A novel path for rapid transverse communication of vestibular signals in turtle cerebellum

Abstract

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

A novel cerebellar commissure and other myelinated axons in the Purkinje cell layer of a pond turtle (Trachemys scripta elegans)

“…This commissure is anatomically distinct from axons that originate from granule cells to form parallel fibers. Consistent with the rapid spike conduction observed in Brown et al (), the PCL's transverse commissure has axons of very large diameter and they are also covered by myelin. Finally, the previous physiological findings that stimulation of CN VIII resulted in a voltage sensitive dye response in the contralateral Cb is consistent with our observation that labeled cell bodies connected to the axons in this commissure were observed bilaterally within both the eighth nerve ganglia and the vestibular nuclear complex.…”

“…Transversely oriented axons were observed in and near the PCL as fluorescent tubes. These FM‐stained tubes (see Figure a inset) are similar to the 1‐μm‐thick tubes in mid‐sagittal sections stained with osmium tetroxide and potassium ferricyanide (figures 11–13 in Brown et al, ).…”

“…Qualitatively, these sets of labeled axons were observed in the transverse plane (Figure ). These commissural axons traversed within the PCL (Figure e) as suggested by the knife cuts made during the previous physiological recordings (see figure 8 of Brown et al, ).…”

“…Myelin in either 30 μm transverse sections (Figure a,c) or 1 μm sagittal sections (Brown et al, ) is suggestive of a Cb commissure. However, viewing myelin in that plane of section does not preclude the possibility of a plexus of axons within the PCL that are not connected directly to the lateral edges of the Cb.…”

“…Such tracts from cerebellar peduncles to and from Cb regions or tracts between Cb regions are often difficult to follow because of the highly foliated nature of most cerebellar cortices. We examined the anatomy of a commissure within the Purkinje cell layer (PCL) that was suggested using physiological recordings of the nonfoliated Cb of the pond turtle ( Trachemys scripta ) (Brown, Martin, Rosenbluth, & Ariel, ).…”