Quantitative analysis of the dendrites of cat phrenic motoneurons stained intracellularly with horseradish peroxidase

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Abstract
All the dendrites (N = 37) generated by four phrenic motoneurons were analyzed following intracellular injection of horseradish peroxidase. The dendritic arbors produced from each of these stem dendrites were studied in detail. The mean number of stem dendrites produced by a phrenic motoneuron was 9.7, their mean diameter was 6.0 μm, and their mean combined diameter was 58.3 μm. The length at which a phrenic motoneuronal dendrite terminated was 1.236 μm, with several end terminals extending more than 2 mm from the cell body. The mean value for the combined lengths of all segments originating from a single stem dendrite was 5.3 mm. A full spectrum of dendritic branching patterns was observed from simple (five unbranched) to complex, the latter producing up to ninth‐order branches. Most terminal and nonterminal dendritic segments tapered, producing a mean diameter reduction of 34%, or approximately 9% per 100‐μm length. All phrenic motoneurons exhibited a steady decrease in the combined dendritic parameter (Σd3/2) with distance from the soma as a result of tapering and end‐branch termination. The mean surface area and volume of a phrenic motoneuronal dendrite were 35.3 × 103 μm2 and 25.9 × 103 μm3, respectively. The dendrites constituted greater than 97% of the total phrenic motoneuronal surface area, with 75% of this area lying outside of a 300‐μm radius from the cell body. The diameter of a stem dendrite was positively correlated with its combined dendritic length, number of terminal branches, dendritic surface area, and volume. Despite this strong correlation, the value of total dendritic surface area calculated using the power equation derived from the dendritic surface area versus stem dendritic diameter plot was not a consistent estimator of the total dendritic surface area directly measured for these four phrenic motoneurons. It is suggested that this inconsistency may be the result of a heterogeneity in the phrenic motoneuronal population and/or in the dendrites projecting to the different terminal fields.

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