Selective suppression of sphincter activation during sacral anterior nerve root stimulation

The purpose of this work was to electrically activate small-diameter motor fibers in the sacral anterior roots innervating the urinary bladder, without activating the large-diameter fibers to the sphincter. Quasitrapezoidal current pulses were applied through tripolar spiral nerve electrodes on selected anterior sacral roots during acute experiments on eight dogs, maintained under pentobarbital anesthesia. Pressures were recorded from the bladder and sphincter with catheter-mounted gauges. Stimulation with biphasic quasitrapezoidal pulses showed decrease in sphincter recruitment with increasing pulse amplitudes. The minimum current amplitude that resulted in maximum sphincter suppression was used to stimulate the roots with trains of 20 Hz pulses, with 60 mL of saline filling the bladder. Pressures were also recorded when 100 micros rectangular pulse trains at 20 Hz, both continuous and intermittent, were applied. Trains of stimuli were applied before and after dorsal root rhizotomy. Suppression of sphincter activation was defined to be a percentage, [(Maximum pressure -Minimum pressure)/Maximum pressure x100. The results from 22 roots in eight animals show that with single pulses, the average percentage suppression of sphincter activation was 76.3% (+/-14.0). The minimum current for maximum sphincter suppression was 1.29 mA (+/-0.62). The average bladder pressure evoked was 50 cm of water during pulse train stimulation, with no significant difference due to pulse type. With pulse trains, the sphincter pressures were significantly higher when the bladder was filled. Evacuation of fluid occurred in three animals with average flow rates of 1.0 mL/s.     

Reduction of bladder outlet resistance by selective stimulation of the ventral sacral root using high frequency blockade: a chronic study in spinal cord transected dogs.

PURPOSE: The use of electrical neural stimulation as treatment for neurogenic bladder is complicated by simultaneous contraction of the striated sphincter. This result is due to the composition of the ventral sacral roots, which contain somatic fibers innervating the external urethral sphincter and preganglionic parasympathetic fibers innervating the detrusor, among others. The somatic fibers have a larger caliber than the parasympathetic fibers and, as large diameter fibers, need a lower stimulus amplitude for activation than smaller fibers. Activation of the smaller fibers is always accompanied by activation of the larger fibers. We studied the effect of chronic application of selective combined low and high frequency current stimulation of the ventral sacral root on bladder evacuation and urethral resistance in chronically spinalized (spinal cord transected above the spinal micturition center) male dogs for daily bladder evacuation. MATERIALS AND METHODS: A total of 14 male mongrel dogs weighing 20 to 25 kg. underwent transection of the spinal cord at the T10 vertebra. The S2 ventral sacral root was wrapped with a bipolar electrode connected to a subcutaneous microstimulator. Daily bladder evacuation by neural stimulation with determination of the voided and residual urine volumes was done for 8 months. The animals were stimulated only with low frequency current during the 1-month spinal shock phase. Selective parameters of combined low frequency stimulation and high frequency blockade currents were subsequently applied for 6 months. For the last study month the animals were again stimulated by low frequency current alone. Bladder and urethral pressure as well as electromyography of the external urethral sphincter and pelvic floor muscles were evaluated monthly. Histopathological testing of the chronically stimulated nerve and external sphincter was performed. RESULTS: Of the 14 dogs 12 completed the followup study. During the shock phase with the application of low frequency only stimulation the animals voided 26% of mean functional bladder capacity with the early return of detrusor activity. Mean detrusor pressure plus or minus standard error of mean was 76.4 +/- 21.6 cm. water, while mean urethral pressure was 83.6 +/- 16.8 cm. water. During the application of selective combined low and high frequency stimulation 7 animals (58%) evacuated the bladder completely with post-void residual urine less than 10% of mean individual functional bladder capacity and 5 (42%) had mean post-void residual urine less than 20% of mean individual bladder capacity. Mean detrusor pressure was 73.5 +/- 20 cm. water and mean urethral pressure was 44 +/- 7.3 cm. water. There was a 45.19% reduction in the mean electromyography activity of the external sphincter. Stimulation of the dogs for the last month by low frequency current resulted in the voiding of 33% of mean bladder capacity with an increase in mean urethral pressure and electromyography activity. CONCLUSIONS: These results of reducing external sphincter activity by the simultaneous application of high frequency current blockade of the somatic fibers and activation of the autonomic fibers of the bladder by low frequency stimulation are promising.     

