The determination of dendrite morphology on lateral rectus motoneurons in cat

Previously developed morphometric analysis of motoneurons (Ulfhake and Kellerth, '81, J. Comp. Neurol. 202: 571-583) was applied to lateral rectus motoneurons (LRMs). Total dendrite size was approximated from a single stem dendrite measurement. Fifteen dendrites from nine LRMs of the principal abducens nucleus intracellularly stained with HRP were morphometrically analyzed. The diameters and lengths along the extent of the dendrite were measured to calculate the surface area, volume, and combined length of the process. Linear correlation of stem dendrite diameter to these size parameters produced r values of .80, .84, and .61, respectively. Although the regression lines could be used to estimate dendrite size from the stem dendrite diameter, two morphologically distinct types were found among the 83 dendrites of the nine cells. Six dendrites differed from the other 77. Therefore, these six and a representative sample of the more common dendrite (nine) were included in the measurements. The rare dendrites consistently branched at about 40 micron from the soma into a rostrally and a caudally directed secondary dendrite. The secondary dendrites branched less and reduced more in diameter by tapering. Also, these dendrites exhibited a higher than expected total dendrite size to stem diameter ratio compared to "regular" dendrites. Statistical correlations of the stem diameter to surface area or volume within each dendrite type showed clear increases in r values from those of all 15. Significant differences were found between the size parameters of the two types. These qualitative and quantitative differences should be considered in accurate motoneuron size determinations in the abducens nucleus.