An HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons

Abstract

The dimensions of the somata and stem dendrites of 57 α‐ and three γ‐ motoneurons, identified as to motor unit type and labeled by intracelluar injection of horseradish peroxidase, were measured in the triceps surae and plantaris motor pools. The somata of type S motoneurons tended to be smaller (mean diameter 47.9 μm) than those of FF and FR units (52.5 and 53.1 μm, respectively) but these mean values were not significantly different and the data distributions showed considerable overlap between the unit types. The mean numbers and diameters of stem dendrites of stem dendrites exhibited somewhat larger differences related to motor unit type and some of these were statistically significant. The total membrane area (A N) of each cell was estimated from measurements of the soma and stem dendrites, by using recent data of Ulfhake and Kellerth ('81) to calculate the membrane area of a dendritic tree from stem dendrite diameter. Mean A N varied with motor unit type in the sequence FF > FR > S (average values: 369 × 100 3 μm 2, 323 × 100 3 μm 2, and 250 × 100 3 μm 2, respectively). There was covariation between A N and the conduction velocity of the motor axon, as well as with the force output from the muscle unit. Comparison of A N and motoneuron input resistance (R N) in 19 α‐motoneurons suggested that the specific resistivity of the cell membrane in type S motoneurons was systematically higher than that characteristic of type FF or FR motoneurons.