A sphincter is a specialized type of circular muscle that closes off an orifice or tube when it's in its normal relaxed state, but opens up when the muscle contracts. The two largest sphincters in the body are in the anus and surrounding the urethra as it leaves the bladder.
The contraction of a sphincter is usually voluntary, but can be involuntary if the bladder or rectum are unusually full.
Like any other muscle, a sphincter can spasm, which often means it cannot contract properly to open the orifice. A sphincter muscle can also become paralyzed.