Microphotographs of the spinal cord.A, Motoneurons labeled retrogradely with fluorogold in a 40-μm-thick horizontal section of the extensor motoneuron pool taken from the C3 segment (midline top and rostral right). B, Histology of the mudpuppy spinal cord stained with cresyl violet in a 40-μm-thick coronal section from the C2 segment. There are ∼80 cells in the gray matter on each side. One to three motoneurons are typically labeled retrogradely on each side, as shown inC and D. C, Two extensor motoneurons are labeled retrogradely in a 40-μm-thick coronal section from the C3 segment. D, At the C3 ventral root entry level, both extensor motoneurons (left) and flexor motoneurons (right) are labeled. The C3 ventral root can be seen in this section. E, Partial reconstruction of a flexor motoneuron showing the morphological details of its processes and orientation in the cord. The dotted line indicates the border of the gray matter.