sphincter

Examples of sphincter in the following topics:

• Defecation Reflex

  • A sufficient increase in fecal material in the rectum causes stretch receptors from the nervous system, located in the rectal walls, to trigger the contraction of rectal muscles, relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external sphincter .
  • The relaxation of the internal anal sphincter causes a signal to be sent to the brain indicating an urge to defecate.
  • The perineal wall is lowered, causing the anorectal angle to decrease from 90 degrees to less than 15 degrees (almost straight), and the external anal sphincter relaxes.
  • The internal and external anal sphincters, along with the puborectalis muscle, allow the feces to be passed by pulling the anus up and over the exiting feces in shortening and contracting actions.

• Urinary Bladder

  • There are two sphincters, or muscular valves, that separate the bladder from the urethra.
  • The sphincters must open before the urine can flow into the urethra.
  • The internal sphincter is under involuntary control and the external sphincter is under voluntary control.
  • When the bladder fills with urine stretch receptors send nerve impulses to the spinal cord, which then sends a reflex nerve impulse back to the internal sphincter valve at the neck of the bladder that causes it to relax and allow the flow of urine into the urethra.
  • The internal urethral sphincter is involuntary and controlled by the autonomic nerves.

• Micturition and the Micturition Reflex

  • The voiding phase: A contracted bladder that forces the external sphincter open and discharges urine through the urethra.
  • The muscles controlling micturition are controlled by the autonomic and somatic nervous systems, which open the two sphincters during the voiding phase of micturition.
  • During the storage phase the internal urethral sphincter is tense and the detrusor muscle is relaxed by sympathetic stimulation.
  • During the voiding phase of micturition, parasympathetic stimulation causes the internal urethral sphincter to relax.
  • The external urethral sphincter (sphincter urethrae) is under somatic control and is consciously relaxed (and thus opened) during micturition.

• Urethra

  • The external urethral sphincter is a striated muscle that allows voluntary control over urination by controlling the flow of urine from the bladder into the urethra.
  • The urethral sphincter  separates the bladder from the urethra.
  • Somatic (conscious) innervation of the external urethral sphincter is supplied by the pudendal nerve, which allows the sphincter to open and close.

• How Skeletal Muscles Are Named

  • The anatomical arrangement of skeletal muscle fascicles can be described as parallel, convergent, pennate, or sphincter.
  • The fibers of the circular or sphincter muscles are arranged concentrically around an opening or recess.

• Blood Flow Through the Body

  • The blood entering some capillary beds is controlled by small muscles called precapillary sphincters .
  • A sphincter is a ringlike band of muscle that surrounds a bodily opening, constricting and relaxing as required for normal physiological functioning.
  • If the precapillary sphincters are open, the blood will flow into the associated branches of the capillary bed.
  • If all of the sphincters are closed, then the blood will flow directly from the arteriole to the venule through the thoroughfare channel.
  • (a) Precapillary sphincters are rings of smooth muscle that regulate the flow of blood through capillaries; they help control the blood flow to where it is needed.

• Gross Anatomy of the Stomach

  • Two smooth muscle valves, or sphincters, keep the contents of the stomach contained:
  • Pyloric sphincter or pyloric orifice, dividing the stomach from the small intestine.

• Overview of Urine Transport, Storage, and Elimination

  • Circular muscles called sphincters help keep urine from leaking.
  • The sphincter muscles close tightly, like a rubber band, around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body.
  • At the same time, the brain signals the sphincter muscles to relax.

• Digestive System: Small and Large Intestines

  • The "C-shaped," fixed part of the small intestine, the duodenum, is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum where it mixes with pancreatic juices.
  • Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary, while the outer sphincter is voluntary.

• Autonomic Interactions