Abstract
Traditionally, coordinated micturition (bladder contractions coupled with a reduction in external urethral sphincter activity) is assumed to require a spinal-brainstem-spinal reflex loop triggered by bladder distention. However, recent data suggest that bladder contractions can also be evoked by activation of urethral afferents, even following spinal cord transection. This project sought to verify that stimulation of pudendal urethral afferents elicits bladder contractions both before and after spinal transection and to determine the required stimulus parameters. Robust bladder contractions were generated by electrical stimulation of pudendal urethral afferent nerve fibers in 3 cats, including post spinal cord transection in one animal. This response was dependent on bladder volume, stimulation frequency, stimulation amplitude, and genital nerve branch activity. Control of bladder activity via urethral afferent stimulation would be a valuable rehabilitation tool for individuals with spinal cord injury, and therefore deserves additional investigation.
Introduction
Neurological disease or spinal cord injury (SCI) can
result in loss of voluntary control of bladder evacuation, bladder
hyper-reflexia, and bladder sphincter dysynergia. These factors often lead to renal complications
and damage, episodes of autonomic dysreflexia, incontinence and urinary tract
infections. Loss of bladder control also has profound social impact,
leads to decreased quality of life and large direct medical costs. The long-term goal of this research is to
develop a neural prosthesis to restore bladder function in persons with
neurological disorders or spinal cord injury.
The traditional view holds that coordinated micturition (bladder contractions coupled with a reduction in external urethral sphincter activity) requires a spinal-brainstem-spinal reflex loop triggered by bladder distention [1]. SCI interrupts this reflex. However, recent data suggest that bladder contractions can also be evoked by activation of pudendal urethral afferents, even following spinal transection.
Fluid flow in the urethra [2] and electrical stimulation of the pudendal urethral sensory branches [3-5] have been shown to activate the bladder. A recent study reported that increases in bladder pressure and reductions in urethral sphincter activity were evoked by electrical stimulation of the urethral sensory branch of the pudendal nerve following acute spinal transection [5].
The objective of this project was to verify that stimulation of urethral afferents elicits bladder contractions, and in particular, determine if this effect is still present after spinal cord transection. An additional goal was to determine the effects of bladder volume and stimulus parameters on the evoked response.
Methods
Three acute experiments were conducted in adult cats. All animal care and procedures were according to NIH Guidelines, publication No. 86-23, revised 1985, and were approved by the Institutional Animal Care and Use Committee. Animals were anesthetized with ketamine (35 mg/kg, IM), and maintained with alpha-chloralose (60 mg/kg IV).
The bladder was cannulated to control bladder volume, and the catheter connected to an external pressure transducer. The pudendal nerve was exposed and the branches identified by anatomical landmarks.
Reflex bladder contractions were generated by filling the bladder with warm saline while obstructing the bladder neck and urethra with a catheter. Bladder volume was adjusted to near the threshold for reflexive bladder contractions.
Bipolar stimulating hook electrodes were placed on the principal sensory branches of the pudendal nerve (urethral, dorsal penile, ischiourethralis) which were stimulated separately using regulated current stimuli consisting of 5 or 10 s trains of 100 µs rectangular pulses applied at 1, 2, 5, 10, and 20 Hz and various stimulus amplitudes.
In one experiment, the external urethral sphincter nerve activity was recorded with a bipolar hook electrode. In one experiment, the spinal cord was transected at approximately T12 and recordings repeated.
Results
In all three experiments, bladder contractions were elicited by stimulation of the pudendal urethral sensory branch. An example is shown in Figure 1. Bladder and urethral sphincter responses were dependent on bladder volume, stimulation frequency, stimulation amplitude and genital branch activation. Bladder volume had to be at or near the volume threshold for reflexive bladder contractions to elicit a response. No responses were observed at low bladder volumes. In the one experiment where urethral sphincter ENG activity was recorded, stimulation at low bladder volumes resulted in a substantial increase in ENG activity.
As shown in Table 1, robust bladder contractions were elicited at low stimulation frequencies and not elicited or inhibited at high stimulation frequencies.
Figure 1. Robust bladder
contractions were elicited by electrical stimulation (bar: 1 Hz, 1.0 mA) of the
pudendal nerve urethral sensory branch.
