ANATOMICAL AND PHYSIOLOGICAL STUDY OF BRAIN-STEM NUCLEI RELAYING DORSAL COLUMN INPUTS IN LAMPREYS

The course and sites of termination of dorsal column fibres in the lamprey brainstem are described along with their brainstem relays projecting to reticulospinal neurons. Dorsal column fibres ascend to the brainstem level where they intermingle with cells located in the alar plate close to the obex, a location that is analogous to that of the dorsal column nucleus in other vertebrates. Some dorsal column fibres continue further rostrally where they reach the octavolateralis and octavomotorii nuclei. Finally, a small contingent of fibres reach the cerebellum. Injections of cobalt-lysine into the posterior rhombencephalic reticular nucleus retrogradely label neurons within the dorsal column nucleus and within the octavolateralis and octavomotorii nuclei. Microstimulation of the dorsal column nucleus on either side elicits monosynaptic inhibitory responses in reticulospinal neurons while stimulation of octavolateralis and octavomotorius nuclei elicits excitation. By using intracellular recordings, it was shown that neurons within these alar plate nuclei receive monosynaptic inputs from the dorsal columns. It is thus proposed that disynaptic inputs from dorsal columns to reticulospinal neurons are relayed by these alar plate neurons: inhibition is relayed mainly by neurons in dorsal column nuclei and excitation by neurons in the octavolateralis and octavomotorii nuclei.