anal sphincter

Examples of anal 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 rectum now contracts and shortens in peristaltic waves, thus forcing fecal material out of the rectum and out through the anal canal.
  • 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.

• Episiotomy

  • This area forms the floor of the pelvis and contains the external sex organs and the anal opening.
  • It can be further divided into the urogenital triangle in front and the anal triangle in back.
  • Tears can involve the perineal skin or extend to the muscles and the anal sphincter and anus.

• Absorption and Feces Formation in the Large Intestine

  • The exit of this waste material is regulated by the anal sphincter.

• Overview of the Spinal Cord

  • The nerves that compose the cauda equina supply the pelvic organs and lower limbs, including motor innervation for the hips, knees, ankles, feet, and internal and external anal sphincters.

• 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, causing it to relax and allow the flow of urine into the urethra.
  • The internal urethral sphincter is involuntary and controlled by autonomic nerves.

• Micturition and the Micturition Reflex

  • 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 remains tense and the detrusor muscle 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 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.

• 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.

• Perineum

  • Ischioanal fossa – a fat-filled space at the lateral sides of the anal canal.
  • It is bounded laterally by obturator internus muscle, medially by pelvic diaphragm and the anal canal.

• 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.