Sizes of soma and stem dendrites in intracellularly labelled alpha-motoneurones of the cat.

The morphology of identified hindlimb motoneurones was studied after intracellular labelling with Procion yellow (59 cells), Procion red (19 cells) or horseradish peroxidase (9 cells). With respect to the measurements performed, all three intracellular labels gave similar results. As judged by their axonal conduction velocity (62-117 m/sec) all included cells were alpha-motoneurones. The motoneuronal cell bodies had cross-sectional areas of 816-3732 sq. microns, corresponding to diameters of about 32-69 microns. On average each neurone had 12 (5-20) dendritic stems. For all cells together, the number of dendritic stems per neurone was not strongly correlated to soma diameter. In the whole material, the dendritic stem diameters varied between about 0.5 and 19 microns. Stem dendrites of 4-5 microns were common in all kinds of cells, whereas thicker dendritic stems were preferentially distributed to cells with larger somas. The maximum as well as the mean stem-dendrite diameter (d) per cell was clearly correlated to, and roughly proportional to, the diameter of the cell body. The sum of the cross-sectional areas of all the dendritic stems emanating from a cell ('sum of dendritic holes' = sigma pi d2/4) was roughly proportional to the volume of the soma. Quadriceps motoneurones had a markedly greater number of dendritic stems per cell (mean 16.9) than other kinds of hindlimb motoneurones studied (mean 11.5; includes motoneurones of the hamstring muscles, triceps surae and intrinsic foot muscles). The many quadruceps dendrites were, however, also relatively thin, and the average ratio between 'sum of dendritic holes' and soma volume was the same for quadriceps motoneurones as for the other cells.