Selective activation of the sacral anterior roots for induction of bladder voiding

AIM: We investigated the efficacy of selective activation of the smaller diameter axons in the sacral anterior roots for electrically induced bladder voiding. MATERIALS AND METHODS: Acute experiments were conducted in five adult dogs. The anterior sacral roots S2 and S3 were implanted bilaterally with tripolar electrodes. Pressures were recorded from the bladder and from the proximal urethra and the external urethral sphincter. A detector and flow meter monitored fluid flow. A complete sacral dorsal rhizotomy was carried out. The effects of two types of pulse trains at 20 Hz were compared; quasitrapezoidal pulses (500 microsec with 500 microsec exponential decay) and interrupted rectangular (100 microsec, 2 sec on/2 sec off). Before rhizotomy, rectangular pulse trains (100 microsec) to activate all fibers were also applied. The experimental design was block randomized before and after rhizotomy. RESULTS: Quasitrapezoidal pulses showed block of sphincter activation with average minimum current for maximum suppression of 1.37 mA. All pulse types evoked average bladder pressures above the basal sphincter closure pressure. The pressure patterns in the proximal urethra closely followed the bladder pressures. Before dorsal rhizotomy, stimulation evoked a superadded increase in sphincter pressures with slow rise time. After rhizotomy, the sphincter pressure patterns followed the bladder pressures during selective activation and voiding occurred during stimulation with quasitrapezoidal trains and in between bursts with interrupted rectangular stimulation. CONCLUSIONS: Selective activation of sacral ventral roots combined with dorsal rhizotomy may provide a viable means of low-pressure continuous voiding in neurological impairment.     

Selective block of urethral sphincter contraction using a modified Brindley electrode in sacral anterior root stimulation of the dog

AIMS: Patients with spinal cord injury often present with dysfunction of urinary bladder and urethral sphincter. One treatment option is sacral rhizotomy and sacral anterior root stimulation with the Finetech Brindley stimulator. However, a major disadvantage is the lack of selective stimulation, resulting in simultaneous contraction of sphincter and bladder followed by unphysiological micturition. This study investigated the possibility of selective bladder stimulation by using a Brindley electrode. METHODS: In 11 male anaesthetized foxhounds, a complete posterior rhizotomy was perormed. The anterior S2 roots were stimulated with different quasi-trapezoidal (QT) pulses (pulse length range, 600-1,400 microsec; stimulation current, 0.1-2.0 mA; frequency, 20 Hz) by using a tripolar Brindley electrode. Sphincter and bladder pressures were measured urodynamically. RESULTS: All 11 animals showed a maximal reduction of the highest sphincter pressure over 80%, and in 6 of 11 trials, the sphincter pressure was inhibited completely (100%). With stimulations at maximal sphincter blockade, the average achievable bladder pressure was 33.48 cm H(2)O higher than the average sphincter pressure, and in three cases, a strong micturition was observed. Selective blockade of the sphincter was possible by applying QT pulses. The bladders remained uninfluenced by this blockade and kept their excitability at any time. CONCLUSIONS: This study shows that selective bladder stimulation with little or no coactivation of the sphincter is possible. A physiological micturition can be achieved by using a tripolar Brindley electrode. Introduction of this stimulation technique into clinical practice should not face major difficulties, considering that the device is an established electrode.     

Site of deafferentation and electrode placement for bladder stimulation: clinical implications.

