Fibre composition of the ventral roots L7 and S1 in the owl monkey (Aotus trivirgatus).

The ventral roots L7 and S1 of the owl monkey Aotus trivirgatus, were examined by electron microscopy. On average, these roots contain 2950 and 1837 myelinated axons respectively. In both roots the myelinated axons have bimodal size distributions, but the S1 root contains more small myelinated axons. Both roots contain a substantial proportion of unmyelinated axon profiles (UAP). In the L7 root the proportion of UAP decreases as the spinal cord is approached, from 19% distally to 5% in the juxtamedullary rootlets. Unmyelinated and very small myelinated CNS-type axons have not been observed in the L7 transitional region. The average S1 root contains some 40% unmyelinated axons at all examined proximo-distal levels. Unmyelinated/very small myelinated axons are easily found on the CNS side of the S1 transitional region, in direct relation to motoraxon bundles. Bundles of unmyelinated and small myelinated axons occur in the ventral pia mater of both segments. The unmyelinated axons in the L7 root of the owl monkey appear to be arranged like those in the feline L7 ventral root, possibly representing afferents. It is likely that most unmyelinated and small myelinated axons in the ventral root S1 are autonomic efferents.