Cerebro-cerebellar projections from the ventral bank of the anterior ectosylvian sulcus in the cat.

1. Stimulation of the ventral bank of the anterior ectosylvian sulcus (AESv) induced marked mossy fibre (MF) and climbing fibre (CF) responses in the cerebellar posterior vermis (lobules VI-VII) and moderate sized ones in the paraflocculus, paramedian lobules and crus I and II of the cat. The relay stations for these responses to the posterior vermis were investigated morphologically and electrophysiologically. 2. It can be considered that the MF responses were relayed at least in part via the dorsolateral, peduncular and paramedian pontine nuclei, since in these nuclei there were units orthodromically responsive to AESv stimulation and antidromically responsive to stimulation of the posterior vermis. The MF responses are thought to be relayed monosynaptically, since the distribution of axon terminals labelled after injection of wheatgerm agglutinin-conjugated peroxidase (WGA-HRP) into the AESv overlapped in these pontine nuclei with that of neurons labelled after injection of WGA-HRP into the posterior vermis. 3. It is thought that the CF responses are relayed in the caudomedial part of the medial accessory olive (MAOcm), because neurons in the MAOcm were orthodromically responsive to AESv stimulation and antidromically responsive to stimulation of the posterior vermis. 4. It is suggested that the cerebro-olivary projection which transmits the orthodromic responses in the MAOcm is indirect, via the superior colliculus (SC), because injection of WGA-HRP into the AESv labelled axon terminals not in the MAOcm but in the SC, and injection of WGA-HRP into the MAOcm gave rise to retrograde labelling of cells in the SC. Synaptic connections between the axon terminals of the cerebrotectal projection and the tecto-olivary neurons were demonstrated by extracellular unit studies in the SC. 5. The hypothesis that the CF responses were transmitted via the SC was supported by the finding that the CF responses disappeared transiently after muscimol or lidocaine was injected into the SC. 6. These findings provide evidence that the MF responses are transmitted at least in part via the cerebro-ponto-cerebellar projection, while the CF responses are relayed via the cerebro-tecto-olivo-cerebellar projection. These cerebro-cerebellar pathways from the AESv are suggested to participate in conducting visual information to the posterior vermis.