Based on the clinical experience of treating neurogenic bladders by the electrical stimulation of the ventral sacral roots, neuroanatomical and neurophysiologic studies were designed to study the mechanism of detrusor-sphincter dyssynergia during electrical stimulation of the sacral roots. An experimental model was developed to decrease the stimulation response of the pelvic floor and external urethral sphincter muscles while preserving bladder contraction. The significance of the site of deafferentation and electrode implantation was evaluated under functional and clinical aspects. Our results indicate that a combination of intradural deafferentation and extradural electrode implantation may offer maximal deafferentation efficiency with minimal surgical risk. Intradural deafferentation is facilitated by a consistent arrangement of sacral roots with the dorsal roots running laterally to the ventral roots at the site of their exit from the dura. Detrusor-sphincter dyssynergia can be reduced by selective division of ventral sacral rootlets innervating the striated musculature of the pelvic floor and the urethral sphincter.     

Urethral pressure profiles in the female canine implanted with sacral anterior nerve root electrodes

 The purpose of this work was to study the pressure distribution along the urethra in female canines with and without electrical stimulation of the sacral anterior roots innervating the bladder and urethra. Urethral pressure profiles were recorded in two orientations, dorsal and ventral, with microtransducer catheters. Two pulse types were applied at 1 Hz, 500 μs quasitrapezoidal pulses to selectively activate the small axons and 100 μs rectangular pulses. Four parameters were measured from each profile; maximum urethral pressure (MUP), bladder pressure (Pv), functional urethral length (FUL), and the position of the maximum peak from the bladder neck (PMP). Two derived measures, the estimated maximum urethral closing pressure (UCP) and the position of the maximum as a percentage of the FUL (PM%) were calculated. There were highly significant differences (P<0.01) in the value and position of the estimated UCP in the two orientations of the transducers. The highest pressures were recorded in the ventral orientation near the terminal portions of the urethra. Principal sphincter activity during electrical stimulation of the ventral sacral roots was also confined to this part. Selective small fiber activation did not result in any significant increase in this peak pressure from passive values.     

Selective Detrusor Activation by Electrical Stimulation of the Human Sacral Nerve Roots

Abstract: The purpose of this study was to investigate the feasibility of selective detrusor activation without activation of the urethral sphincter by sacral root stimulation in patients. The sacral roots were stimulated using a tripolar electrode. An anodal block was used to prevent the urethral sphincter from contraction. Using square current pulses (700 μs, 6–7 mA), no increase in intraurethral pressure was measured, while a normal increase in intravesical pressure occurred. The minimum pulse duration to obtain a complete block was 550 μs. The study shows that anodal blocking of action potentials is possible in humans and can result in selective detrusor activation when used in sacral root stimulation.     

Functional magnetic stimulation of the spinal cord--a urodynamic study in healthy humans

AIMS: To study the effects of functional magnetic stimulation of the spinal cord in healthy subjects on somatic and autonomic pathways innervating the anal and the external urethral sphincter, bladder, bladder neck, and rectum. METHODS: Eight healthy male volunteers gave their written informed consent and underwent functional magnetic stimulation of the thoracolumbar and sacral spinal cord. A two-channel microtip pressure transducer catheter was placed rectally measuring the abdominal and anal sphincter pressure. A three-channel microtip pressure transducer catheter was inserted into the urethra measuring the bladder, the bladder neck, and the external urethral sphincter pressure. A comprehensive protocol of single and repetitive magnetic stimulations was performed. Frequency, location, and duration of stimulation were varied while the intensity of stimulation was adapted to the maximum the subjects could tolerate. In four subjects, the degree of bladder filling was changed and the protocol was repeated when the subjects reported a full bladder and desire to void. RESULTS: Continuous magnetic stimulation of the thoracolumbar spinal cord and the sacral roots applied with different frequencies (5, 15, 30, 60, 100 Hz) and different duration of stimulation (10, 30, 120 sec) evoked sphincter contraction of both anal and urethral sphincters. The stimulation could not evoke contractions of the bladder, the bladder neck, or the rectum. Also with filled bladder and present desire to void, the magnetic stimulation could not activate autonomic pathways innervating these structures. CONCLUSIONS: Considering our results, we suggest that in individuals with preserved sensibility magnetic stimulation of the spinal cord with intensities below the pain threshold is ineffective in activating autonomic nerve fibres innervating bladder, bladder neck, and rectum.     

