Morphological analysis of external urethral and external anal sphincter motoneurones of cat

A previous electrophysiological study suggested that there is a common correlation among axonal conduction velocity, input resistance, and size of motoneurones regardless of the muscle type innervated by a motoneurone and that the dendritic arborization is less developed in sphincter than in hindlimb motoneurones. To verify the previous suggestions, cell bodies, axons, and dendrites of external urethral sphincter (EUS) and external anal sphincter (EAS) motoneurones were studied after intracellular labelling with horseradish peroxidase in cats anaesthetised with pentobarbitone. All the cell bodies were located in Onuf's nucleus. The soma diameter (19.7-50.0 microns) was positively correlated with the axonal conduction velocity, and the plots of these two variables were extrapolations from similar plots obtained for hindlimb alpha-motoneurones, supporting the suggestion of the preceding report. Half of motoneurones had recurrent axon collaterals, which terminated within Onuf's nucleus and the ventral border of lamina VIII. The diameter of the cell body with collaterals was significantly larger than that without collaterals. Dendrites extended in five directions: dorsal, medial, lateral, rostral, and caudal. Dorsally directed dendrites of both EUS and EAS motoneurones coursed in lamina VII toward the intermediolateral nucleus, and dendrites of EAS motoneurones further extended toward the intermediomedial nucleus. Long dendrites directed rostrally or caudally within Onuf's nucleus, more prominently in EUS motoneurones. EAS motoneurones had longer end branches and relatively larger number of end branches and summed length and surface area of a dendrite compared with EUS. Among relations, the dendritic surface area and the dendritic volume were tightly correlated with the diameter of the first-order dendrite. The latter relation was almost identical between EUS, EAS, and hindlimb alpha- and gamma-motoneurones. The large values for the dendritic-to-soma surface area ratio (31 in EUS, 50 in EAS) indicated the well-developed dendritic arborization comparable to hindlimb motoneurones. The ratio of sum sigma (daughter diameter)3/2 to the 3/2 power of the parent diameter was 0.99 in EUS and 1.08 in EAS motoneurones, indicating that the 3/2 power constraint at branching points is well satisfied, and Rall's equivalent-cylinder model is applicable to sphincter motoneurones.