Light microscopical study of dendrites and perikarya of interneurones mediating la reciprocal inhibition of cat lumbar alpha-motoneurones

Summary Interneurones which mediate disynaptic inhibition from la muscle spindle afferents of the quadriceps nerve to lumbar alpha-motoneurones were stained with intracellular injection of horseradish peroxidase. Seven best stained and most satisfactorily preserved cells were selected for analysis, and the light microscopic morphology of their cell bodies and dendrites were quantitatively investigated in parasagittal sections. The perikarya were located dorsal or dorso-medial to the motoneurones; they had mean diameters of 51 × 27 m and a mean volume of 35820 m³. The cells had 3 to 7 dendrites, which were arranged asymmetrically around the parent somata. The dendrites extended mainly in the dorso-ventral direction, in which the mean tip to tip distance for each cell was 1742 m. The dendrites had few spines and they branched almost only in bifurcations. On the average, each process divided 3.5 times and in each cell they gave rise to 14.9 branching points as well as a total combined length of more than 7000 m. Primary dendrites had a mean length of 193 m which was generally shorter than the lengths of the branches of higher order. A more detailed analysis of two cells revealed the mean width of primary dendrites to be 5.6 m while that of the 5th order processes was 1.5 m. The mean tapering of individual dendritic branches per unit length was 17%, being somewhat more pronounced for the distally located segments, while at branching points the sum of daughter processes approximately equalled the diameter of the parent process. The surface area and volume of the dendrites constituted 90% and 83% of the total surface area and 46% and 37% of the total volume of the two cells, respectively, excluding the axons. The Ia interneurones differed considerably among themselves with respect to the quantitively investigated parameters. They resembled the inhibitory Renshaw cells of the cat with regard to the number of dendrites, the poverty of spines, and the relationships between cell body diameter and width of primary dendrites.