On the role of anatomy in learning by the cerebellar cortex

Interconnected Network of Ganglion-Like Neural Cell Spheres Formed on Hydrozoan Skeleton

Identifying scaffolds supporting in vitro reconstruction of active neuronal tissues in their 3-dimensional (3D) conformation is a major challenge in tissue engineering. We have previously shown that aragonite coral exoskeletons support the development of neuronal tissue from hippocampal neurons and astrocytes. Here we show for the first time that the porous aragonite skeleton obtained from bio-fabricated hydrozoan Millepora dichotoma supports the spontaneous organization of dissociated hippocampal cells into highly interconnected 3D ganglion-like tissue formations. The ganglion-like cell spheres expanded hundreds of microns across and included hundreds to thousands of astrocytes and mature neurons, most of them having only cell-cell and no cell-surface interactions. The spheres were linked to the surface directly or through a neck of cells and were interconnected through thick bundles of dendrites, varicosity-bearing axons, and astrocytic processes. Thus, M. dichotoma exoskeleton is a novel scaffold with the unprecedented ability to support a highly ordered organization of neuronal tissue. This unexpected organization opens new opportunities for neuronal tissue regeneration, because the spheres resemble in vivo nervous tissue having high volume of cells associated primarily through cell-cell rather than cell-matrix interactions.

Feedforward inhibition controls the spread of granule cell-induced Purkinje cell activity in the cerebellar cortex

Synapses associated with the parallel fiber (pf) axons of cerebellar granule cells constitute the largest excitatory input onto Purkinje cells (PCs). Although most theories of cerebellar function assume these synapses produce an excitatory sequential "beamlike" activation of PCs, numerous physiological studies have failed to find such beams. Using a computer model of the cerebellar cortex we predicted that the lack of PCs beams is explained by the concomitant pf activation of feedforward molecular layer inhibition. This prediction was tested, in vivo, by recording PCs sharing a common set of pfs before and after pharmacologically blocking inhibitory inputs. As predicted by the model, pf-induced beams of excitatory PC responses were seen only when inhibition was blocked. Blocking inhibition did not have a significant effect in the excitability of the cerebellar cortex. We conclude that pfs work in concert with feedforward cortical inhibition to regulate the excitability of the PC dendrite without directly influencing PC spiking output. This conclusion requires a significant reassessment of classical interpretations of the functional organization of the cerebellar cortex.

Mathematical constraints for building learning rules in the Purkinje cell system

We first present a method to mathematically build a learning rule for closed-loop neural networks. This rule is then applied to climbing fibers in the cerebellar cortex. Our analytical study is based on previous experimental non-analytical studies, which suggests that climbing fibers carry out an error signal to the brain. Thus, our goal is to find the class of functions for the activity propagated by climbing fibers, allowing the output of the Purkinje cell to converge towards a desired output. These functions must tend towards zero when the objective is reached. Our techniques are generalized to other network models.