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

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 >1 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.