Organization of neurons forming the femoral, sciatic, common peroneal and tibial nerves in rats and monkeys.

Our study shows that, using Elliott's method, 6 major groups of neurons can be identified in the lumbosacral cord segments of both monkeys and rats. These are the medial (m), ventromedial (vm) anterolateral (al), central (c), posterolateral (pl) and post-posterolateral (ppl) groups; they are, except for minor differences, similarly arranged in neonatal, immature, and mature monkeys and rats. Localization of neurons by the horseradish peroxidase method reveals that the sciatic nerve (ScN) neurons are distributed to L4-L7 segments in all monkeys studied and also to the rostral part of S1 segment in some monkeys. They occur in the ppl, pl, c and al groups. Of the ScN neurons, the common peroneal nerve (CPN) neurons occur more peripherally and slightly more rostrally than the tibial nerve (TN) neurons. ScN neurons in rats extend between the caudal parts of L3 and L6 segments but may extend to the rostral part of S1 segment. Their distribution to the ppl, pl, c and al groups resembles that in the monkey except for minor differences. The distribution of CPN and TN neurons in the ScN neurons in the rat also resembles that in the monkey. The distribution of ScN neurons is similar in the neonatal, immature and mature rats and monkeys except for minor differences. Femoral nerve (FN) neurons in both monkeys and rats extend between L2 and L4 segments. However, in monkeys, FN neurons start to appear only in the caudal part of L2 segment and may extend to the rostral part of L5 segment. In rats, they appear in the rostral part of L2 segment and end about the middle of L4 segment. In both rats and monkeys, FN neurons occur in the al, pl and c groups; in in addition they also occur in the ppl group in the monkey. There is minor difference in the distribution of FN neurons in the al, pl and c groups in both rats and monkeys. There is postnatal growth in length of the spinal segments containing ScN neurons in both rats and monkeys, but no significant difference in the number of labeled ScN neurons in the neonatal, immature and mature rats and monkeys. There is also no significant difference in the counts of labeled right and left ScN neurons. The histograms showing the number of labeled ScN neurons against their diameters are very similar in both rats and monkeys. In both animals, the diameters of ScN neurons vary widely between 3 and 53 microns.