bims-climfi 2019-04-21 papers

Issue of 2019–04–21
two papers selected by
Jun Maruta, Mount Sinai Health System

Elife. 2019 Apr 17. pii: e44964. [Epub ahead of print]8

Selective targeting of unipolar brush cell subtypes by cerebellar mossy fibers.

In vestibular cerebellum, primary afferents carry signals from single vestibular end organs, whereas secondary afferents from vestibular nucleus carry integrated signals. Selective targeting of distinct mossy fibers determines how the cerebellum processes vestibular signals. We focused on vestibular projections to ON and OFF classes of unipolar brush cells (UBCs), which transform single mossy fiber signals into long-lasting excitation or inhibition respectively, and impact the activity of ensembles of granule cells. To determine whether these contacts are indeed selective, connectivity was traced back from UBC to specific ganglion cell, hair cell and vestibular organ subtypes in mice. We show that a specialized subset of primary afferents contacts ON UBCs, but not OFF UBCs, while secondary afferents contact both subtypes. Striking anatomical differences were observed between primary and secondary afferents, their synapses, and the UBCs they contact. Thus, each class of UBC functions to transform specific signals through distinct anatomical pathways.

Keywords: cerebellum; granule cell; mossy fiber; mouse; neuroscience; optogenetics; unipolar brush cell; vestibular

DOI: https://doi.org/10.7554/eLife.44964

J Neurophysiol. 2019 Apr 17.

How Cerebellar Motor Learning Keeps Saccades Accurate.

Robijanto Soetedjo, Yoshiko Kojima, Albert F Fuchs.

The neuronal substrate underlying the learning of a sophisticated task has been difficult to study. However, the advent of a behavioral paradigm that deceives the saccadic system into thinking it is making an error has allowed the mechanisms of the adaptation that corrects this error to be revealed in a primate. The neural elements that fashion the command signal for the generation of accurate saccades involve subcortical structures in the brainstem and cerebellum. Here we show that sites in both those structures also are involved with the gradual adaptation of saccade size, a form of motor learning. Pharmacological manipulation of the oculomotor vermis (lobules VIc and VII) impairs mechanisms that either increase or decrease saccade size during adaptation. The net saccade-related simple spike (SS) activity of its Purkinje cells is correlated with the changes in saccade characteristics that occur during adaptation. These changes in SS activity are driven by an error signal delivered over climbing fibers, which create complex spikes whose probability of occurrence reflects the motor error between the actual and desired saccade size. These climbing fibers originate in the part of the inferior olive that receives projections from the superior colliculus (SC). Disabling the SC prevents adaptation and stimulation of the SC just after a normal saccade produces a surrogate error signal that drives adaptation without an actual visual error. Therefore, the SC not only provides the initial command that generates a saccade as shown by others, but also the error signal that ensures that saccades remain accurate.

Keywords: Adaptation; Cerebellum; Saccade; Superior Colliculus

DOI: https://doi.org/10.1152/jn.00781.2018