The internal and external sphincter muscles control the opening and closing of the anus; these muscles, which are sensitive membranes made up of many nerve endings, facilitate pleasure or pain during anal sex.
The Human Sexuality: An Encyclopedia states that "the inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and that "the rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand".
By contrast, each person's sphincter muscles react to penetration differently, the anal sphincters have tissues that are more prone to tearing, and the anus and rectum, unlike the vagina, do not provide natural lubrication for sexual penetration.
Researchers say adequate application of a personal lubricant, relaxation, and communication between sexual partners are crucial to avoid pain or damage to the anus or rectum.
but it is typically described as being located behind the female pubic bone surrounding the urethra and accessible through the anterior wall of the vagina; it and other areas of the vagina are considered to have tissue and nerves that are related to the clitoris.