The degree to which the cerebellum supports onset and offset of muscle contractions, as well as timing of contractions, must be learned by the cerebellum. Typically, when a person first performs a new motor act, the degree of motor enhancement by the cerebellum at the onset of contraction, the degree or inhibition at the end of con traction, and the timing of these are almost always incorrect for precise performance of the movement. However, after the act has been performed many times, the individual events become progressively more precise, sometimes requiring only a few movements before the desired result is achieved, but at other times requiring hundreds of movements.
How do these adjustments come about? The exact answer is not known, although it is known that sensitivity levels of cerebellar circuits progressively adapt during the training process, especially the sensitivity of the Purkinje cells to respond to the granule cell excitation. Furthermore, this sensitivity change is brought about by signals from the climbing fibers entering the cerebellum from the inferior olivary complex.
Under resting conditions the climbing fibers fire about once per second, but they cause extreme depolarization of the entire dendritic tree of the Purkinje cell, lasting for up to a second, each time they fire. During this time, the Purkinje cell fires with one initial strong output spike followed by a series of diminishing spikes. When a person performs a new movement for the first time, feedback signals from the muscle and joint proprioceptors will usually denote to the cerebellum how much the actual movement fails to match the intended movement, and the climbing fiber signals alter the long-term sensitivity of the Purkinje cells in some way. Over a period, this change in sensitivity, along with other possible “learning” functions of the cerebellum, is believed to make the timing and other aspects of cerebellar control of movements approach perfection. When this state has been achieved, the climbing fibers no longer need to send “error” signals to the cerebellum to cause further change.
