Responses of interpositus neurones to nerve stimulation in chloralose anaesthetized cats

Abstract

Eccles et al. 6 have recently described the responses of neurones of the nucleus interpositus to stimulation of forelimb and hindlimb cutaneous afferents in decerebrate cats. The responses included two phases of excitation associated with cell discharges with latencies of about 6 and 19 msec. These were separated and succeeded by phases of inhibition. The first excitation was attributed to the action of collaterals of cuneo-cerebellar or direct spino-cerebellar axons and the second to collaterals of reticulo-cerebellar and olivo-cerebellar axons. The inhibitions were attributed to the action on the nuclear cells of the impulses from Purkinje cells 5 discharged by the afferent volleys. The present paper describes firing patterns of interpositus neurones following nerve stimulation in cats anaesthetised with a-chloralose. A somewhat surprising finding was that the cells did not always display the high level of spontaneous activity which previous authors have reported for intracerebellar nuclear neurones in unanaesthetised 15, decerebrate 6 or barbiturate anaesthetised s preparations. In addition the responses to nerve stimulation included a hitherto undescribed component consisting of a high-frequency burst of action potentials. The experiments were performed on 17 cats initially anaesthetised with ether or halothane followed by 60 mg/kg of recrystallised a-chloralose dissolved in propane1,2-diol. Some animals were paralysed with gallamine triethiodide and artificially respired. Recordings were usually begun about 7 h after the initial dose of chloralose. The left deep radial (DR), sural (SUR), femoral (FEM), gastrocnemius-soleus (GS) nerves and both superficial radial (SR) nerves were dissected and stimulated with single shocks at a repetition rate of 1/2 sec. Stimulus strengths were usually in the range 1-4 times the threshold of the most excitable fibres in the nerve, though stronger stimuli were occasionally used. Interpositus axons were stimulated in the region of the right red nucleus using a concentric stainless steel electrode. In 7 experiments a similar electrode was placed in the rostral part of the right inferior olivary nucleus.