Modulation of excitability as an early change leading to structural adaptation in the motor cortex.

The excitability of the motor cortex is a function of single cell excitability, synaptic strength, and the balance between excitatory cells and inhibitory cells. Sustained periods of sensory stimulation enhance the excitability in the motor cortex. This adaptation, which represents an early change in cortical network function effective in motor learning and recovery from a motor deficit, is followed by longer-lasting changes, such as modifications in cortical somatotopy, and by structural plasticity. Interventions aiming at increasing excitability also positively affect learning processes. Recent studies highlight that the cerebellum, especially the interpositus nucleus, plays a key function in the adaptation of the motor cortex to repeated trains of peripheral stimulation. Interpositus neurons, which receive inputs from the sensorimotor cortex and the spinal cord, are involved in somesthetic reflex behaviors and assist the cerebral cortex in transforming sensory signals to motor-oriented commands by acting via the cerebello-thalamo-cortical projections. Moreover, climbing fibers originating in the inferior olivary complex and innervating the nucleus interpositus mediate highly integrated sensorimotor information derived from spinal modules. The intermediate cerebellum allows the motor cortex to tune the gain of polysynaptic responses originating from the spinal cord after repetitive trains of peripheral stimulation, allowing an online calibration of cutaneo-muscular responses.