====== Anal sphincter ======

The external anal sphincter (or sphincter ani externus ) is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.
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The internal anal sphincter, IAS, (or sphincter ani internus) is a muscular ring that surrounds about 2.5–4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter.

It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6 mm from the orifice of the anus.
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If a patient has the ability to contract the [[anal sphincter]] voluntarily or to feel a pinprick or touch around the anus, the injury is considered a [[incomplete spinal cord injury]].

The nerves in this area are connected to the very lowest region of the spine, the [[sacral region]], and retaining sensation and function in these parts of the body indicates that the spinal cord is only partially damaged. This includes a phenomenon known as [[sacral sparing]].
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The [[filum]] is differentiated from [[nerve root]]s by presence of characteristic squiggly vessel on surface
of filum. Also, under the microscope, the filum has a distinctively whiter appearance than the [[nerve
root]]s, and ligamentous-like strands can be seen running through it. NB: intra-op Electrostimulation
and recording of [[anal sphincter]] EMG are more definitive.