sphincter
Meaning of sphincter: A ring of muscle whose resting state is closed, opening only when the body signals that something should pass.
Definition of sphincter
A sphincter is a ring of muscle encircling a tube or opening in the body, held contracted to keep the passage shut and relaxing to let contents through. Some work automatically and cannot be controlled at will, while others answer to voluntary command. More than a dozen exist, from the ring around the pupil of the eye to those guarding the stomach, bowel and bladder.
How does a sphincter work?
The opposite way round from most muscles. A muscle in the arm rests loose and tightens to do its job, while a sphincter rests tight and its job is to let go at the right moment. Holding that closure takes a constant low-level nerve signal, and opening requires an active instruction to relax as material arrives. That arrangement explains the two ways they fail. One is failure to relax on cue, which blocks the passage. The other is loss of the resting tone, which lets contents through when they should be held.
Where are the main ones?
| Sphincter | Location | Control |
|---|---|---|
| Upper esophageal | Top of the gullet, behind the voice box | Reflex, with some voluntary influence |
| Lower esophageal | Junction of the gullet and the stomach | Automatic |
| Pyloric | Outlet of the stomach | Automatic |
| Sphincter of Oddi | Where the bile and pancreatic ducts enter the bowel | Automatic |
| Internal anal | Just inside the anus | Automatic |
| External anal | Around the anal opening | Voluntary |
| Urethral | Below the bladder | Automatic and voluntary parts |
What goes wrong with them?
The two directions of failure produce opposite problems. A sphincter that will not relax obstructs, as in achalasia, where the ring at the base of the gullet stays shut and swallowed food will not pass, or in the thickened stomach outlet of an infant, which causes forceful vomiting. A sphincter that no longer holds leaks, giving heartburn when the ring at the base of the gullet slackens, and loss of control of urine or stool when the anal or urethral muscles are damaged. Childbirth, surgery and nerve disease are the usual routes to that damage. Preserving these muscles is stated openly in treatment planning, which is why chemoradiation is used in anal cancer partly to avoid an operation that would take the anal sphincter, and why continence is discussed before an open prostatectomy.
Also known as
Used in a sentence
Tone of the anal sphincter was preserved on examination.
Good to know
A sphincter rests contracted and has to be actively relaxed, so failure shows either as obstruction when it will not open or as leakage when the resting tone is lost.
Word origin
Where “sphincter” comes from
Sphincter comes from the Greek sphinkter, a band or binder, from sphingein, meaning to bind tight.