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The organization of the dorsal root entry zone in cats and monkeys

In five adult cats and three adult monkeys the dorsal rootlets at cervical and various lumbar levels were examined by both light and electron microscopy. In both species, the rootlets could be divided into three anatomically distinct parts: a peripheral, a transitional, and a central zone. The anatomy of the peripheral and transitional parts of the cat were identical with those of the monkey. The peripheral part of the rootlet in both these species had many of the ultrastructural characteristics of a peripheral nerve: and, as in the peripheral nerve, the small caliber axons were randomly distributed throughout the rootlet. In the transitional part of the rootlet, where the anatomical characteristics change to those typifying the tracts of the central nervous system, the unmyelinated axons and small caliber myelinated axons become organized into bundles at the periphery of the fascicles of large myelinated axons. In the central part there were differences between the two species in the distribution of the unmyelinated axons. In the cat no reorganization of the fibers of the central part of the rootlet could be found: the rootlets entered the cord and passed through the dorsal part of Lissauer's tract, and the bundles of unmyelinated axons simply merged with the tract as they came into contact with it. In the monkey, however, the dorsal root enters dorsal to Lissauer's tract and as each part of the rootlet passed from transitional to central, the small caliber axons took a ventrolateral course within the rootlet. As the rootlet merged with the dorsal columns, these axons formed a single bundle on the ventral and lateral surface of the rootlet which then merged with Lissauer's tract. These results indicate that a “lateral division” of the dorsal root is present in the monkey, but not the cat.