Organization of external urethral sphincter recruitment during micturition in the female rat.

Abstract

The goal of this study was to examine the activation patterns of individual external urethral sphincter (EUS) motor units (MUs) during micturition in rats. To this end, bladder intravesical pressure (BP) and EUS EMG activity were recorded in urethane anesthetized adult female Sprague Dawley rats during continuous infusion of saline into the bladder. Individual EUS MU activity was recorded using a tungsten microelectrode (1 MΩ) inserted in the EUS or by placing an L6 ventral root filament, dissected in continuity, on platinum electrodes. Four different EUS MU recruitment patterns were observed. (1) Low threshold/sustained MUs were recruited prior to bladder contraction and exhibited sustained firing. (2) Medium threshold/not sustained MUs were recruited at the beginning of bladder contraction and stopped firing at the end of bladder contraction. (3) Medium threshold/sustained MUs were recruited at the beginning of bladder contraction and exhibited sustained firing. (4) High threshold/burst only MUs were recruited shortly before EUS bursting and stopped firing shortly after EUS bursting. Every EUS MU exhibited EUS bursting at the peak of bladder contraction. Among medium threshold MUs, derecruitment varied inversely with conduction velocity with slowly conducting units exhibiting longer sustained activity. This project is supported by the New York State Spinal Cord Injury Research Board.