External urethral sphincter motor unit recruitment patterns during micturition in the spinally

28 29 In the rat, external urethral sphincter (EUS) activation during micturition consists of three 30 sequential phases: 1) an increase in tonic EUS activity during passive filling and active 31 contraction of the bladder (guarding reflex), 2) synchronized phasic activity (EUS bursting) 32 associated with voiding, and 3) sustained tonic EUS activity that persists after bladder 33 contraction. These phases are perturbed following spinal cord injury. The purpose of the present 34 study was to characterize individual EUS motor unit (MU) patterns during micturition in the 35 spinally intact and transected adult rat. EUS MU activity was recorded either from the L5 or L6 36 ventral root (intact) or EUS muscle (transected) during continuous flow cystometry in urethane37 anesthetized adult female Sprague Dawley rats. Using bladder pressure threshold and timing of 38 activation, four distinct patterns of EUS MU activity were identified in the intact rat: Low 39 Threshold Sustained, Medium/High Threshold Sustained, Medium/High Threshold Not 40 Sustained and Burst Only. In general, these MUs displayed little frequency modulation during 41 active contraction, generated high frequency bursts of action potentials during EUS bursting, and 42 varied in terms of the duration of sustained tonic activity. In contrast, three general patterns of 43 EUS MU activity were identified in the transected rat: Low Threshold, Medium Threshold, and 44 High Threshold. These MUs exhibited considerable frequency modulation during active 45 contraction of the bladder, no bursting behavior and little to no sustained firing. The prominent 46 frequency modulation of EUS MUs is likely due to the enhanced guarding reflex seen in EUS 47 whole-muscle EMG recordings in transected rats (D’Amico et al. 2011). In addition, EUS MU 48 recruitment in transected rats more closely followed predictions by the size principle than in 49 intact rats. This may reflect the influence of local synaptic circuits or intrinsic properties of EUS 50 motoneurons that are active in intact rats but attenuated or absent in transected rats. 51