Membrane properties of external urethral and external anal sphincter motoneurones in the cat

1. Intracellular recordings were made from external urethral sphincter (EUS) and external anal sphincter (EAS) motoneurones in the cat spinal cord under pentobarbitone anaesthesia. EUS and EAS motoneurones were located in segments S1 and S2 in the lateral part of the ventral horn corresponding to column Y of Romanes in the cat or group X of Onuf in man. 2. The axonal conduction velocity of sphincter motoneurones, calculated from the latency of the antidromic action potential and the conduction distance, ranged from 16 to 80 ms-1, much slower than that of hindlimb motoneurones. The duration of the spike after-hyperpolarization (AHP) was in a similar range to that of hindlimb motoneurones. The antidromic latency, the duration of the action potential and the duration of the AHP were positively correlated with one another. 3. The input resistance ranged from 2.6 to 9.0 M omega and was positively correlated with the latency of the antidromic spike. The plots of input resistance versus conduction velocity in sphincter motoneurones were distributed around the extrapolated regression line determined for hindlimb motoneurones, indicating that there is a common correlation amongst conduction velocity, input resistance, and size of motoneurones regardless of the muscle type innervated by a motoneurone. 4. The regression line relating AHP duration and input resistance in sphincter motoneurones was quite different from that in hindlimb motoneurones in its slope, indicating that the AHP duration does not depend solely on the size of the motoneurone. 5. The voltage responses to injection of steps of hyperpolarizing current developed a time-dependent depolarizing 'sag' at higher current levels. The delay in onset and the time constant of decay of this depolarizing sag depended upon the peak amplitude of the hyperpolarizing response. The slope resistance in the I-V curve decreased in the hyperpolarizing direction in all neurones examined, indicating the existence of anomalous rectification analogous to the Q current IQ. In some sphincter motoneurones, an increase of slope resistance in the I-V curve was observed when the membrane was further hyperpolarized. 6. The membrane time constant was positively correlated with input resistance, suggesting that motoneurones with high input resistance have high specific membrane resistivity. The dendritic-to-soma conductance ratio (p) and electrotonic length of dendrite (L) were estimated according to Rall's model. p was smaller than that of hindlimb motoneurones, suggesting less developed dendritic arborization. L was similar to that of hindlimb motoneurones. 7. No differences were observed, in any of the above characteristics of motoneurones, between EUS and EAS.