Sulkijalihasten

Sulkijalihasten are a group of circular muscles that surround and regulate the openings or passages of several tubular structures in the body. They maintain a baseline tone to prevent leakage and relax to permit the passage of contents. Sphincters can be composed of smooth muscle (involuntary) or skeletal muscle (voluntary), and many are controlled by autonomic nerves, with some also under somatic control.

Digestive system: The esophagus contains the upper esophageal sphincter (UES), largely skeletal muscle, and the lower esophageal sphincter (LES), smooth muscle, which prevent reflux. The pyloric sphincter at the distal stomach controls gastric emptying into the duodenum. The ileocecal sphincter lies at the junction of ileum and cecum and regulates flow between small and large intestine. The internal anal sphincter is smooth muscle; the external anal sphincter is skeletal muscle and under voluntary control; together they regulate defecation.

Other sphincters include the sphincter of Oddi (controls bile and pancreatic juice flow into the duodenum) and the internal urethral sphincter (urethral neck, smooth muscle) with the external urethral sphincter (skeletal muscle) providing additional control of urination. The pupil sphincter (sphincter pupillae) of the iris constricts the pupil in response to light or parasympathetic stimulation.

Control and clinical relevance: Autonomic innervation from parasympathetic and sympathetic fibers regulate smooth-sphincters, while somatic pathways via the pudendal nerve regulate the external anal and external urethral sphincters. Sphincter dysfunction can cause reflux, dysphagia, fecal or urinary incontinence, or obstructive symptoms. Treatments include pharmacologic modulation, surgical repair, or biofeedback-based therapy.