Normal dendritic morphology of frog spinal motoneurons: a Golgi study

Normal dendritic morphology of frog (Rana pipiens) lumbar motoneurons was studied using Golgi silver impregnation. Branching characteristics and quantitative measurements of dendrites were obtained using computer-aided serial reconstruction of a typical lumbar motoneuron over seven adjacent 80-micrometer transverse sections. Dendrites were classified based upon site of dendrite origin from the soma and distribution of the dendritic array within the spinal cord. Eight possible sites of dendritic origin from the soma were identified. Two dendrites, D1 and D2, are planar dendrites which arise from the dorsal aspect of the soma. They are moderately complex, reaching branch order 5-6, and are oriented predominantly in the transverse plane. Input to these dendrites is primarily segmental via dorsal root projections. Three dendrites, D3, D4, and D5, arise laterally from the soma and extend through the lateral funiculus toward the subpial region. Two dendrites, D6 and D7, arise ventrally. D6 extends ventrolaterally and is a simple dendrite reaching branch order 3-4. D7 aborizes extensively in the ventral funiculus and in the central gray, reaching a branch order of 8-9. This dendrite extends rostrally and caudally over a distance of at lest 560 micrometer. Another dendrite (D8) arises from the medial aspect of the soma and projects toward the central canal. Four sites (D1, D2, D6, and D7) almost invariably give rise to dendrites. Dendrites arise at D4 in 66% of the cells examined. Dendrites are found at D3, D5, and D8 much less frequently (6-21%). Total dendritic length (12,043 micrometer) and lengths of the individual dendrites, branch length versus branch order, and number of branches at increasing radii were examined, and Sholl analysis was performed.