These parasympathetic neurons make the rectum contract and the internal anal sphincter relax.
这些副交感神经元可以使直肠收缩,并使肛门内括约肌松弛。
英语百科
Internal anal sphincter
The internal anal sphincter, IAS, (or sphincter ani internus) is a muscular ring that surrounds about 2.5–4.0cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter.
It is about 5mm thick, and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6mm from the orifice of the anus.