Slow waves in the olfactory bulb and their relation to unitary discharges

Abstract

1. 1. In terms of the general distribution of extracellular currents, spontaneous activity, induced waves and potentials evoked by antidromic stimulation of the lateral olfactory tract (LOT) are similar in the olfactory buld. In each case, the main current flow is axial with respect to the neurons, and not tangential. 2. 2. The induced waves recorded from inside the bulb are in phase with those recorded from the surface when the microelectrode is above the mitral layer, while they are 180° out of phase below the mitral layer. The induced waves are markedly reduced in amplitude at the level of the mitral cells. 3. 3. Evoked potentials produced by stimulation of LOT are characterized by a sharp negative deflection which can be recorded through all bulbar layers and which represents the antidromic compound action potential. It is followed by a slower, negative wave, indicating inward currents in the glomeruli and external plexiform layer. As the recording microelectrode passes through the mitral layer, the recorded negative wave is reduced in amplitude and gives place to a positive deflection which increases in amplitude and duration as the microelectrode goes deeper in the granule layer. 4. 4. Large voltage gradients of up to 20 mV/mm occur across the mitral layer both during the evoked potential and during the induced wave of Adrian. 5. 5. The results suggest that there are at least two time-locked virtual generators involved in the production of the slow waves, one above and one below the mitral layer. 6. 6. Action potentials (from mitral, tufted and granule cells) are correlated with the negative-going phase of the induced waves recorded from the same microelectrode. In the granule layer the induced waves are inverted in phase with respect to other layers and consequently the granule cells tend to fire out of phase with mitral and tufted cells. 7. 7. Mitral cells are driven on the first negative component of the potential evoked in the superficial layers of the bulb by stimulation of the LOT. Tufted cells discharge on the second negative wave. Granule cells are often inhibited during the sharp negative and large positive potential recorded in the granule layer. 8. 8. The positive component of the evoked potential recorded in the granule layer and the induced wave recorded at the same level are most easily attributed to synaptic and postsynaptic events, since neither of them could be correlated to action potentials at the same level.