Anatomic and functional properties of bulboglandularis striated muscle support its contribution as sphincter in female rabbit micturition

Multiparity Reduces Urethral and Vaginal Pressures Following the Bulboglandularis Muscle Stimulation in Rabbits

Unlike male mammals showing a well-delimited external urethral sphincter, female mammals have urogenital sphincters shaped by muscles like the urethrovaginal sphincter. Childbirth-related injuries affect morphometry and function of urogenital sphincters in women, which frequently underlies pelvic floor disorders, including stress urinary incontinence and pelvic organ prolapse. The bulboglandularis muscle (Bgm) seems to shape a urogenital sphincter in rabbits. We determined herein the effect of multiparity on urethral and vaginal pressures generated by the Bgm stimulation in age-matched nulliparous and multiparous chinchilla-breed rabbits to stimulate the Bgm with trains of ascending frequencies (from 1 to 100 Hz; 4 s duration each). Subsequently, the Bgm was excised, measured in width, and weighed. Significant differences (P ≤ 0.05) were determined with Mann-Whitney U or Student t-tests or repeated measures two-way ANOVA followed by Tukey tests. Spearman's partial coefficients were calculated to investigate the correlation between the highest pressure (urethral or vaginal) and the Bgm width. Multiparity reduced the weight and the width in the Bgm origin and medial regions. Urethral and vaginal pressures increased in response to the electrical stimulation of Bgm with frequencies from 20 to 100 Hz. Multiparas showed significant reductions in both types of pressures. We detected a strong correlation (conditioned by multiparity) between the medial Bgm width and the highest vaginal pressure. Our present findings demonstrate that multiparity impairs the function of Bgm, resulting in diminished urethral and vaginal pressures. Furthermore, the significant narrowness of the Bgm was correlated with the vaginal pressure recorded.

Targeted neuromodulation of pelvic floor nerves in aging and multiparous rabbits improves continence

Pelvic floor muscle stretch injury during pregnancy and birth is associated with the incidence of stress urinary incontinence (SUI), a condition that affects 30-60% of the female population and is characterized by involuntary urine leakage during physical activity, further exacerbated by aging. Aging and multiparous rabbits suffer pelvic nerve and muscle damage, resulting in alterations in pelvic floor muscular contraction and low urethral pressure, resembling SUI. However, the extent of nerve injury is not fully understood. Here, we used electron microscopy analysis of pelvic and perineal nerves in multiparous rabbits to describe the extent of stretch nerve injury based on axon count, axon size, myelin-to-axon ratio, and elliptical ratio. Compared to young nulliparous controls, mid-age multiparous animals showed an increase in the density of unmyelinated axons and in myelin thickness in both nerves, albeit more significant in the bulbospongiosus nerve. This revealed a partial but sustained damage to these nerves, and the presence of some regenerated axons. Additionally, we tested whether electrical stimulation of the bulbospongiosus nerve would induce muscle contraction and urethral closure. Using a miniature wireless stimulator implanted on this perineal nerve in young nulliparous and middle age multiparous female rabbits, we confirmed that these partially damaged nerves can be acutely depolarized, either at low (2-5 Hz) or medium (10-20 Hz) frequencies, to induce a proportional increase in urethral pressure. Evaluation of micturition volume in the mid-age multiparous animals after perineal nerve stimulation, effectively reversed a baseline deficit, increasing it 2-fold (p = 0.02). These results support the notion that selective neuromodulation of pelvic floor muscles might serve as a potential treatment for SUI.

Lumbosacral ventral root avulsion alters reflex activation of bladder, urethra, and perineal muscles during micturition in female rabbits

AIM

To determine the effect of the lumbosacral ventral root avulsion (VRA) on the reflex activation of bladder, urethra, and activation of perineal muscles during micturition in female rabbits.

METHODS

We allocated 14 virgin female rabbits to evaluate, first, the gross anatomy of lumbosacral spinal cord root (n = 5) and, second, to determine the effect of VRA on perineal muscles during micturition (n = 9). We recorded cystometrograms, urethral pressure, and electromyograms of the bulbospongiosus (Bsm) and ischiocavernosus (Ism) muscles before and after the L6-S2 VRA. Standard variables were measured from each recording and analyzed to identify significant differences (P < .05).

RESULTS

We found that the L6-S2 VRA affected directly the bladder and urethral function and reduced the duration and the frequency of the bursting of Ism and Bsm muscles during voiding. The Ism and Bsm showed a phasic activation, of different frequencies, during the voiding phase and the L6-S2 VRA inhibited the co-contraction of the Ism and Bsm-bladder-urethra.

CONCLUSIONS

The Ism and Bsm are activated at different frequencies to trigger the voiding phase. The L6-S2 VRA affected the activity pattern of both perineal muscles. These modifications affected the bladder and urethra function. It is possible that the restoration of the activation frequency of perineal muscles contributed for an efficient bladder contraction.

ORIGIN AND ANTIMERIC DISTRIBUTION OF THE OBTURATOR NERVES IN THE NEW ZEALAND RABBITS

Abstract New Zealand rabbits are widely used as experimental models and represent an important casuistic in veterinary practices. The musculoskeletal conformation of rabbits frequently leads to the occurrence of lumbosacral lesions with neural involvement. In order to contribute to the comparative anatomy and the understanding of these lesions, the origin and distribution of the obturator nerves of 30 New Zealand rabbits (15 males and 15 females) previously fixed in 10% formaldehyde were studied by dissection. The obturator nerves were originated from the ventral spinal branches of L6 and L7 in 63.3% of the cases, L5 and L6 in 13.4%, only L7 in 13.4%, L7 and S1 in 6.6 % and of L6, L7 and S1 in 3.3%. The spinal segment that most contributed to the formation of the nerve was L7 (86.6% of the nerves). The obturator nerves emitted in all the specimens, a variable number of branches for the internal obturator, external obturator, pectineum, adductor and gracilis muscles. No significant differences were observed between the frequencies of the origin and muscular branches of the obturator nerves when comparing sex and antimers.

Anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits

AIM

To determine the anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits.

METHODS

Chinchilla adult anesthetized female rabbits were used. Anatomical, electrophysiological, and histological studies were performed.

RESULTS

L7, S1, and some fibers from S2 and S3 form the lumbosacral trunk, which gives origin to the sciatic nerve and innervation to the gluteal region. From S2 to S3 originates the pudendal nerve, whose branches innervates the striated anal and urethra sphincters, as well as the bulbospongiosus, ischiocavernosus, and constrictor vulvae muscles. The sensory field of the pudendal nerve is ∼1800 mm² and is localized in the clitoral sheath and perineal and perigenital skin. The organization of the pudendal nerve varies between individuals, three patterns were identified, and one of them was present in 50% of the animals. From S3 emerge the pelvic nerve, which anastomoses to form a plexus localized between the vagina and the rectum. The innervation of the pelvic floor originates from S3 to S4 fibers.

CONCLUSIONS

Most of the sacral spinal nerves of rabbit are mixed, carrying sensory, and motor information. Sacral nerves innervate the hind limbs, pelvic viscera, clitoris, perineal muscles, inguinal and anal glands and perineal, perigenital, and rump skin. The detailed description of the sacral nerves organization, topography, and axonal components further the knowledge of the innervation in pelvic and perinal structures of the female rabbit. This information will be useful in future studies about the physiology and physiopathology of urinary, fecal, reproductive, and sexual functions.