The objective of the present experiments was to study the cortical influence from sensorimotor area I (SM I) and from somatosensory area II (S II) on single neurones of the pontine nuclei (PN) in cats under N2--thiamylal anaesthesia. 2. Extracellular single unit recordings revealed a considerable convergence from S II and SM I. Out of ninety-one PN neurones (identified as ponto-cerebellas neurones by antidromic stimulation of the contralateral brachium pontis), fifty-seven neurones were influenced by stimulation of at least one cortical site. Slightly less than half of these neurones (twenty-five) had a convergent input from SM I and S II; twenty-three PN neurones were excited by SM I only and nine PN neurones by S II only. The proportion of PN neurones excited via collaterals of cortico-spinal neurones was small and restricted to those neurones which had an input from SM I. 3. Sixty per cent of the ponto-cerebellar neurones were reliably activated by natural stimulation such as tapping or passive manipulations of limbs of various joints. The vast majority (thirty-three out of thirty-six PN neurones) which had receptive fields were also influenced by electrical stimulation of one or both cortical areas. The long latency and low probability of discharge to peripheral nerve stimulation suggest a complex, probably transcortical, pathway from the periphery to the PN. 4. The distribution of latencies to both cortical and brachium pontis stimulation indicates that the PN are a relay for fast and slow cerebro-cerebellar connexions. 5. The convergence from cortical areas on PN indicates that the neurones influenced from somatic areas SM I and S II transmit integrated patterns of activity to the cerebellum.