Role of the striated urethral sphincter in the voiding cycle of the decerebrated dog

The role of the striated urethral sphincter on the dynamics of micturition was studied using 4 decerebrated dogs. The voiding cycles were analyzed by pressure flow EMG and pressure flow plot studies before and after the administration of suxamethonium. In the control condition, highly reproducible reflex micturition with bladder contraction and spasmodic rhythmic sphincter contractions was demonstrated. After the administration of suxamethonium reflex micturition occurred, but there was decreased bladder emptying as well as absence of spasmodic rhythmic sphincter contractions. The striated urethral sphincter would seen to play an important role in bladder emptying of decerebrated dogs.     

Feasibility of controlled micturition through electric stimulation.

Historically, man has been aware of bioelectric phenomena for some 4,000 years. Yet it has only been during the last 20 years that technology has advanced to the stage where controlled bladder emptying has become feasible. A great deal of interest followed the introduction of transistor and bladder stimulation via the principle of radio frequency induction. Spinal cord, sacral, and pelvic nerve and direct bladder stimulation have all been attempted. Only direct bladder stimulation in lower motor neuron situations has shown any promise. The many difficulties associated with bladder stimulation include simultaneous sphincter contraction, pain, electrode and insulation difficulties, and fibroplasia due to movement of electrodes placed in pliable tissues. In addition, the role of the prostate, increased urethral length, and erection responses in the male have received little investigation. These problems are outlined and experimental observations of attempts to achieve controlled micturition in canines areresented. These studies were carried out over a 3-year period, and emphasize responses to stimulation of the spinal cord and sacral roots. It was concluded that the most efficient manner by which to effect simulated micturition is via stimulation of the ventral sacral root dominant for bladder responsiveness, and combine this with selective division of somatic fibers of only the root being stimulated.     

Pelvic nerve innervation of the external sphincter of urethra as suggested by urodynamic and horse-radish peroxidase studies

In view of the fact that the detrusor vesicae and external urethral sphincter perform closely synergic functions in micturition, experiments were conducted to explore the action of the pelvic efferent neurons on the external urethral sphincter. The pelvic efferent neurons are generally recognized, by urodynamic assessments and histochemical study with the technique of retrograde axonal transport of horse-radish peroxidase, to innervate the vesical detrusor. In 7 of 15 adult dogs studied, the external urethral sphincter continued to show a normal synergic electromyogram pattern with enhanced electrical activity on vesical distention and disappearance of discharges on vesical contraction even after bilateral transection of the pudendal nerves. The electrical discharges ceased in the sphincter only after subsequent bilateral pelvic neurotomy. Horse-radish peroxidase-positive cells were demonstrated in the intermediolateral and intermediomedial nuclei and in the Onuf nucleus of the sacral cord in approximately half the dogs whose pelvic nerve was injected with the plant peroxidase. The results suggest that the pelvic nerve may contain somatic fibers innervating the external urethral sphincter.     

Electrical stimulation induced sphincter fatigue during voiding.

