Selective "labeling' by transsynaptic degeneration of substantia gelatinosal cells: an attempt to decipher intrinsic wiring in the Rolando substance of primates.

Transganglionic degenerative atrophy of primary sensory terminals in the head of the spinal dorsal horn that follows transection of the segmentally related peripheral sensory nerve, induces transsynaptic degeneration of substantia galatinosal (SG) cells, as was studied here in monkeys. On the basis of increased electron density as a selective "labelling', SG cells are shown to be postsynaptic both to thin (A delta and C) and to thick (A beta) afferents as well as to descending (or propriospinal) fibres, while their recurrent axon collaterals establish inhibitory synapses upon somata of neighbouring SG cells and upon A beta terminals. SG cells are envisaged as biasing elements with a contrast-enhancing function that, by means of inhibitory dendro-dendritic and axo-axonal synapses, realize a gating mechanism.