External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat

Abstract

In the rat, external urethral sphincter (EUS) activation during micturition consists of three sequential phases: 1) an increase in tonic EUS activity during passive filling and active contraction of the bladder (guarding reflex), 2) synchronized phasic activity (EUS bursting) associated with voiding, and 3) sustained tonic EUS activity that persists after bladder contraction. These phases are perturbed following spinal cord injury. The purpose of the present study was to characterize individual EUS motor unit (MU) patterns during micturition in the spinally intact and transected adult rat. EUS MU activity was recorded from either the L5 or L6 ventral root (intact) or EUS muscle (transected) during continuous flow cystometry in urethane-anesthetized adult female Sprague-Dawley rats. With the use of bladder pressure threshold and timing of activation, four distinct patterns of EUS MU activity were identified in the intact rat: low threshold sustained, medium/high threshold sustained, medium/high threshold not sustained, and burst only. In general, these MUs displayed little frequency modulation during active contraction, generated high-frequency bursts of action potentials during EUS bursting, and varied in terms of the duration of sustained tonic activity. In contrast, three general patterns of EUS MU activity were identified in the transected rat: low threshold, medium threshold, and high threshold. These MUs exhibited considerable frequency modulation during active contraction of the bladder, no bursting behavior and little to no sustained firing. The prominent frequency modulation of EUS MUs is likely due to the enhanced guarding reflex seen in EUS whole muscle electromyogram recordings in transected rats (D'Amico SC, Schuster IP, Collins WF 3rd. Exp Neurol 228: 59-68, 2011). In addition, EUS MU recruitment in transected rats more closely followed predictions by the size principle than in intact rats. This may reflect the influence of local synaptic circuits or intrinsic properties of EUS motoneurons that are active in intact rats but attenuated or absent in transected rats.

Initiation of Bladder Voiding with Epidural Stimulation in Paralyzed, Step Trained Rats

“…5B). These evoked potentials with a long latency may represent the activation of complex interneuronal networks that enable bursting activity of the EUS similar to that observed during voiding in intact rats [18]. Stimulation at higher frequencies (40 Hz) at the same sites on the spinal cord (between L2 and S1) facilitates partial weight-bearing bipedal [10] and quadrupedal [27] stepping of spinal rats on a treadmill belt.…”

“…Under anesthesia, a PE 50 catheter was inserted via the urethra [17] and secured in place using surgical tape. The EUS muscle was implanted acutely with Teflon coated stainless steel wires (AM systems) as described previously [18]. Once the animal was conscious, they were suspended vertically using the body weight support system with their feet off the treadmill surface (Movie S2).…”

Neural Control of the Lower Urinary Tract

“…14) (97, 98, 104, 194). In the rat recordings of single EUS motor units as well as intracellular recordings from EUS motoneurons revealed tonic and burst firing in some units and primarily burst firing in others, raising the possibility of two separate excitatory inputs to EUS motoneurons that allow some to contribute to storage and voiding and others to contribute primarily to voiding (129). …”

Electrical Stimulation of the Spinal Dorsal Root Inhibits Reflex Bladder Contraction and External Urethra Sphincter Activity: Is This How Sacral Neuromodulation Works?

“…We have firstly, to our knowledge, tested the inhibitive effect of SNM not only in the bladder, but also in the EUS activity. The EUS activation during micturition consists of 3 phases in rats: an increased tonic EUS activity during the filling phase; synchronized phasic activity (EUS bursting) in voiding phase and sustained tonic EUS activity after the micturition [22]. It is important to note that these differing patterns of EUS activities were generated from different segments of the SC: tonic activity at L6-S1 and bursting activity between T8-9 and L3-4 [23].…”