Cervical motoneuron topography reflects the proximodistal organization of muscles and movements of the rat forelimb: A retrograde carbocyanine dye analysis

Behavioral evidence reveals that the laboratory rat and other rodent specie s display skilled paw and digit use in handling food during eating and skil led limb use in reaching for food in formal laboratory skilled reaching tes ts that is comparable to that described in carnivores and primates. Because less is known about the central control of skilled movements in rodents th an in carnivores or primates, the purpose of the current study was to exami ne the relation between the rat's spinal motoneurons and the individual for elimb muscles that they innervate. In two experiments, 14 forelimb muscles (in the shoulder and the upper and lower arm segments) were injected with c arbocyanine dye tracers. The topography of spinal motoneurons was reconstru cted by using fluorescence microscopy. Motor neurons were found to be organ ized in columns throughout the length of the cervical and upper thoracic ar ea, with 1) extensor motoneurons located more laterally than flexor motoneu rons, 2) rostral motoneurons innervating more proximal muscles than caudal motoneurons, and 3) more dorsally located motoneurons innervating more dist al muscles. These results reveal that the topography of rodent cervical spi nal cord motoneurons is very similar to that of carnivores and of primates, which also are characterized by well-developed, skilled movements. In addi tion, the proximal-distal organization of motoneuron columns parallels the proximal-to-distal pattern of forelimb movement used by the rat when reachi ng. The data fi om this study enable the development of predictions about t he specific movements that would be compromised by experimental transection s or other injuries at different levels of the spinal cord in rat models of spinal injury. J. Comp. Neurol. 419:286-296, 2000. (C) 2000 Wiley-Liss, In c.