The stimulation of the sacral nerves to induce voiding is often associated with simultaneous contraction of the striated sphincter rendering micturition difficult or impossible. Rhizotomy of some sacral nerves was found to be necessary to facilitate voiding with stimulation. An main objective in the present experiment was to evaluate the feasibility of achieving the same result using electrical stimulation to fatigue the sphincter. In order to compare the effect of rhizotomy and fatiguing striated sphincter, the bladder outlet resistance was measured. S2 nerves were stimulated with 3 V, 35 Hz and 100 microseconds duration for 5 to 10 sec. Following S2 nerves stimulation the pudendal nerve was stimulated till we obtained sphincteric fatigue. The optimal parameter to induce sphincter fatigue were 3 V, 100-500 Hz and 100 microseconds. for 15-20 sec. The combined pressure-flow studies showed that fatiguing the sphincter via the pudendal nerve using these parameters was as good as cutting it in achieving bladder emptying with stimulation.     

Control and coordination of bladder and urethral function in the brainstem of the cat

Stimulation and lesion experiments in the pontine tegmentum of 34 cats, with urodynamic measurements both pre- and peroperatively and during up to 4 months of follow-up, have confirmed the existence of two different brainstem regions concerned with lower urinary tract function. In the dorsolateral pontine tegmentum, a compact, dorsomediolaterally located M region, corresponding to Barrington's micturition centre, reacts specifically to electrical stimulation, causing prompt relaxation of the anal and urethral striated sphincters followed after about 2 s by detrusor contraction, as in normal micturition. Bilateral lesions in this M region lead to a 2–9 week period of urinary retention, during which detrusor activity is depressed and the bladder capacity increases. Stimulation in a larger, more diffuse, more laterally located L region elicits sphincter activity: contraction of the striated urethral sphincter together with an increase in the anal-sphincter EMG, or relaxation of the striated urethral sphincter together with either a decrease or an increase in the anal-sphincter EMG. Provided lethal respiratory complications can be avoided, bilateral lesions in this L region lead after a few days to a state, lasting up to 2 months, in which there is urinary incontinence accompanied by a decrease in the bladder capacity and detrusor overactivity. Neither type of lesion leads to true detrusor-urethra dyssynergia. However, because the amplitude of anal-sphincter EMG sometimes increases when the striated urethral sphincter relaxes, observations of the anal-sphincter EMG can misleadingly suggest dyssynergia. Brainstem mapping of the results of stimulation suggests a motor pathway running from the M to the L region and another descending caudally from the L region. The observations suggest that the M region forms a true micturition centre, facilitating the detrusor voiding contraction and also (via the presumptive connection with the L region) ensuring synergic sphincter relaxation. The L region appears not only to relay this voiding sphincter relaxation, but to be responsible for control of the pelvic floor and its sphincters in general, and for helping to maintain urinary continence.     

Effect of cerebellectomy on reflex micturition in the decerebrate dog as determined by urodynamic evaluation

The effect of cerebellectomy on reflex micturition in the decerebrate dog was investigated by cystometric and striated urethral sphincter EMG studies. Before and after cerebellectomy, reflex micturition consisting of bladder contraction and spasmodic contraction of the striated urethral sphincter occurred when a critical degree of filling was reached. Cerebellectomy showed no influence on the striated urethral sphincter EMG activity. However, cerebellectomy produced a significant decrease in threshold volume and threshold pressure during the collecting phase, and also in the contraction pressure and voided volume of the emptying phase. The present study suggests that the cerebellum plays an inhibitory role in the collecting phase and a facilitatory role in the emptying phase during the entire reflex micturition cycle of the decerebrate dog. Further study will have to be done concerning the neurotransmission mechanism that causes these different effects in the collecting and emptying phases.     

Abnormalities of the innervation of the urethral striated sphincter musculature in incontinence

Perineal nerve and transcutaneous spinal cord stimulation have been used to study 17 patients with idiopathic neurogenic faecal incontinence, 12 of whom also had urinary incontinence. Significant increases in spinal, perineal and pudendal nerve motor latencies were demonstrated in all 17 patients. These results suggest that there is damage to the nerves innervating both the urethral and perianal sphincter musculature in these patients, including those with isolated faecal incontinence. There was evidence of both a distal (perineal nerve) and a proximal (sacral root) component to the damage to the nerve supply of the urethral striated sphincter muscle in half of the patients.     

