Multiparity affects conduction properties of pelvic floor nerves in rabbits

Abstract

Introduction: Women often develop pelvic floor dysfunction due to damage to the pelvic musculature during childbirth; however, the effect on pelvic floor nerves function is less understood. This study used adult rabbits to evaluate the electrophysiological and histological characteristics of the bulbospongiosus (Bsn) and pubococcygeus nerves (Pcn) in multiparity.

Methods: Compound nerve action potentials (CNAP) were compared between age-matched nulliparous and multiparous animals and associated to the histological characteristics of myelinated axons from the Bsn and Pcn nerves. The extensor digitorum longus nerve (EDLn) was used as negative control. Data were analyzed with unpaired two-tailed Student's t test or Mann-Whitney U test to determine significant differences between groups.

Results: The onset and peak latencies, duration, and conduction velocity of the motor fibers in these pelvic nerves were not significantly different between nulliparous and multiparous animals. However, the peak-to-peak amplitude and area of the CNAP in both Bsn and Pcn were reduced in multiparous rabbits. Histology showed a higher percentage of axons with myelin disorganization caused by multiparity in these pelvic nerves. Together, the data indicate a reduction in the number of functional pelvic axons due to multiparity. As expected, no effect of parity was observed in the EDLn controls.

Conclusions: Present findings demonstrated that multiparity affects myelination and consequently conduction properties in the small pelvic floor nerves.

Keywords: bulbospongiosus muscle; micturition; myelin; pubococcygeus muscle; reproduction; urinary incontinence.

Figure 1

(a) Schematic view showing the anatomical location of bulbospongiosus (Bsn) and pubococcygeus (Pcn) nerves in the female rabbit; dotted lines delimit the lumbosacral plexus; the extensor digitorum longus nerve (EDLn) location is showed in the Inset. (b and c) representative photographs of dissected Bsn and Pcn

Figure 2

Multiparity affects specifically the CNAP evoked in Bsn and Pcn but not in the EDLn of rabbits. Representative CNAP traces induced by single current pulses applied with different strengths (1, 3 and 5 times threshold, ×T) to Bsn (a), Pcn (b), and EDLn (c) in nulliparous (N; gray lines and arrows) and multiparous rabbits (M, black lines and arrowheads). Traces represent the mean of 16 traces per nerve

Figure 3

Multiparity decreases the CNAP amplitude of perineal and pelvic nerves in rabbits. Data are means ± SEM (n = 6 per group) of the CNAP amplitude provoked by electrical current pulses of gradually increased strength (from 1 to 6 times threshold, ×T) applied to the Bsn (a), Pcn (b), and EDLn (c) from nulliparous (N) and multiparous (M) rabbits. *p < 0.05; **p < 0.01

Figure 4

Multiparity decreases the CNAP area of perineal and pelvic nerves in rabbits. Data are means ± SEM (n = 6 per group) of the CNAP area evoked in Bsn (a), Pcn (b), and EDLn (c) from nulliparous (N) and multiparous (M) rabbits. *p < 0.05; **p < 0.01

Figure 5

Multiparity increases the occurrence of axons with myelin disorganization in perineal and pelvic nerves of rabbits. Representative photomicrographs from Bsn (a–c), Pcn (d–f), and EDLn (g–i) transverse sections stained with Toluidine blue from nulliparas (N) and multiparas (M). Data are medians ±minimal to maximal values for Bsn (j) and Pcn (k). Symbols indicate abnormal myelinated fibers: myelin disruption (asterisk), invagination (ampersand), and apparent separation of the myelin sheaths (arrow). Scale bar, 10 μm