The dorsal column nuclei of the frog.

Surface potentials and field potentials recorded from the medulla in response to cutaneous and mixed nerve stimulation revealed that fibers of the dorsal white column project onto two separate neuron groups in the medulla representing the dorsal column nuclei. The hind-limb was represented in the medial group and the fore-limb in the lateral group in the projection. The fore-limb nerves projected to a region extending from the medulla to the 4th segment of the spinal cord. Primary afferent depolarization and depression of synaptic activities were shown by direct and indirect stimulation of dorsal column fiber terminals in the dorsal column nuclei. Conditioning volleys set up in hind-limb nerves had no effect on test responses evoked by stimulation of forelimb nerves, and vice versa. Slow negative potentials with decreasing latencies were recorded from the posterocentral nucleus of the thalamus in response to stimulation of the 2nd dorsal root, the dorsal column and the dorsal column nuclei, respectively. The physiological results were correlated with histological observations using the cobalt labelling method. It was concluded that the amphibian dorsal column-medial lemniscus system is closely comparable with that found in the mammalian brain.