Puborectalis

The puborectalis is the most medial and functionally significant part of the levator ani muscle group. It forms a U-shaped sling around the anorectal junction and plays a critical role in maintaining fecal continence by actively maintaining the anorectal angle. Despite its small size, it is one of the most important muscles in pelvic floor function, particularly in defecation mechanics, continence, and pelvic organ support. Dysfunction of this muscle can lead to conditions like incontinence, obstructed defecation, or pelvic organ prolapse.

Structure

The puborectalis is a paired skeletal muscle that originates from the posterior surface of the pubic body, just lateral to the pubic symphysis. The muscle fibers course posteriorly and medially, looping around the anorectal junction to merge with the fibers from the opposite side, forming a muscular sling. It does not insert onto a bone but rather interlaces with the external anal sphincter and the connective tissue surrounding the rectum.

Anatomical Course

• Originates from the inner surface of the pubic bone
• Courses medially and posteriorly, looping around the posterior surface of the anorectal junction
• Joins with its contralateral counterpart to form a sling
• Blends posteriorly with the external anal sphincter and perineal body

Location

The puborectalis lies within the midline of the pelvic diaphragm, encircling the rectum just at the junction where it becomes the anal canal. It is positioned anterior to the coccyx and posterior to the pubic symphysis, and is the most medial of the levator ani subdivisions. It lies inferior and medial to the pubococcygeus and superior to the external anal sphincter.

Relations

Function

• Maintains the Anorectal Angle: The primary function of the puborectalis is to form a sling around the anorectal junction, pulling it anteriorly and creating an acute angle (usually around 80°) between the rectum and anal canal. This anorectal angle is key for continence.
• Fecal Continence: By maintaining the anorectal angle, the puborectalis helps close the anal canal and resist the downward force of abdominal pressure.
• Defecation Control: During defecation, the puborectalis relaxes, straightening the anorectal angle and allowing passage of feces through the anal canal.
• Pelvic Organ Support: Contributes to the muscular hammock that supports pelvic viscera such as the rectum, bladder, uterus, and prostate.

Innervation

• Nerve to levator ani (S3–S4): Primary motor supply
• Inferior rectal branch of pudendal nerve (S2–S4): May contribute

Vascular Supply

• Inferior gluteal artery
• Internal pudendal artery
• Accompanied by corresponding veins that drain into the internal iliac vein

Microscopic Structure

Puborectalis is composed of striated skeletal muscle fibers, allowing voluntary control. Like other pelvic floor muscles, it shows fatigue-resistant fiber characteristics due to its constant role in maintaining tone. Muscle fibers are interspersed with connective tissue and closely associated with autonomic nerve terminals, reflecting its partial involuntary tone even at rest.

Development

The puborectalis muscle arises from the mesoderm of the myotomes in the sacral region during embryogenesis. It differentiates alongside the rest of the levator ani group and develops connections to the rectum and anal canal. During fetal development, it becomes functionally active by birth and continues to mature through infancy and early childhood as continence develops.

Role in Imaging

Puborectalis can be visualized on MRI and ultrasound. In MRI defecography, its tone and symmetry are assessed in cases of obstructed defecation syndrome. Loss of puborectalis support may appear as rectocele or descending perineum. Its avulsion from the pubis can be diagnosed with 3D ultrasound in postpartum women and is a risk factor for prolapse.

Clinical Significance

• Fecal Incontinence: Damage or denervation of the puborectalis, such as from obstetric trauma or pudendal nerve injury, can impair anorectal angle control and lead to passive fecal leakage.
• Obstructed Defecation: In some cases, the puborectalis fails to relax adequately during defecation (dyssynergic defecation), leading to straining and incomplete evacuation. This is often treated with biofeedback therapy.
• Pelvic Organ Prolapse: Avulsion or weakening of puborectalis support contributes to descent of pelvic organs like the rectum or uterus. This is especially common in multiparous women.
• Levator Ani Syndrome: Chronic pelvic pain syndrome characterized by tender, hypertonic puborectalis muscle with no visible pathology. Pain may worsen on sitting or after bowel movements.
• Postpartum Muscle Avulsion: Avulsion of puborectalis from the pubis during childbirth is detectable on imaging and correlates with long-term pelvic floor disorders.