The organization of primary afferents in the subnucleus caudalis of the trigeminal: A light and electron microscopic study of degeneration

Abstract

Summary The organization of primary afferents from the cranial and upper cervical root systems in the subnucleus caudalis and uppermost cervical dorsal horn has been investigated in cats. Following transection of a single or multiple sensory roots the distribution of primary fibers in the spinal tract and tract of Lissauer and of terminal knobs in the nucleus was studied by light and electron microscopic methods. A considerable proportion of the small myelinated fibers and most of the unmyelinated fibers in the spinal tract of V are not of dorsal root origin. Fibers of the trigeminal root reach the lower half of the C 2 segment in minimal numbers. The marginal zone receives primary axodendritic contacts on small dendrites and none of the knobs in the characteristic clusters are of primary origin. Marginal neuron somata do not appear to receive primary afferents. The marginal plexus has a large primary contingent and it is concluded that it serves as system for redistributing fibers in the tract on their way to the nucleus. The substantia gelatinosa contains numerous primary endings which are distributed in a patchy manner; the majority establish axodendritic synapses. A large proportion of these primary endings are postsynaptic to knobs which contain flattened vesicles and rarely spherical vesicles. Occasional primary endings are presynaptic to normal knobs which contain spherical vesicles. Primary endings on gelatinosa neurons are present though infrequent. In the nucleus proprius, small primary knobs synapse on medium and small caliber dendrites but not on neuron somata. Large endings showing pleomorphism of synaptic vesicles which may be indicative of a degenerative process were observed in synaptic contact with small and medium dendrites and with cell bodies. Both types of endings were seen to be postsynaptic to small knobs which contained either spherical or flattened vesicles. The functional significance of these obsevations is discussed.