Frequency-dependent selection of reflexes by pudendal afferents in the cat

Abstract

Activation of urethral or genital afferents of the pudendal nerve can elicit or inhibit micturition, and low frequency stimulation of the compound pudendal nerve (PN) is known to produce a continence response. The present study demonstrates that PN stimulation also can elicit a micturition-like response and that the response to PN stimulation is dependent on stimulation frequency. We measured the changes in bladder pressure and external urethral sphincter (EUS) electroneurogram (ENG) evoked by PN stimulation before and up to 16 h after spinal cord transection (SCT) in cats anaesthetized with alpha-chloralose. Low frequency (10 Hz) stimulation elicited a continence-like response, including inhibition of the bladder and activation of the EUS, but mid-frequency (33 Hz) stimulation produced a micturition-like response, including excitation of the bladder without activation of the EUS. The dependence of the response on stimulus frequency was linked to interpulse interval as the same number of pulses at 10, 33 and 100 Hz produced different responses. Stimulation of the PN at 33 Hz produced bladder contractions before and 8 h after SCT provided the bladder contained a minimum volume of fluid. Only mid-range frequency stimulation with sufficient stimulus train duration produced a reduction in EUS ENG activity before and after SCT. In addition to a continence-like response, PN stimulation can also elicit a micturition-like response, and this response is dependent on stimulation frequency, stimulus train duration, and bladder volume. The ability to control the two principal functions of the bladder by pudendal nerve stimulation is an exciting prospect for neurorehabilitation.

Figure 1. Bladder responses evoked by PN stimulation for 5 s (A, B and C) and 25 s (D, E and F) at stimulation frequencies 10 Hz (A and D), 33 Hz (B and E), and 100 Hz (C and F) in the same animal

Short duration stimulation at all frequencies produced bladder inhibition, but the bladder response to 25 s stimulus trains was dependent on stimulation frequency. Stimuli were trains of 100 μs, 900 μA pulses, and bladder volume was above the threshold for distension-evoked reflex contractions.

Figure 2. Bar graphs of the character of the bladder responses evoked by PN stimulation at 10 Hz (white bar), 33 Hz (black bar), and 100 Hz (grey bar) as a function of stimulus train duration

A, the percentage of trials where stimulation generated sustained bladder contractions depended on the stimulation frequency, stimulus train duration, and the interaction between frequency and duration. B, the percentage of trials in which PN stimulation inhibited the bladder depended on stimulation frequency and the interaction between stimulus frequency and train duration, but not on train duration alone. C, the percentage of trials where PN stimulation evoked neither a sustained bladder contraction nor sustained inhibition of the bladder was dependent on stimulus duration, but on neither stimulus frequency nor the interaction between stimulus frequency and train duration. D, bladder inhibition in those trials where PN stimulation was delivered during elevated bladder pressure. The numbers above the bars are the number of trials across cats. *Stimulus parameters evoking a significantly greater percentage of sustained bladder contractions (P < 0.05, non-parametric comparison based on Bonferroni inequalities (Gibbons, 1993). All stimulation amplitudes were between 2 × and 4 × PN_thr and all bladder volumes were above threshold for distension-evoked contractions.

Figure 3. Bladder pressure (top trace) and ENG from the contralateral deep perineal nerve evoked by PN stimulation

