Revision of Jn-00986-2009r1

26 Voltage–sensitive dye activity within the thin, unfoliated turtle cerebellar cortex (Cb) was 27 recorded in vitro during eighth cranial nerve (nVIII) stimulation. Short latency responses were localized 28 to the middle of the lateral edges of both ipsilateral and contralateral Cb (vestibulocerebellum; vCb). 29 Even with a severed contralateral Cb peduncle, stimulation of the nVIII ipsilateral to the intact 30 peduncle evoked contralateral vCb responses with a mean latency of only 0.25 ms after the ipsilateral 31 responses, even though the distance between them was ~5 mm. We investigated whether a rapidly 32 conducting commissure exists between each vCb by stimulating one of them directly. Responses in both 33 vCb spread sagittally but, surprisingly, there was no sequential activation along a transverse Cb beam 34 between them. In contrast, stimulation medial to either vCb evoked transverse beams that required ~20 35 ms to cross the Cb. Therefore, the rapid commissural connection between each vCb is not mediated by 36 slowly conducting parallel fibers. Also, the vCb was not strongly activated by climbing fiber stimulation, 37 suggesting that inputs to vCb involve distinct cerebellar circuits. 38 Responses between the two vCb remained following knife cuts through the rostral and caudal Cb 39 along the midline, through both peduncles, and even shallow midline cuts to the middle Cb through its 40 white matter and granule cell layer. Commissural responses were still observed only with a narrow 41 transverse bridge between each vCb or in thick transverse Cb slices. HRP transport from one vCb labeled 42 transverse axons traveling within the Purkinje cell layer that were larger than parallel fibers and lacked 43 varicosities. In sagittal sections, cross-section profiles of myelinated axons were observed around 44 Purkinje cells midway between the rostral and caudal Cb. This novel pathway for transverse 45 communication between lateral edges of turtle Cb suggests that afferents may directly conduct vestibular 46 information rapidly across the Cb to coordinate vestibulomotor reflex behaviors. 47 48