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Department of Physiology and Biophysics, University of Lund, Sweden.

The purpose of the present work was to identify sites in the cerebellar cortex which are likely to control eyeblink. This work was motivated by findings suggesting that the cerebellum is involved in the learning and/or performance of the classically conditioned eyeblink response. The identification was based on climbing fibre input to the cortex and on the effects of electrical stimulation of the cerebellar cortex in cats decerebrated rostral to the red nucleus. The cerebellar surface was searched for areas receiving short latency climbing fibre input on periorbital electrical stimulation. Four such areas were found in the c1 and c3 zones of lobules VI and VII in the anterior lobe of the cerebellum and in the c3 zone in the paramedian lobule. Electrical stimulation of the cerebellar cortex with trains (150-400 Hz) of at least 10 ms duration evoked two types of EMG response in the orbicularis oculi muscle. An early response, time-locked to the onset of the stimulation, was unrelated to climbing fibre input and a delayed response, time-locked to the termination of the stimulation, could only be evoked from areas which received short latency climbing fibre input from the eye, that is, the c1 and c3 zones. The delayed responses had long latencies (up to 50 ms) after the termination of the stimulus train and could be delayed further by prolonging the stimulation. Both types of response were abolished by injections of small amounts of lignocaine into the brachium conjunctivum. A number of characteristics of the delayed responses are described. They could be inhibited by a further shock to the same area of the cerebellar cortex. Their latency could be increased by increasing the stimulation frequency. The period between stimulation and appearance of the response often showed a decrease in spontaneous EMG activity. There was a close topographical correspondence between input and output. Delayed responses could be evoked from all four of the areas in the c1 and c3 zones which have climbing fibre input from the periorbital area. They could not be evoked from other areas. In contrast, early responses were only evoked from areas without such climbing fibre input. It is proposed that the delayed responses were generated by activation of Purkinje cell axons leading to hyperpolarization and a subsequent rebound depolarization and activation of cells in the interpositus nucleus. The cortical areas are therefore probably involved in the control of the orbicularis oculi muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

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