Comparison of identified mitral and tufted cells in freely breathing rats: I. Conduction velocity and spontaneous activity.

The spontaneous activity and impulse conduction velocities of mitral and tufted cells were compared in the entire main olfactory bulb of freely breathing, anesthetized rats. Single units in the mitral cell body layer (MCL) and external plexiform layer (EPL) were identified by antidromic activation from the lateral olfactory tract (LOT), electrode track reconstructions based on dye marking, and the waveform of LOT-evoked field potentials. Using the track reconstructions, EPL units were further subdivided into glomerular border (GB) and not at the glomerular border (notGB) cells. For conduction velocity, significant differences were only found between MCL and GB units and not between MCL and all EPL units or between MCL and notGB units. For spontaneous activity, no significant differences were found between the different unit groups regarding the mean, maximum, or relative maximum rate per 100-ms bin. By contrast, they showed a differential modulation of their firing activity by respiration. GB but not notGB units had a significantly higher mean rate during the respiratory cycle than MCL units with significantly more activity during inspiration. Thus, mitral and tufted cells are similar in their impulse conduction velocity and spontaneous activity, though the more superficially placed GB cells exhibit differences. A comparison of odor responses in these cell types in the companion paper also points to differences between mitral and superficial projection tufted cells.