Submitted on July 14, 2008
Revised on October 28, 2008
Accepted on November 6, 2008
Topography and Response Timing of Intact Cerebellum stained with Absorbance Voltage-Sensitive Dye
Michael E. Brown1 and Michael Ariel1*
1 Saint Louis University
* To whom correspondence should be addressed. E-mail: arielm{at}slu.edu.
Physiological activity of the turtle cerebellar cortex (Cb), maintained in vitro, was recorded during microstimulation of inferior olive. Previous single electrode responses to such stimulation showed similar latencies across a limited region of Cb, yet those recordings were from variable recording depths and lacked spatial and temporal resolution. The topography and timing of these responses were reexamined using photodiode optical recordings. Because turtle Cb is thin and unfoliated, its entire surface can be stained by a voltage-sensitive dye and transilluminated to measure changes in its local absorbance. Microstimulation of the inferior olive evoked wide-spread depolarization from the rostral to the caudal edge of the contralateral Cb. The response time course measured at a single photodiode matched that of single microelectrode responses in the corresponding Cb locus. The largest and most readily-evoked response was a sagittal band centered ~0.7 cm from the midline. Focal white-matter microstimulation on the ventricular surface also activated sagittal bands, whereas stimulation of adjacent granule cells evoked a radial patch of activation. In contrast, molecular layer microstimulation evoked transverse beams of activation, centered on the rostrocaudal stimulus position, which traveled bi-directionally across the midline to the lateral edges of the Cb. A timing analysis demonstrated that both inferior olive and white-matter microstimulation evoked responses with a nearly simultaneous onset along a sagittal band, while molecular-layer microstimulation evoked a slowly propagating wave traveling approximately 25 cm/s. The response similarity to inferior olive and white-matter microstimulation suggest that the responses to white-matter microstimulation are dominated by activation of its climbing fibers. The Cb's role in the generation of precise motor control may result from these temporal and topographic differences in orthogonally oriented pathways. Optical recordings of the turtle's thin flat Cb can provide insights into that role.
