Innervation of the levator ani muscles in the female squirrel monkey

The purpose of this study was to characterize the innervation of the levator ani muscles in the female squirrel monkey and to investigate its usefulness as an animal model of pelvic organ prolapse.

Eleven nulliparous female squirrel monkeys with no pelvic organ prolapse were used in this study. Detailed pelvic dissections were conducted (n = 3), and the Koelle stain for acetylcholinesterase was used to identify the motor endplate zone in the levator ani muscles (n = 2). Unilateral levator ani (n = 4) and pudendal (n = 2) neurectomies were performed; changes in levator ani muscle mass and myocyte diameter were examined 14 days after neurectomy. Nerve biopsy specimens from each animal were processed for microscopy.

The levator ani nerve originated from the S2 spinal root and entered the pelvic cavity adjacent to the pelvic nerve between the flexor caudalis brevis and iliocaudalis muscles. The levator ani nerve then projected caudally and bifurcated to penetrate the iliocaudalis and pubocaudalis. A single motor endplate zone in each muscle correlated with the point of levator ani nerve penetration. The pudendal nerve originated from the S1-S2 spinal roots to innervate the urethral and anal sphincters, clitoris, and perineum, but not the iliocaudalis or pubocaudalis. Significant atrophy and myocyte shrinkage occurred in the iliocaudalis and pubocaudalis ipsilateral to the levator ani nerve transection (P <.05). Pudendal neurectomy produced no levator ani muscle changes.

Intrapelvic skeletal muscles in the female squirrel monkey are similar to humans and have distinct innervation with no contribution from the pudendal nerve. The squirrel monkey is likely to be a useful model of pelvic organ prolapse and warrants further study.