SPONTANEOUS ACTIVITY OF FIRST-ORDER AND SECOND-ORDER NEURONS IN THE FROG OLFACTORY SYSTEM

The spontaneous activity of first-order neurons (neuroreceptors of the mucosa) and second-order neurons (mitral cells of the bulb) was recor ded extracellularly in the frog olfactory system. To assess the influe nce of peripheral inputs upon mitral cells, the bulb was either normal ly connected or partially deafferented. Our first set of findings conc ern the firing behavior. We found that most neurons generated interspi ke intervals (ISIs) that were stationary in mean and variance, and wer e not serially correlated at first and second order. Individual spikes in mitral cells and bursts of spikes in neuroreceptors were found to be generated by a Poisson process. Stochastic modeling suggests that t he Poissonian behavior depends on the mean value of the membrane poten tial at the axon hillock. In these models, the mean potential in mitra l cells would be far below the firing threshold and in neuroreceptors it would fluctuate at random between two states, one close to resting potential (between bursts) and the other close to the firing threshold with occasional crossings (within bursts). Secondly, partially deaffe rented mitral cells had significantly higher activity and lower varian ce than mitral cells receiving normal afferent input. This effect give s evidence that peripheral inputs influence mitral cells at rest not o nly through direct excitation but also through indirect inhibition exe rted by local neurons. Thus, the unstimulated state of the olfactory b ulb would not be qualitatively different from its stimulated state in the sense that both states involve the same types of synaptic interact ions. Consequently, understanding the synaptic relationships that take place in the bulb network can benefit from studies of its spontaneous activity.