Dynamic modulation of mossy fiber system throughput by inferior olive synchrony: A multielectrode study of cerebellar cortex activated by motor cortex

We investigated the effects of climbing fiber synchrony on the temporal dyn amics of mossy fiber system throughput in populations of cerebellar Purkinj e cells (PCs). A multielectrode technique was used in ketamine-anesthetized rats that allowed both complex and simple spikes (CSs and SSs) to be recor ded from multiple PCs simultaneously in lobule crus IIa. Stimulation of the tongue area of the primary motor cortex (TM1) was used to evoke cerebro-ce rebellar interaction. At the single PC level, robust shortterm interactions of CSs and SSs were observed after TM1 stimulation that typically consiste d of an immediate depression and subsequent enhancement of SS firing after the occurrence of a CS. Such modulations of SS rate in a given PC were as r obustly correlated to the CSs of simultaneously recorded PCs as they were t o the CS on its own membrane-and did not require a CS on its own membrane-i ndicating a network basis for the interaction. Analyses of simultaneously r ecorded PCs using the normalized joint perievent time histogram demonstrate d that CS and SS firing were dynamically correlated after TM1 stimulation i n a manner that indicated strong control of mossy fiber system throughput b y CS synchrony. For less than or equal to 300 ms after TM1 stimulation, mos t PCs showed episodic modulations in SS rate that appeared to be entrained by the population rhythm of climbing fiber synchrony. SS rhythmicity also w as modulated dynamically by CSs, such that it was depressed by CSs and faci litated by their absence. Like the modulations in SS rate, a given PC's mod ulation in SS rhythmicity did not require it to fire a CS but was, on those instances, equally correlated to the synchronous CSs of other PCs. The dat a indicate that the climbing fiber system controls the temporal dynamics of SS firing in populations of PCs by using synchrony to engage intracerebell ar circuitry and modulate mossy fiber system throughput.