Abstract
Cerebellar Purkinje cells (PC) fire action potentials at high, sustained rates. Changes in spike rate that last a few tens of milliseconds encode sensory and behavioral events. Here we investigated spontaneous fluctuations of PC simple spike rate at a slow time scale of the order of 1 s. Simultaneous recordings from pairs of PCs that were aligned either along the sagittal or transversal axis of the cerebellar cortex revealed that simple spike rate fluctuations at the 1 s time scale were highly correlated. Each pair of PCs had either a predominantly positive or negative slow-rate correlation, with negative correlations observed only in PC pairs aligned along the transversal axis. Slow-rate correlations were independent of faster rate changes that were correlated with fluid licking behavior. Simultaneous recordings from PCs and cerebellar nuclear (CN) neurons showed that slow-rate fluctuations in PC and CN activity were also highly correlated, but their correlations continually alternated between periods of positive and negative correlation. The functional significance of this new aspect of cerebellar spike activity remains to be determined. Correlated slow-rate fluctuations seem too slow to be involved in the real-time control of ongoing behavior. However, slow-rate fluctuations of PCs converging on the same CN neuron are likely to modulate the excitability of the CN neuron, thus introduce a possible slow modulation of cerebellar output activity.
Highlights
Purkinje cells (PCs), the principal neurons of the cerebellar cortex, generate action potentials at baseline rates between 30 and 200 spikes/s (Thach, 1970; Bryant et al, 2010; Cao et al, 2012a)
We found PC pairs with positive slow-rate correlations along both spatial orientations, while negative slow-rate correlations were only observed in PC pairs aligned along the transversal axis (Figure 2A, upper row)
We report that pairs of PCs in the awake mouse cerebellum express slow fluctuations in their simple spike rates which for any given pair of PCs are either highly correlated or anticorrelated
Summary
Purkinje cells (PCs), the principal neurons of the cerebellar cortex, generate action potentials at baseline rates between 30 and 200 spikes/s (Thach, 1970; Bryant et al, 2010; Cao et al, 2012a). How the activity of CN neurons is controlled by the converging inputs from PC is a question of central importance to understanding the neuronal principles of cerebellar function. In vitro and modeling studies have focused mostly on temporal synchronization of PC simple spike activity within millisecond time windows (Gauck and Jaeger, 2000; Steuber et al, 2011; Person and Raman, 2012). The analysis of cerebellar spike trains and their relationship with sensory and motor events is based on millisecond temporal resolution of spike times or of the length of inter-spike intervals (instantaneous rate). PCs and CN neurons continually fire at high rates, and this ongoing spike activity shows slow, spontaneous rate fluctuations at a time scale of ∼1 s, that are not obviously linked to any sensory or motor event. In the terminology of Eccles et al (1967) the transversal pairs are ‘‘on beam’’ neighbors as they receive excitatory inputs from overlapping populations of parallel fibers, whereas sagittal PC pairs are ‘‘off beam’’ neighbors who do not receive shared excitatory inputs
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