Which structure is involved in the innervation of the internal anal sphincter?

The internal anal sphincter is innervated primarily by the autonomic nervous system, specifically by sympathetic fibers. This sphincter is composed of smooth muscle and is responsible for involuntary control over the anal canal. The sympathetic nerves come from the hypogastric plexus, facilitating the contraction of the internal anal sphincter and thereby helping maintain continence.

While the pudendal nerve, mentioned in another option, does innervate the external anal sphincter and provides somatic control over voluntary closure, it is not involved in the innervation of the internal anal sphincter, which is under autonomic control. The vagus nerve is not associated with lower rectal or anal innervation, and the phrenic nerve primarily innervates the diaphragm, making these options irrelevant for the context of this question.