Overactive bladder inhibition in response to pelvic floor muscle exercises

A recent study by the senior author demonstrated that striated urethral sphincter contraction effected the inhibition of vesical contraction and suppression of the desire to micturate, an action suggested to be mediated through the "voluntary urinary inhibition reflex". We hypothesized that the effect of pelvic floor muscle (PFM) exercises on the overactive bladder was mediated through this reflex action. The current communication investigates this hypothesis. A total of 28 patients (mean age 44.8+/-10.2 years, 18 men, 10 women) with overactive bladder and 17 healthy volunteers (mean age 42.6+/-9.8 years, 12 men, 5 women) were enrolled in the study. The vesical and posterior urethral pressures were determined before and after vesical filling reached the volume at which urge in control subjects, and involuntary voiding in the patients, occurred. Intra-abdominal pressure was recorded to obtain detrusor pressure readings. The bladder was refilled to the above volume and the subject asked to hold PFM contractions for 10 s during which the vesical and posterior urethral pressures were recorded. In healthy volunteers, the mean detrusor and posterior urethral pressures at urge to void were 30.6+/-4.8 SD and 18.7+/-3.3 cm H(2)O, respectively. On PFM contraction, the detrusor pressure declined to 11.6+/-1.4 cm H(2)O (P<0.01) and urethral pressure increased to 139.8+/-17.4 cm H(2)O (P<0.001). In patients, the mean detrusor and posterior urethral pressure readings when the bladder was filled to the volume which induced involuntary incontinence, were 28.2+/-4.2 and 17.3+/-3.4 cm H(2)O, respectively; on PFM contractions, the detrusor pressure decreased to 10.6+/-2.1 cm H(2)O (P<0.01), while urethral pressure increased to 86.6+/-7.9 cm H(2)O (P<0.001) and voiding did not occur. In conclusion, PFM contractions led to a decline of detrusor and increase of urethral pressures and suppressed the micturition reflex. These contractions appear to induce their effect by preventing internal sphincter relaxation produced by the micturition reflex. Failure of the internal sphincter to relax seems to cause reflex detrusor relaxation, an action presumably mediated through the "voluntary urinary inhibition reflex". The results of the current study encourage the treatment of overactive bladder with PFM contractions.     

Urinary bladder control by electrical stimulation: Review of electrical stimulation techniques in spinal cord injury

Evacuation of urine in paraplegics without the need for catheters would be possible when voiding could be induced by eliciting a bladder contraction. A challenging option to obtain detrusor contraction is electrical stimulation of the detrusor muscle or its motor nerves. This article reviews the 4 possible stimulation sites where stimulation would result in a detrusor contraction: the bladder wall, the pelvic nerves, the sacral roots, and the spinal cord. With respect to electrode application, sacral root stimulation is most attractive. However, in general, sacral root stimulation results in simultaneous activation of both the detrusor muscle and the urethral sphincter, leading to little or no voiding. Several methods are available to overcome the stimulation-induced detrusor-sphincter dyssynergia and allow urine evacuation. These methods, including poststimulus voiding, fatiguing of the sphincter, blocking pudendal nerve transmission, and selective stimulation techniques that allow selective detrusor activation by sacral root stimulation, are reviewed in this paper.     

Micturition in thoracic spinal cord injured cats with autografting of the adrenal medulla to the sacral spinal cord.

