SPHINCTER ANI INTERNUS

GENERAL INFORMATION

The sphincter ani internus is an involuntary smooth muscle located around the anal canal. It plays a crucial role in maintaining continence by providing a constant tone to keep the anus closed. This muscle is part of the pelvic floor and works with other muscles in the region to control bowel movements. Unlike the external anal sphincter, the internal sphincter functions automatically, contracting and relaxing without conscious effort.

ORIGIN

The sphincter ani internus originates from the circular smooth muscle fibres surrounding the anal canal, beginning at the rectoanal junction.

INSERTION POINT

This muscle inserts into the perianal skin and blends with the fibres of the external anal sphincter.

MAJOR ARTERIES

• Inferior Rectal Artery: Branch of the internal pudendal artery supplying the sphincter ani internus.
• Middle Rectal Artery: Provides blood flow to the middle section of the rectum and surrounding muscles.

NEURAL INNERVATION

• Autonomic Nervous System (Sympathetic and Parasympathetic): The sympathetic fibres (via the hypogastric plexus) keep the muscle contracted, while the parasympathetic fibres (via the pelvic splanchnic nerves, S2-S4) cause relaxation during defecation.

TRIGGER POINT

While the sphincter ani internus is an involuntary muscle, dysfunction or chronic tension can result in pain, discomfort, or difficulty with bowel movements. Conditions like chronic constipation or rectal surgery may lead to hypertonicity or spasms in this muscle. That cause discomfort or a feeling of incomplete bowel evacuation.

CONCENTRIC FUNCTION

The sphincter ani internus contracts to maintain constant tone, preventing involuntary release of faeces and maintaining continence.

ECCENTRIC FUNCTION

The sphincter ani internus relaxes eccentrically during defecation to allow stool to pass through the anal canal while ensuring a gradual opening of the anus.

ISOMETRIC FUNCTION

This muscle provides continuous, isometric contraction at rest to maintain closure of the anal canal, preventing leakage of faeces or gas between bowel movements.

RELATED MUSCLE – SCIENTIFIC NAMES

• Sphincter Ani Externus: Works voluntarily to maintain continence alongside the internal sphincter.
• Puborectalis: Helps maintain continence by creating a kink in the rectum.
• Levator Ani: Supports pelvic organs and assists with continence.
• Coccygeus: Stabilises the pelvic floor and helps with the support of pelvic structures.
• Obturator Internus: Aids in lateral rotation and stabilisation of the pelvic floor.

ANTAGONIST

• Parasympathetic Nervous System: Causes the relaxation of the sphincter ani internus during defecation.
• Pelvic Floor Muscles: Relax to allow the sphincter ani internus to open during bowel movements.
• Rectal Musculature: Contracts during defecation while the internal sphincter relaxes.
• Sphincter Ani Externus: Can contract voluntarily to counteract relaxation of the internal sphincter.
• Anal Muscles (Subcutaneous): Work with the external sphincter to regulate the opening and closing of the anus.

COMMON INJURIES

• Anal Sphincter Dysfunction: Occurs when the sphincter ani internus loses its ability to maintain tone, leading to faecal incontinence. That often caused by nerve damage or weakening of pelvic floor muscles.
• Chronic Constipation: Repeated straining can lead to overuse and dysfunction of the internal sphincter, resulting in muscle fatigue or hypertonicity, causing pain or difficulty in passing stool.
• Sphincter Spasm (Proctalgia Fugax): Sudden, involuntary spasms of the internal sphincter that can cause intense rectal pain, often occurring without warning or apparent cause.
• Haemorrhoids: Swelling of blood vessels near the anus can affect sphincter function, leading to discomfort and altered muscle tone.
• Rectal Prolapse: Weakening of the pelvic floor muscles can lead to the prolapse of rectal tissues, often involving dysfunction of the sphincter ani internus.

EXERCISES

• Kegel Exercises: Contract the pelvic floor muscles (including the sphincter ani internus) for 5-10 seconds, then relax. Repeat to strengthen the muscle and improve bowel control.
• Pelvic Floor Contractions: Lie on your back and gently contract the pelvic muscles as if stopping the flow of urine. This exercise engages the internal sphincter and enhances control.
• Bridge Pose with Pelvic Engagement: Lift your hips while lying on your back, focusing on contracting the pelvic floor muscles to strengthen the internal sphincter.
• Deep Breathing with Pelvic Focus: Perform deep abdominal breathing while contracting the pelvic floor muscles on each exhale, helping to build muscle endurance.
• Pelvic Tilts: Tilt the pelvis upwards while lying on your back, engaging the pelvic floor and strengthening the sphincter ani internus and surrounding muscles.

STRETCHES

• Child’s Pose: Sit back on your heels while extending your arms forward, gently stretching the pelvic muscles and relaxing the sphincter ani internus.
• Seated Forward Bend: Sit with legs extended, reaching forward to stretch the lower back, hamstrings, and pelvic muscles, helping relieve tension in the sphincter.
• Happy Baby Pose: Lie on your back, holding your feet and pulling your knees toward your chest. This movement stretches the pelvic muscles and helps alleviate tightness in the internal sphincter.
• Pelvic Floor Relaxation: Sit cross-legged and focus on relaxing the pelvic floor muscles with deep breathing, allowing the sphincter ani internus to release tension.
• Hip Flexor Stretch: Stretch the hip flexors by lunging forward, which can help relax the pelvic floor muscles and improve flexibility around the internal sphincter.