Sphincter ani internus muscle

The Sphincter ani internus (or internal anal sphincter) is a muscular ring that surrounds about 2.5 cm of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus.

It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm from the orifice of the anus.

Actions

Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Parasympathetic fibers from the pelvic segments of the spinal cord supply the internal sphincter. It is not innervated by the pudendal nerve, which carries somatic (motor and sensory)fibers that provide the innervation to the external anal sphincter.^[1]

Additional images

References

External links

• LUC spha
• pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (rectum)

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.