Structure, innervation, mechanical properties and reflex activation of a striated sphincter in the vestibule of the cat vagina

Abstract

Vaginal constriction might be important for reproduction in mammals, but existing information is both limited and controversial. This paper shows the structure, mechanical properties, innervation and reflex response of a striated sphincter in the vestibule of the cat vagina. A Foley catheter coupled to a pressure transducer detected in the lumen of the vestibule a pressure wave that was induced by stimulation of the external branch of the motor pudendal nerve. The peak pressure of the wave induced by bilateral stimulation (30.6 cm H 2 O) was about double of the peak pressure wave induced unilaterally. The tetanus/twitch amplitude ratio was 4.5. The sphincter that produces the increase in vaginal pressure fatigues slowly. Digital, point-to-point summation of unilateral waves was greater than the wave induced bilaterally. Summation of the pressure wave induced by the separate stimulation of the terminal motor branches was also greater than the wave induced by the entire motor nerve. This might reflect multiple innervation of muscle fibres. Single, controlled probing of the vaginal vestibule induced a reflex discharge in the motor nerve. Repetitive probing (10 Hz) induced a motor nerve post-discharge lasting O1 min. The vaginal sphincter is two-half rings of striated muscle fibres in the wall of the vaginal vestibule; the fibres end freely in the dorsal and ventral midlines. Penetration of the vestibule by the penis might trigger sustained contraction of the vaginal sphincter.

Functional Coupling Between Motor and Sensory Nerves Through Contraction of Sphincters in the Pudendal Area of the Female Cat

“…In a prior study, probing the vaginal vestibule with brief (Ͻ20 ms) controlled strokes (intact nerves) induced in MPN a short-latency (Ͻ2 ms) discharge that preceded the reflex response, with a latency of about 8 ms (Lagunes-Córdoba et al 2009). It was postulated that this short latency might result from the firing of muscle afferents.…”

“…In addition, afferent fibers from muscle also fired. We cannot discern whether these afferents originated in the anal or the vaginal sphincter, or in the ischiocavernosus muscle (see Lagunes-Córdoba et al 2009). It appears that muscle contraction and nerve response in the overlying skin is a single but compound action.…”

“…The pressure in the lumen of the vaginal vestibule and of the anal canal was measured as previously described (Lagunes-Córdoba et al 2009). Briefly, a bulb filled with water and tightly plugged to a Statham transducer was introduced 16 -21 mm into the vagina and 20 -30 mm into the anal canal (Fig.…”

“…1A), which was sectioned as far distal as possible, and a lateral, thicker branch. According to most authors, the latter branch innervates the external anal sphincter (see, e.g., Krier 1985; Paroschy and Shefchyk 2000) but, in addition, also innervates the vaginal constrictor sphincter and the ischiocavernosus muscle (Lagunes-Córdoba et al 2009). A schema of all these nerves in the ischiatic cavity is shown in Fig.…”

“…Thus Pv and Pac might also be delayed, which is important in the present study. To test for a possible delay of the pressure waves, we compared Pv with the tension twitch (Tv) of the vaginal constrictor sphincter (VCS; Lagunes-Córdoba et al 2009). The system recording tension with OCsg has no delay.…”