Sphincters! Could they be the most interesting muscles?

Let’s face it, sphincters can be quite amusing!  Sphincters look pretty funny, but when they open and close, forget about it!  Sphincters can be quite useful, as well.  They act like doors helping to control the passage of things from one region to another.  You might be asking yourself, “What are the sphincters of the human body?”  Probably the most famous sphincter is the anal sphincter.  Did you know it’s actually made of 2 sphincters consisting of 2 different types of muscle tissue?  There’s an internal anal sphincter which is made of smooth muscle, so it’s involuntarily controlled.  That’s right!  It can open and close whenever it wants to!  Luckily for us, we have an external anal sphincter which is made of skeletal muscle.  Skeletal muscle is voluntarily controlled which means we can control the closure of this muscle, giving us the ability to wait to release the contents of our bowels when the time and place are appropriate.  The following is a list of other sphincters in the human body you can impress your friends with the next time the opportunity presents itself:

  • Preputial sphincter or Peripenic muscle: (smooth) functions to contract penile foreskin around the head of the penis.
  • Internal urethral sphincter: (smooth) part of the urinary bladder. It controls the passage of urine from the bladder into the urethra.
  • External urethral sphincter: (skeletal) found in the musculature that makes up the floor of the pelvis [urogenital diaphragm]. It allows us to control when and where to release the urine from our body.
  • Orbicularis oris: (skeletal) allows us to close the opening of our mouth. Upon closer look, the muscle fibers are not all organized in a circular fashion.  The classification of this as a sphincter is debatable.
  • Orbicularis oculi: (skeletal) the circular muscle that closes and squints the eye it encircles.
  • Sphincter pupillae or Pupillary sphincter or Pupillary constrictor: (smooth) circular muscle in the iris that encircle the pupil causing constriction upon contraction.
  • Ciliary muscles: (smooth) they suspend the lens behind the pupil. Their contraction and relaxation will change the shape of the lens creating the “fine focus” function in the eye.
  • Upper esophageal sphincters or Inferior pharyngeal sphincter: (smooth) controls passage of substances from the pharynx into the esophagus.
  • Lower esophageal sphincter or Cardiac sphincter: (smooth) located just before the esophagus enters the stomach.
  • Pyloric valve or sphincter: (smooth) helps to control the release of contents from the stomach into the duodenum.
  • The sphincter of Oddi in the Hepatopancreatic ampulla: (smooth) helps to control bile and pancreatic juices’ entrance into the duodenum.
  • Accessory pancreatic sphincter or Sphincter of Helly: (smooth) sometimes present in the accessory pancreatic ampulla, it helps to control passage of pancreatic secretions from the accessory pancreatic duct into the duodenum.
  • Ileocecal valve or sphincter: (smooth) located where the ileum of the small intestine meets the large intestine at the region known as the cecum.
  • Precapillary sphincters: (smooth) located at the entrance of capillary beds [mainly the mesenteric circulation] controlling blood flow into them.

Probably more information you ever wanted to know about sphincters, but there it is.  Did I remember all of them?  If not, feel free to add to the list.  Cheers!

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