Neuroscience Letters, 127 (1991) 87-90
87
Elsevier Scientific Publishers Ireland Ltd. ADONIS 0304394091002772 NSL 07794
Membrane electrical properties of external urethral and external anal sphincter somatic motoneurons in the decerebrate cat S. Hochman*, B. Fedirchuk and S.J. Shefchyk Departments of Medicine and Physiology, Faculty of Medicine, University of Manitoba, Winnipeg (Canada) (Received 9 January 1991; Revised version received 25 February 1991; Accepted 4 March 1991)
Key words: Pudendal; Motor unit; Continence; Micturition; Defecation Membrane electrical properties of motoneurons innervating the striated muscle of the external urethral and anal sphincters were examined in the decerebrate cat. Both populations of motoneurons had similar electrical properties (mean pooled values; conduction velocity 48 m/s, membrane time constant 3.3 ms, afterhyperpolarization (AHP) duration 97 ms, membrane input resistance 2.2 MI2, rheobase 3.3 hA, and threshold voltage 8.1 mV). Although a portion of cells in both subpopulations of sphincter motoneurons displayed subthreshold conductances (i.e. sag, anomalous rectification), the incidence was higher in the external urethral sphincter motoneurons. The sphincter motoneuron membrane properties were likened to 'slow' hindlimb motoneurons and the low rheobase values suggest that the sphincter motoneurons are easily recruited.
A significant component of bladder and bowel continence is achieved by the actions of the striated muscle of the external urethral and external anal sphincters. The sphincter muscles are innervated by a group of sacral ventral horn motoneurons referred to as Onuf's nucleus [21]. The present study describes a variety of membrane electrical properties of these sphincter motoneurons and is a necessary step in understanding the neural control of the sphincters. These data have been presented in abstract form I11]. Twelve male cats (2.5-3.5 kg) were used in the present study: 11 were decerebrated at the precollicular postmammillary level, one was anaesthetized with chloralose. Initial surgery was carried out using halothane anesthesia carried in a mixture of nitrous oxide and oxygen. Pudendal muscle nerve branches to the external urethral sphincter (EUS) and external anal sphincter (EAS) were cut and mounted on silver bipolar stimulating electrodes. A laminectomy exposed the $3 to L6 spinal cord segments. The pudendal nerves and the exposed spinal cord were bathed in mineral oil. Body temperature was maintained at about 38°C by radiant heat. Ca-
*Current address: Department of Physiology, University of Toronto, Toronto, Canada. Correspondence: S. Shefchyk, Department of Physiology, Room 421 Basic Medical Science Bldg., University of Manitoba, 770 Bannatyne Ave., Winnipeg, Canada R3E 0W3.
rotid artery blood pressure and expired C O 2 w e r e maintained within physiological limits and the cats were artificially respired following paralysis with gallamine triethiodide. The details of intracellular recording and the methods of estimating the selected membrane properties are described in Hochman and McCrea [12]. Motoneurons were impaled in the S1 spinal segment and antidromically activated from either the branch of the pudendal nerve innervating the EUS, EAS or the common pudendal nerve trunk. Only motoneurons with a stable action potential with heights >i 60 mV were included in the present study. Intracellular records were digitized at a rate of 15000 samples per second and averages were obtained from 16 to 256 traces. The membrane electrical properties examined were; whole cell input resistance, membrane time constant, equivalent cylinder electrotonic length, rheobase, threshold voltage (actual and calculated from the product of rheobase and input resistance), afterhyperpolarization (AHP) duration, AHP peak voltage, and axonal conduction velocity. Records were obtained from 31 motoneurons; 14 EUS, 15 EAS, and 2 from the common pudendal nerve. The mean and standard deviations of the action potential height in the EUS and EAS motoneuron subpopulations were similar (see Table I) thereby allowing a comparison of their membrane properties without concern for differences arising secondarily to electrode impalement injury [9, 12].
88 TABLE I SUMMARY AND COMPARISON OF THE ELECTRICAL PROPERTIES OF THE EUS AND EAS MOTONEURONS Mean values and S.D. are presented with sample sizes in brackets. All pudendal Action potential height (mV) Conduction velocity (m/s)
EUS
71+11 (31) 48+15 (31)
EAS
71__+10 (14) 47+14 (17)
74+11 (15) 50+16 (12)
Membrane time constant (ms) 3.3 _+1.2 (7) Input resistance (Mr2) 2.2 +0.7 (26) Electrotonic length 1.41_+0.20 (7)
2.1 +0.2* (3) 2.0 _+0.7 (13) 1.37_+0.05 (3)
4.4 _+0.7* (3) 2.5 _+0.5 (ll) 1.35+0.12 (3)
Rheobase(nA)
3.3 +1.9 (22) 8.1 +4.9 (19) 7.2 ±2.2 ([7)
3.1 +1.7 (10) 7.1 +3.0 (9) 5.2 _+1.4 (9)
3.6 _+2.1 (10) 8.9 +6.5 (8) 9.9 _+5.5 (8)
97+23 (25)
97+22
100+25 03)
Threshold voltage (mV) Rheobase × resistance (mV)
AHP duration (ms)
AHP peak voltage (mV)
1.9 _+1.0 (25)
(ll) 1.5 _+0.7 ([2)
2.2 _+1.0 (13)
*Statistically significant, P < 0.05; Student's t-test.
