Average transmission distance from mitral-tufted to granule cells in olfactory bulb

Abstract

The EEG waves and averaged evoked potentials (AEPs) in the olfactory bulb oscillate sinusoidally at frequencies between 40 and 80 c/sec. The mechanism in negative feedback interaction between a mass of excitatory neurons (the mitral-tufted cells) and a mass of inhibitory neurons (the granule cells) in the bulb. The time required for each synaptic action is one quarter of a cycle of the oscillation or 3–6 msec. Thus a maximum in activity of inhibitory neurons, which is manifested by a maximum of surface-negative potential in the EEG or AEP, is preceded 3–6 msec earlier by a maximum in firing rates of mitral-tufted cells and is followed 3–6 msec later by a maximum in firing rates of mitral-tufted cells. In this study the distance was measured for the spread of activity during two successive synaptic events comprising one half cycle. A focal response was evoked in the bulb by electrical stimulation of the primary olfactory nerve (PON) and localized by recording the AEP at the bulbar surface. Preceding the first negative peak (N1) of the AEP there was a focus of mitral-tufted cells excited by the PON volley. Following N1 there was a larger focus of inhibited mitral-tufted cells. The foci were identified by measuring poststimulus time histograms of neurons at multiple sites in and around the foci. The mean (± S.E.) difference between the radii of the excitatory and inhibitory foci was 569 ± 35 μ. It is inferred that the mean radius of spread from any part of a focus to its surround for each quarter cycle of activity is on the order of 270 μ. For a typical focus 2 mm in radius and lasting 4 or 5 cycles, the effects of inout to any part of the focus may quickly appear in all parts of the focus. If olfactory information is processed in this distributed manner rather than by networks of small numbers of neurons, the distance of spread is a crucial parameter in the mechanism.