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

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 (AN) for motoneurons of type S (slow‐twitch) motor units was about 22% smaller than AN for cells of type F units (including both FF and FR motor unit types in this category) (480.1 × 103μml;m2 vs. 617.7 ×103 μm2, 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 AN for a sample of 79 type‐identified motoneurons. Mean estimated AN values were significantly different for the F and S motoneuron groups, despite a large overlap in AN values between these groups.

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