Membrane area and dendritic structure in type‐identified triceps surae alpha motoneurons

Abstract

The size and branching structure of the dendritic tree were studied in nine type‐identified triceps surae α‐motoneurons that were labeled intracellularly with horseradish peroxidase and reconstructed from serial sections in the light microscope. The average total membrane area (A N) for motoneurons of type S (slow‐twitch) motor units was about 22% smaller than A N for cells of type F units (including both FF and FR motor unit types in this category) (480.1 × 10 3 μml;m 2 vs. 617.7 ×10 3 μm 2, respectively). Systematic correlations were found between stem dendrite diameter and three measures of dendritic size: dendrite membrane area, combined dendritic length, and number of terminations. All of these correlations were significantly different for the dendrites of F and S montoneurons. Power‐function relations between stem diameter and dendritic membrane area were used to estimate A N for a sample of 79 type‐identified motoneurons. Mean estimated A N values were significantly different for the F and S motoneuron groups, despite a large overlap in A N values between these groups. The branching structure of dendrites of F and S motoneurons also showed clear differences. Type S motoneuron dendrites showed less‐profuse branching and a more‐even radial distribution of branch points than found in type F cells. Examination of two forms of the “3/2 power rule” for the relation between the diameters of parent and daughter dendritic branches at branch points showed that the dendrites of type S motoneurons conform less well with the anatomical constraints necessary to represent binary branching trees as equivalent cylinders than do dendrites of type F cells. There was no systematic difference between F and S motoneuron dendrites in the degree of asymmetry of first‐order daughter trees. The results overall indicate that the dendrites of F and S motoneuron groups are structurally different, giving rise to a systematic difference in A N between these groups. Such structural differences suggest that the F and S groups of α‐motoneurons can be viewed as intrinsically distinct cell types and not just large vs. small variants of the same cell species.