Somatotopic organization of the primary sensory trigeminal neurons in the hagfish, Eptatretus burgeri.

Primary sensory trigeminal projections were investigated in the hagfish following application of horseradish peroxidase (HRP) to the sensory branches. In our control preparations we were able to distinguish five sensory ganglia and their respective nerves. HRP application confirmed the almost exclusive relation of each of these nerves to their respective ganglia, with very little overlap. In normal frontal sections of the medulla oblongata, five columns of fibers surrounded by neuronal cell bodies could be clearly distinguished, but the number is probably fortuitous, for there was no one-on-one relationship with the five trigeminal ganglia. From their peripheral connections, we surmised that columns 1 and 3 handle general cutaneous sensation, columns 2, 4, and 5 handle taste sensation, and column 5 handles general mucous cutaneous sensation conveyed by utricular ganglion cells. Dorsally located columns received projections from nerves with dorsal peripheral connections, and more ventrally located columns received projections from nerves with ventral peripheral connections. This relation is the reverse of that seen in other vertebrates.