Table I presents the means and standard deviations for the various properties of EUS, EAS, and pooled sphincter motoneurons. The mean membrane properties of EUS and EAS motoneurons are statistically indistinguishable except for membrane time constant which was twice as long in EAS motoneurons (4.4 ms) than in EUS motoneurons (2.1 ms) (P
87
Elsevier Scientific Publishers Ireland Ltd. ADONIS 0304394091002772 NSL 07794
Membrane electrical properties of external urethral and external anal sphincter somatic motoneurons in the decerebrate cat S. Hochman*, B. Fedirchuk and S.J. Shefchyk Departments of Medicine and Physiology, Faculty of Medicine, University of Manitoba, Winnipeg (Canada) (Received 9 January 1991; Revised version received 25 February 1991; Accepted 4 March 1991)
Key words: Pudendal; Motor unit; Continence; Micturition; Defecation Membrane electrical properties of motoneurons innervating the striated muscle of the external urethral and anal sphincters were examined in the decerebrate cat. Both populations of motoneurons had similar electrical properties (mean pooled values; conduction velocity 48 m/s, membrane time constant 3.3 ms, afterhyperpolarization (AHP) duration 97 ms, membrane input resistance 2.2 MI2, rheobase 3.3 hA, and threshold voltage 8.1 mV). Although a portion of cells in both subpopulations of sphincter motoneurons displayed subthreshold conductances (i.e. sag, anomalous rectification), the incidence was higher in the external urethral sphincter motoneurons. The sphincter motoneuron membrane properties were likened to 'slow' hindlimb motoneurons and the low rheobase values suggest that the sphincter motoneurons are easily recruited.
A significant component of bladder and bowel continence is achieved by the actions of the striated muscle of the external urethral and external anal sphincters. The sphincter muscles are innervated by a group of sacral ventral horn motoneurons referred to as Onuf's nucleus [21]. The present study describes a variety of membrane electrical properties of these sphincter motoneurons and is a necessary step in understanding the neural control of the sphincters. These data have been presented in abstract form I11]. Twelve male cats (2.5-3.5 kg) were used in the present study: 11 were decerebrated at the precollicular postmammillary level, one was anaesthetized with chloralose. Initial surgery was carried out using halothane anesthesia carried in a mixture of nitrous oxide and oxygen. Pudendal muscle nerve branches to the external urethral sphincter (EUS) and external anal sphincter (EAS) were cut and mounted on silver bipolar stimulating electrodes. A laminectomy exposed the $3 to L6 spinal cord segments. The pudendal nerves and the exposed spinal cord were bathed in mineral oil. Body temperature was maintained at about 38°C by radiant heat. Ca-
*Current address: Department of Physiology, University of Toronto, Toronto, Canada. Correspondence: S. Shefchyk, Department of Physiology, Room 421 Basic Medical Science Bldg., University of Manitoba, 770 Bannatyne Ave., Winnipeg, Canada R3E 0W3.
rotid artery blood pressure and expired C O 2 w e r e maintained within physiological limits and the cats were artificially respired following paralysis with gallamine triethiodide. The details of intracellular recording and the methods of estimating the selected membrane properties are described in Hochman and McCrea [12]. Motoneurons were impaled in the S1 spinal segment and antidromically activated from either the branch of the pudendal nerve innervating the EUS, EAS or the common pudendal nerve trunk. Only motoneurons with a stable action potential with heights >i 60 mV were included in the present study. Intracellular records were digitized at a rate of 15000 samples per second and averages were obtained from 16 to 256 traces. The membrane electrical properties examined were; whole cell input resistance, membrane time constant, equivalent cylinder electrotonic length, rheobase, threshold voltage (actual and calculated from the product of rheobase and input resistance), afterhyperpolarization (AHP) duration, AHP peak voltage, and axonal conduction velocity. Records were obtained from 31 motoneurons; 14 EUS, 15 EAS, and 2 from the common pudendal nerve. The mean and standard deviations of the action potential height in the EUS and EAS motoneuron subpopulations were similar (see Table I) thereby allowing a comparison of their membrane properties without concern for differences arising secondarily to electrode impalement injury [9, 12].
88 TABLE I SUMMARY AND COMPARISON OF THE ELECTRICAL PROPERTIES OF THE EUS AND EAS MOTONEURONS Mean values and S.D. are presented with sample sizes in brackets. All pudendal Action potential height (mV) Conduction velocity (m/s)
EUS
71+11 (31) 48+15 (31)
EAS
71__+10 (14) 47+14 (17)
74+11 (15) 50+16 (12)
Membrane time constant (ms) 3.3 _+1.2 (7) Input resistance (Mr2) 2.2 +0.7 (26) Electrotonic length 1.41_+0.20 (7)
2.1 +0.2* (3) 2.0 _+0.7 (13) 1.37_+0.05 (3)
4.4 _+0.7* (3) 2.5 _+0.5 (ll) 1.35+0.12 (3)
Rheobase(nA)
3.3 +1.9 (22) 8.1 +4.9 (19) 7.2 ±2.2 ([7)
3.1 +1.7 (10) 7.1 +3.0 (9) 5.2 _+1.4 (9)
3.6 _+2.1 (10) 8.9 +6.5 (8) 9.9 _+5.5 (8)
97+23 (25)
97+22
100+25 03)
Threshold voltage (mV) Rheobase × resistance (mV)
AHP duration (ms)
AHP peak voltage (mV)
1.9 _+1.0 (25)
(ll) 1.5 _+0.7 ([2)
2.2 _+1.0 (13)
*Statistically significant, P < 0.05; Student's t-test.
Table I presents the means and standard deviations for the various properties of EUS, EAS, and pooled sphincter motoneurons. The mean membrane properties of EUS and EAS motoneurons are statistically indistinguishable except for membrane time constant which was twice as long in EAS motoneurons (4.4 ms) than in EUS motoneurons (2.1 ms) (P
