Plantar motoneuron columns in the rat

In the rat, the numbers and locations of motoneurons innervating the short plantar muscles of the hindlimb (supplied by the medial and lateral plantar nerves, as well as a branch of the sural nerve) were determined by using both horseradish peroxidase (HRP) and fluorochromes as retrograde labels. Topographical organization within the plantar motor nucleus was examined by exposing individually the cut ends (encapsulated in low melting‐point paraffin) of medial plantar, lateral plantar, and sural nerves to HRP. In addition, double‐labeling experiments were conducted in which the medial plantar nerve was labeled with one fluorochrome (either true blue or diamidino yellow) and the lateral plantar nerve with another. The plantar motor pool is located in the extreme dorsolateral portion of the ventral horn, usually concentrated in the fifth lumbar (L5) spinal segment. Labeled motoneurons extended caudally into the sixth lumbar (L6) segment and rostrally into portions of the fourth lumbar (L4) segment. Motoneurons of the medial plantar, lateral plantar, and sural nerve have overlapping territories. Sural motoneurons (about 70 cells per side) are generally confined to L5, medial plantar motoneurons (about 180 cells per side) tend to be concentrated in caudal L5 and rostral L6, whereas the lateral plantar motoneurons (about 310 cells per side) extend throughout the entire length of the plantar motor pool. The distribution of motoneuronal cell size is unimodal (mean cross‐sectional area = 610 ± 150 μm2). Cell bodies of plantar motoneurons tend to have similar geometries in all three major planes of sectioning. In all, the combined plantar plus sural nerve population amounts to about 560 motoneurons on each side of the spinal cord. On the basis of these data, and those published by others, the innervation of the small muscles of the foot accounts for about 25% of the motor axons carried by the entire sciatic nerve.