Olfaction in Lamprey Pallium Revisited – Roles of Mitral and Tufted Cells

The presence of two separate afferent channels from the olfactory glomeruli to different targets in the brain is unravelled in the lamprey. It represents the oldest group of now living vertebrates. The mitral cells send axonal projections directly to the piriform cortex in the ventral pallium, whereas the smaller tufted cells, identified here in the lamprey, project separately and exclusively to a relay nucleus called the dmtn. It is located at the interface between the olfactory bulb and pallium and in turn it projects to a circumscribed area in the anteromedial part of the ventral pallium. The tufted cells are activated with short latency and terminate with mossy fibres on the dmtn cells and elicit large unitary EPSPs. In all synapses along this tufted cell pathway there is no concurrent inhibition, in contrast to the mitral cell pathway. This is very similar to recent findings in rodents establishing two separate exclusive projection patterns, suggesting an evolutionary conserved organisation.