PURPOSE: The role of noradrenergic projection from the pontine micturition center to the sacral spinal cord during micturition was examined in thoracic spinal cord injured cats after autografting the adrenal medulla to the sacral spinal cord. MATERIALS AND METHODS: In 13 female cats the lower thoracic cord was transected and the right adrenal gland was removed under halothane anesthesia. The resected adrenal medulla was divided into several small pieces, which were subsequently autografted to the sacral spinal cord in 7 cats. Another 6 cats underwent sham operation and served as controls. Continuous cystometry and electromyography of the external urethral sphincter were performed every 2 weeks postoperatively without anesthesia. At week 8 the sacral spinal cord was removed and immunohistochemical testing was done to assess tyrosine hydroxylase immunoreactivity. RESULTS: At week 6 the relative mean duration of detrusor-external sphincter coordination plus or minus standard error during bladder contraction was 62.4% +/- 4.9% in adrenal grafted cats, which was significantly (p = 0.0485) longer than in controls (34.2% +/- 12.6%). However, maximum bladder contraction pressure, bladder contraction duration and post-void residual urine volume were not significantly different in the 2 groups. Tyrosine hydroxylase immunoreactive cells were observed in and on the sacral spinal cord in adrenal grafted animals but not in controls. CONCLUSIONS: Autografting the adrenal medulla to the sacral spinal cord prolonged detrusor-external sphincter coordination during bladder contraction in thoracic spinal cord injured cats, although other urodynamic parameters did not change. Therefore, noradrenergic projections to the sacral spinal cord may relax the external urethral sphincter during bladder contraction.     

Innervation of the female human urethral sphincter: 3D reconstruction of immunohistochemical studies in the fetus

OBJECTIVES: The precise location, origin and nature of nerve fibers innervating the urethral sphincter have not been clearly established. Classical anatomical studies based on cadaver dissections have provided conflicting results concerning the location of pudendal and autonomic nerve fibers. This study was designed to identify nerve fibers innervating the urethral sphincter and to provide a three-dimensional representation of their tissue relations in the female human fetus. MATERIALS AND METHODS: Histology and immunohistochemistry (Masson's Trichromic, Luxol Fast Blue, Protein S 100 immunostaining and smooth fiber actin immunostaining) were performed on the external urethral sphincter of ten female fetuses with a crown-rump length of 112 to 340mm. Three-dimensional reconstructions of the urethral structure and innervation were obtained from serial sections using Surf Driver 3.5.3 software (David Moody and Scott Lozanoff). RESULTS: Three-dimensional reconstructions of the same sections with different stains demonstrated the precise structure of the muscle layers (smooth and striated muscle fibers) and nerve fibers (myelinated and unmyelinated) and their relations with the urethra and vaginal wall. The proximal third consisted of a circular smooth muscle sphincter, the middle third consisted of two circular layers of smooth and striated muscle fibers and the distal third consisted of a circular layer of smooth muscle fibers surrounded by an omega-shaped layer of striated muscle fibers. In the proximal third of the urethral sphincter, myelinated fibers were identified running with unmyelinated fibers from the pelvic plexus. These fibers were closely related to the lateral and anterior aspects of the vagina. Unmyelinated fibers entered the smooth muscle part of the sphincter at 4 o'clock and at 8 o'clock. Most myelinated fibers entered the sphincter at 3 o'clock and at 9 o'clock. CONCLUSION: Histological and immunohistochemical three-dimensional reconstruction of the anatomical structures of the urethral sphincter provides a better understanding of the origin and nature of the Innervation participating in urinary continence. It provides a very informative view of the three-dimensional arrangement of sphincter muscle layers.     

Urethral sphincteric responses to sacral root stimulation.

Deafferentation and stimulation of sacral nerves in patients with complete spinal cord lesions is a good model to study the parasympathetic influence on urethral behavior. During intradural sacral deafferentation from S2 to S4/S5 in preparation for implantation of the Finetech-Brindley anterior sacral root stimulator, sacral roots were stimulated with 3 and 30 Hz and 3 and 10 V and the bladder and urethral responses noted. Stimulation of the posterior roots at 3 Hz gives a urethral relaxation without bladder contraction. At this frequency, anterior sacral root stimulation has no effect on bladder nor urethral pressure. Stimulation of the anterior roots at 30 Hz produces simultaneous bladder contraction and urethral relaxation. These findings support the presence of an afferent parasympathetic inhibitory mechanism in the spinal micturition center in the human.