GENERAL INFORMATION
The sphincter ani internus is the region between the pubic symphysis and the coccyx, located between the thighs. It contains various structures, including muscles, nerves, blood vessels, and genitals. The perineum plays a role in supporting pelvic organs and is involved in sexual function, urination, as well as defecation.
SPHINCTER ANI INTERNUS
The sphincter ani internus is a smooth muscle located in the perineum. It forms a circular band within the walls of the anal canal and is responsible for involuntary control of the anal opening.
ORIGIN
The sphincter ani internus muscle originates from the circular smooth muscle fibres within the walls of the anal canal.
INSERTION POINT
It inserts into the mucosa lining of the anal canal.
MAJOR ARTERIES
The internal anal sphincter receives its blood supply from the superior rectal artery. It is a branch of the inferior mesenteric artery.
NEURAL INNERVATION
The sphincter ani internus is innervated by the autonomic nervous system, specifically the
sympathetic as well as parasympathetic fibres.
TRIGGER POINT
Trigger points in the internal anal sphincter muscle can cause pain as well as dysfunction, such as anal fissures or involuntary spasms.
CONCENTRIC FUNCTION
The sphincter ani internus muscle remains in a state of tonic contraction, providing continuous closure of the anal canal at rest.
ECCENTRIC FUNCTION
The internal anal sphincter relaxes during defecation to allow the passage of stool through the anal canal.
ISOMETRIC FUNCTION
The sphincter ani internus maintains a constant level of contraction to maintain anal tone. It also prevents leakage of stool or gas.
RELATED MUSCLE SCIENTIFIC NAMES
- Sphincter ani internus: This muscle provides involuntary control over the anal opening, maintaining continence at rest.
ANTAGONIST MUSCLES
- External anal sphincter: This voluntary muscle works alongside the internal sphincter to control the passage of stool.
- Puborectalis: This muscle helps maintain continence by creating a sling around the rectum, contributing to the closure of the anal canal.
COMMON INJURIES
- Anal fissures: Small tears in the lining of the anus, causing pain as well as bleeding.
- Internal haemorrhoids: Swollen veins inside the rectum, leading to discomfort as well as bleeding.
- Anal sphincter dysfunction: Issues with the sphincter ani internus’s ability to contract or relax properly. It also causes continence problems.
EXERCISES
- Pelvic floor muscle exercises: Improve perineal muscle strength and control through targeted contractions and relaxations.
- Biofeedback training: Enhance awareness and coordination of the pelvic floor muscles using feedback devices.
- Squats: Engage the perineal muscles and improve pelvic stability by lowering the body into a seated position and standing up.
STRETCHES FOR SPHINCTER ANI INTERNUS
- Butterfly stretch: Stretches the inner thighs and perineal muscles by sitting with the soles of the feet together and gently pressing the knees towards the ground.
- Happy baby pose: Releases tension in the perineum and lower back by lying on the back and holding the feet with the hands while drawing the knees towards the chest.
- Seated forward fold: Stretches the hamstrings and perineal area by sitting with the legs extended and reaching forward towards the feet.
- Child’s pose: Stretches the perineum and relaxes the pelvic floor muscles by kneeling and bending forward to rest the forehead on the ground.