RHYTHMIC DISCHARGE OF CLIMBING FIBER AFFERENTS IN RESPONSE TO NATURAL PERIPHERAL STIMULI IN THE CAT

The rhythmicity of inferior olivary neurons evoked by natural ipsilateral forepaw inputs was evaluated in the climbing fiber afferent discharge of Purkinje cells recorded in the cerebellar cortex of the decerebrate, unanesthetized cat. Almost 50% of all Purkinje cells responding to the forepaw stimulus with an increase in complex spike activity exhibited periodic discharge, with the dominant periodicity being between 100-160 ms. In 10 of 25 neighboring, simultaneously recorded Purkinje cells the forepaw stimulus evoked similar periodicity in their complex spike discharge. For some cells 2 peaks of complex spike activity were evoked by a forepaw stimulus without an obvious 3rd peak. By altering the stimulus duration the 2nd peak of the response was shown to be temporally uncoupled to the off phase of the displacement for many cells. The interdependence of the trials contributing to the periodic peaks in the peristimulus time histogram (PSTH) was examined by a separation technique. This analysis indicated that the complex spikes contributing to a specific peak in the PSTH were generated with a high degree of independence (i.e., in different trials) from the complex spikes contributing to any other peak. The independence of the rhythmic complex spike peaks apparently is due to the long relative refractoriness following a complex spike in a single cell. The probability of a complex spike occurring at the next 1 or 2 cycles is decreased significantly. An inferior olivary neuron fibers usually at only 1 of the various peaks in response to a single presentation of the forepaw stimulus. This hypothesis predicts that stimuli evoking a complex spike at the initial peak in a high percentage of trials should give rise to less periodicity. This prediction was tested by comparing the presence of absence of evoked oscillation with the probability of evoking a complex spike in the first peak of the PSTH. Cells exhibiting a probability for complex spike discharge of > 50% in the 1st peak showed much less periodicity than cells with a complex spike occurring in < 50% of the trials in the 1st peak. The results are discussed in the context of the inferior olive being viewed as a population of coupled elements with a tendency to oscillate. The natural forepaw stimulus is hypothesized as synchronizing the phases of spontaneously oscillating climbing fiber afferents, resulting in the observed periodicity in the complex spike PSTH.