ENG from the contralateral deep perineal nerve evoked by PN stimulation is indicated with bars below the pressure and rectified and integrated ENG traces in A–C and arrows below the rectified ENG traces in D–K. A, EUS ENG increased above baseline and bladder pressure remained at baseline during PN stimulation at 10 Hz. B, EUS ENG activity increased initially during PN stimulation at 33 Hz but decreased to baseline as bladder pressure increased. C, EUS ENG activity increased initially during PN stimulation at 100 Hz and returned to baseline shortly after stimulation onset, and PN stimulation at 100 Hz had little influence on bladder pressure. D–F, stimulation evoked compound nerve action potentials (CNAPs) in the contralateral deep perineal nerve approximately 7–15 ms after the first pulse at the onset of stimulation at all frequencies (10, 33 and 100 Hz). Stimulus pulses at 10 Hz continued to evoke CNAPs 7.5 s after stimulation onset (G), but stimulation at 33 Hz (H) or 100 Hz (I) did not. A burst of EUS activity is observable in response to the 250th pulse of 10 Hz stimulation (J) but not in response to the 250th pulses of either 33 Hz (H) or 100 Hz (K) stimulation. Stimuli were 25 s trains of 100 μs, 900 μA pulses, and bladder volume was above the threshold for distension-evoked reflex contractions. Traces D–K show ENG responses to individual stimuli averaged over 3 sweeps. *The 250th pulse of 33 Hz stimulation occurs 7.5 s after stimulation onset; thus, trace H depicts the ENG response to both because they are the same pulse. The grey arrows (F, I and K) represent succeeding stimulus pulses shown in the 30 ms time frame because the interpulse interval of 100 Hz stimulation is 10 ms.

Figure 4. The mean and standard deviation of bladder pressure (A) and rectified and integrated ENG (B) from the contralateral deep perineal nerve evoked by PN stimulation as a function of interval during stimulation at 10 Hz, 33 Hz and 100 Hz

A, stimulation at 10 Hz decreased bladder pressure during all intervals greater than 1 s (P > 0.05 for each interval, Wilcoxon test, n = 18 for each interval across 3 cats with a minimum of 5 trials per interval per cat), while 33 Hz stimulation evoked progressively larger increases in bladder pressure during each interval > 1 s. Stimulation at 100 Hz evoked no significant change from baseline pressure during any interval. B, stimulation at all frequencies elicited an initial increase in ENG during the first second of stimulation which remained above baseline during all intervals of stimulation at 10 Hz, but progressively decreased during 33 Hz stimulation, and dropped to baseline in response to 100 Hz stimulation. Stimuli were 20–40 s trains of 100 μs pulses at 2–4 × PN_thr, and bladder volume was above the threshold for distension-evoked reflex contractions. *Statistically significant increase or decrease from baseline with P < 0.05. †Significant difference from pressures produced by other stimulation frequencies during the same interval of stimulation with P < 0.05.

Figure 5. Bladder pressure and rectified and integrated ENG from the deep perineal nerve evoked by PN stimulation 8 h after SCT

PN stimulation is indicated with bars below the pressure and ENG traces. A, PN stimulation at 10 Hz generated an increase in deep perineal nerve ENG, which remained above baseline while bladder pressure remained at baseline. B, PN stimulation at 33 Hz initially produced an increase in ENG activity, but the ENG activity decreased below baseline as bladder pressure increased and remained elevated. C, PN stimulation at 100 Hz also increased ENG activity initially, and the ENG activity remained above baseline as bladder pressure decreased transiently. Stimuli were 15 s trains of 100 μs, 900 μA pulses, and bladder volume was above the threshold for distension-evoked reflex contractions prior to spinal transection.

Figure 6. The mean and standard deviation of bladder pressure (A) and rectified and integrated ENG (B) from the contralateral deep perineal nerve evoked by PN stimulation after acute spinalization as a function of interval during stimulation at 10 Hz, 33 Hz, and 100 Hz

A, 10 Hz stimulation did not elicit a significant change in bladder pressure during the initial intervals and only evoked a small increase in bladder pressure during the successive intervals. However, 33 Hz stimulation evoked progressively larger increases in bladder pressure during each interval > 1 s. Stimulation at 100 Hz produced small increases in bladder pressure during each interval > 1 s. B, stimulation at all frequencies elicited an initial increase in ENG during the 1st second of stimulation, which remained above baseline during all intervals of stimulation at 10 Hz, but decreased below baseline during 33 Hz stimulation and dropped to baseline in response to 100 Hz stimulation. Stimuli were 20–40 s trains of 100 μs pulses at 2–4 × PN_thr, and bladder volume was above the threshold for distension-evoked reflex contractions prior to SCT. *Statistically significant increase or decrease from baseline with P < 0.05. †Significant difference from pressures produced by other stimulation frequencies during the same interval of stimulation with P < 0.05.