Table 1. Effects of pudendal afferent stimulation frequency on bladder pressure for experiments 1, 2 and 3 respectively. Robust bladder contractions were defined as sustained evoked bladder pressures >15 cm H2O.
|
|
Stimulation frequency (Hz) |
|||||
|
|
1 |
2 |
5 |
10 |
20 |
40 |
|
Robust |
1,2,3 |
2,3 |
3 |
2,3 |
|
|
|
Weak |
|
|
1,2 |
3 |
|
|
|
No effect |
|
|
|
|
3 |
3 |
|
Inhibited |
|
|
|
1,2 |
1,2 |
2 |
Following spinal transection, reflex bladder contractions were eliminated. However electrical stimulation of the urethral sensory nerve still evoked robust bladder contractions (Figure 2).
Figure 2.
Bladder pressure response to sensory branch stimulation (bar: 20 Hz, 3.0
mA) approximately 3.5 hrs after spinal cord transection.
Activation of the dorsal penile nerve via mechanical stimulation of the penis abolished bladder contractions evoked by urethral sensory nerve stimulation as shown in Figure 3. Urethral sphincter branch ENG activity increased during mechanical stimulation of the penis.
Figure 3.
Bladder contractions evoked by urethral sensory nerve stimulation were
abolished by mechanical stimulation of the penis. The thin bar represents electrical
stimulation (1 Hz, 150 μA) of the urethral sensory branch. The thick bar represents mechanical
stimulation of the penis. The bladder
contraction returned after cessation of penile stimulation while the urethral
sensory branch was still activated, and ceased at the end of urethral sensory
branch stimulation.
Discussion
These results confirm that bladder contractions may be generated by
stimulation of the pudendal sensory branches, even after SCI. These results are consistent with results
from other studies.
Studies in cats and rats indicate that fluid flow in the urethra evokes firing in pudendal sensory nerve fibers innervating the urethra [2]. This sensory signal, in turn, initiates bladder contractions in the quiescent bladder and augments ongoing contractions in the active bladder [6-8]. This urethro-bladder reflex is preserved following spinal transection and mediated by pudendal nerve afferent inputs to the sacral spinal cord and efferent outputs in the pelvic and pudendal nerves [6-7].
Electrical stimulation of the urethral
sensory branch of the pudendal nerve also leads to excitation of the bladder
[3-5] and prolonged stimulation leads to long-term augmentation of the
urethro-bladder reflex [4]. Shefchyk and
Buss [5] recently reported that this effect was preserved following acute
spinal transection, however the bladder pressure increases were relatively weak
at 1 hour post transection, when spinal shock may have been present. The sustained pressures evoked in the current
experiments (Figure 2) were less than that obtained pre-transection, yet
substantially greater than that reported in [5]. Bladder volume and pressure were limited by
leakage around the urethral catheter during filling and during elicited bladder
contractions.
These results suggest that the neuronal circuitry required for reflex bladder contractions exist within the spinal cord, may be accessed via urethral afferents, and that a direct descending pathway is not required for reflex bladder activation.
The excitatory urethro-bladder reflex is strongly dependent on bladder volume. At low bladder volumes, urethral sensory stimulation does not evoke a bladder contraction and evokes a strong increase in urethral sphincter activity. At higher bladder volumes, urethral stimulation elicits bladder contractions and relaxation of the urethral sphincter. This response has been shown with fluid stimulation [7] and electrical stimulation [3].
Low frequency (<5 Hz) stimulation produced robust bladder contractions, while higher frequency (>10 Hz) stimulation had no effect or produced inhibition of the bladder. Bladder inhibition may be a manifestation of the inhibitory urethro-vesical reflex mediated by urethral c-fibers [9], which may be activated by higher amplitude or higher frequency stimulation.
Previous data indicate that electrical stimulation of the genital branch (dorsal penile nerve or clitoral nerve) of the pudendal nerve or the whole pudendal nerve [10] can inhibit or abolish hyper-reflexive bladder contractions [10]. The inhibitory response evoked by genital nerve stimulation appears to be stronger than the excitatory evoked by urethral sensory branch stimulation. Mechanical stimulation of the penis eliminated bladder contractions elicited by urethral sensory branch stimulation as shown in Figure 3. These results suggest that selective activation of the urethral sensory branch without activation of the genital branch must occur to elicit bladder contractions.
Control of bladder activity via urethral afferent stimulation may be a valuable rehabilitation tool for individuals with SCI, and therefore warrants additional investigation.
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