Loss of primary afferent nerve terminals in the brainstem after peripheral nerve transection: an anatomical study in monkeys
The effect of chronic transection of the sciatic and saphenous nerves on the receptive fields of dorsal horn neurons in the L7 segment has been reinvestigated in six cats anesthetized with chloralose. Following nerve transection only a narrow lateral band of dorsal horn contained neurons with light touch receptive fields; these were situated on the proximal part of the hind limb. Dorsal horn neurons situated more than about 0.25 mm medial of the lateral edge (at the level of lamina IV) of the dorsal horn lost their light touch receptive fields, and did not acquire new light touch RFs on the proximal part of the hind limb for as long as 49 days after nerve transection. There was thus no sign of the extensive mediolateral reorganization of somatotopy described by some previous workers. Many affected neurons throughout laminae IV to VI became phasically responsive to mechanical stimulation of unidentified mechanoreceptors in deep tissue (e.g., muscle, tendon, joints, and fasciae) of the proximal part of the limb. Some of these neurons had quite low thresholds to mechanical distortion. A small proportion of neurons in medial lamina V and VI may acquire large, high-threshold cutaneous mechanoreceptive fields on the proximal part of the limb. The relation of these time-dependent changes to the known distribution of primary afferent fibers within the dorsal horn is discussed.