Morphology and ultrastructure of physiologically identified substantia gelatinosa (lamina II) neurons with axons that terminate in deeper dorsal horn laminae (III-V).

In order to determine their local circuit function, we have examined physiologically characterized, intracellularly labeled neurons in laminae I and II with light and electron microscopes. Single neurons in the spinal substantia gelatinosa (lamina II) of the cat and monkey were recorded intracellularly and characterized physiologically. Following characterization, the neurons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. After fixation and sectioning, histochemical reaction allowed visualization of the neuron soma, dendrites, and axon. The four nociceptive neurons reported here (three from cats and one from a monkey) had axons that distributed terminal collaterals to deeper laminae of the spinal cord, including laminae III, IV, and V. Electron microscopy of the axons demonstrated that the parent axons were myelinated and that the terminal collaterals established synaptic contact with neurons in the deeper laminae. These results suggest that some substantia gelatinosa neurons relay nociceptive information to neurons in deeper regions of the spinal dorsal horn via myelinated axons.