Analysis of the circuitry responsible for primary afferent depolarization in the trigeminal spinal nucleus caudalis of cats.

Depth analysis was performed on the field potential evoked by stimulation of the infraorbital nerve in the trigeminal spinal nucleus caudalis and the subjacent lateral reticular formation of cats. It was shown by dye marking of the recording positions that each subnucleus of the nucleus caudalis (subnucleus marginalis, gelatinosus and magnocellularis) and the reticular formation could be differentiated from one another by the characteristics of the peripherally evoked field potentials. Responses of neurons were extracellularly recorded in the subnuclei gelatinosus and magnocellularis of the nucleus caudalis and in the reticular formation to stimulation of the trigeminal sensory branches (the frontal, infraorbital and lingual nerves), the nucleus ventralis posteromedialis of the thalamus and the cerebral cortex. The properties of the neurons were studied in relation to their thresholds, latencies, receptive fields (sensory branches effective for spike generation) and frequency-following capacities. These responses were then compared with properties of the PAD induced in the fibers terminating in the nucleus caudalis by similar peripheral and central stimulation. It was found that the neurons in the subnucleus magnocellularis were the most likely candidates for the interneurons mediating the peripherally evoked disynaptic PAD in the trigeminal nerve fibers terminating in the nucleus caudalis.