The distribution of dorsal root axons to laminae IV, V, and VI of the macaque spinal cord: A quantitative electron microscopic study

Abstract

The projections of dorsal root axons to the deeper laminae (IV, V, and VI) of the Macaque spinal cord were examined by the use of experimentally induced degeneration following dorsal rhizotomy or by injection of dorsal root ganglia with tritiated amino acids followed by light and electron mi-croscopic autoradiography. Following dorsal rhizotomy, neurofilamentous degeneration of synaptic profiles occurs in each of the three deep laminae, more commonly in laminae IV and V than in lamina VI. The neurofilamentous degeneration is seen both in central glomerular (C) profiles and in many of the round vesicle (R) profiles. Neurofilamentous degeneration occurs as early as 18 hours following rhizotomy and the degenerating terminals are most numerous at 3–4 days postrhizotomy. None are seen after 7 days survival. The neurofilamentous profiles form axodendritic and, occasionally, axosomatic synapses with neurons of the dorsal horn. They are also seen to be postsynaptic to flat vesicle (F) profiles in axoaxonal synapses. A second type of degeneration, electron-lucent degeneration, is seen in laminae V and VI, and only occasionally in lamina IV. The lucent degeneration occurs somewhat later after rhizotomy than does the neurofilamentous degeneration and reaches its peak at 5 days postrhizotomy. No lucent terminals are seen after 7 days survival. Electron-dense degeneration, so common in lamina II, is not seen in the deeper dorsal horn. Autoradiographic techniques show that both C and R terminals are labelled in the deeper dorsal horn. Both of these terminals form axodendritic synapses and a significant number are found to be postsynaptic in axoaxonal synapses. Most of the C terminals degenerate following rhizotomy or are labelled following injection of the parent dorsal root ganglia with tritiated amino acids. Approximately one-fifth of the R profiles are derived from dorsal roots. F profiles do not appear to be of dorsal root origin in any case. It is concluded that neurofilamentous alterations represent the degeneration of larger-diameter (Aβ) axons which distribute to the deeper dorsal horn and that electron-lucent degeneration represents the termination of Aδ fibers. Electron-dense degeneration thought to represent the termination of nonmyelinated axons (C fibers) in the superficial dorsal horn is not seen in the deeper dorsal horn and it is concluded that C fibers do not project to the deeper laminae.