter innervation from sympathetic and parasympathetic systems, since sometimes in case of damage to one of these systems, compensatory action of the other is observed; for example, when the n. hypogastrici (sympathetic) is damaged, compensatory action of the parasympathetic nervous system occurs, etc. In vertebrates, the development of sphincters coincides with the beginning of differentiation of one or another system both in structure and function. In the digestive system, for example, sphincters begin to differentiate when the system is clearly divided into sections. In the urogenital system of higher vertebrates, the following stages of sphincter development are outlined. In amphibians, both urogenital and anal openings are covered by one common sphincter, while in reptiles both systems already have separate sphincters. The same cycle is observed in humans during his ontogenetic development. In a 2-month embryo, there is a sphincter of the cloaca, into which both the urogenital and intestinal systems open. At the 3rd month, there is already a clear division of the common cloacal sphincter into the muscles of the urogenital sinus and the sphincter of the anal opening.