[Mechanisms of locomotor control in the cerebellum]

Animals as well as humans adapt their locomotor patterns to suit different situations. To perform smooth and stable locomotion, they coordinate not only parts of a limb but also different limbs. The cerebellum is important for sensorimotor control and plays a crucial role in intra- and inter-limb coordination. Cerebellar gait ataxia is characterized by postural deficiencies and decomposition of movements. During locomotion, the vermis and the intermediate region of the cerebellum receive information through the spinocerebellar pathways about the ongoing activities in the spinal stepping generator and the somatosensory receptors. The information is conveyed by mossy fiber afferents to Purkinje neurons via granule cells and their axons, i.e., parallel fibers. Purkinje neurons transform the mossy fiber input signals to output signals that in turn modulate activities in the brainstem descending tract neurons of the brainstem that are involved in locomotion. Further, Purkinje neurons receive enhanced climbing fiber signals during perturbed locomotion. These climbing fiber signals may induce synaptic plasticity at the parallel fiber-Purkinje neuron synapses. Long-term depression (LTD) occurs in parallel fiber-Purkinje neuron synapses and is regarded as the cellular basis for the learning mechanism of the cerebellar neuronal circuit. The activation of parallel fibers releases glutamate and nitric oxide, and the released glutamate activates the glutamate receptors in the Purkinje neurons. mGluR1, a subtype of the metabotropic glutamate receptors, is highly expressed in Purkinje neurons. In addition, delta 2 glutamate receptor is expressed in only Purkinje neurons throughout the brain. Genetically targeted mice for these glutamate receptors and/or pharmacological blocking studies have been promoted to determine the functional linkage between the molecules at the cellular level and the adaptability of locomotion at the behavioral level. This article highlights some recent advances in the understanding of the role played by the cerebellum in the adaptive control of locomotion.