Numbers of axons in lateral and ventral funiculi of rat sacral spinal cord.

The present study determines the numbers of myelinated and unmyelinated axons in the ventral and lateral funiculi of rat sacral spinal cord. On average, there are 55,000 myelinated and 110,000 unmyelinated axons in the lateral funiculus and 26,000 myelinated and 9,000 unmyelinated axons in the ventral funiculus at these levels. These figures combined with data from earlier studies of the posterior funiculus and the tract of Lissauer give approximate figures of 88,500 myelinated and 131,500 unmyelinated axons for the entire white matter of one side of the rat sacral spinal cord. Thus unmyelinated axons predominate in the white matter of the rat sacral spinal cord. The majority of axons, particularly the unmyelinated axons, are located in the lateral funiculus. The axons are concentrated in the dorsolateral part of the lateral funiculus, and so the dorsal part of the lateral funiculus, often referred to as the dorsolateral funiculus, contains more than half the fibers in the white matter of the spinal cord. A small nick in the dorsal and lateral part of the lateral funiculus, which is often done for various experimental reasons, could thus remove 40% of the axons in the white matter of rat sacral spinal cord. The data reported in the present paper will serve as a basis for future studies on the white matter of the isolated spinal cord.