Two muscle rings called sphincters surround the anal opening. Each functions independently.

If you insert a finger about one half-inch into your anus and press your fingertip against the side, you can clearly feel the two sphincter muscles. There is less than a quarter-inch between them. The external sphincter is controlled by the central nervous system - just like the muscles of the hand, for example. You can readily tense and relax this sphincter whenever you want.
The internal sphincter is quite different. This muscle is controlled by the involuntary or autonomic part of the nervous system, which governs such functions as heartbeat and stress response. The internal sphincter reflects and responds to fear and anxiety during anal sex. It will cause the anus to tense up automatically even if the passive partner is trying to relax. Thus, precautions about safety and comfort are essential here. Even if a person does feel comfortable during anal sex, he or she may still need to learn voluntary control over his or her internal sphincter in order to relax it at will. Doing so requires regularly inserting a finger, perhaps in the shower each day, and feeling the internal sphincter.

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