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that had originally turned on the movement. Therefore, this helps to turn off the movement
after a short time. The time of activation of the Purkinje cells depends on the number of
granule cells that have been stimulated and therefore the parallel fibers.

So, the complete cerebellar circuit causes a rapid turn-on agonist muscle contraction at the beginning
of a movement and also causes a precisely turn-off of the same agonist contraction after a given time
period.

3.2. Circuit for antagonist muscles.

It’s important to know that throughout the spinal cord
there are reciprocal agonist/antagonist circuits for
virtually every movement that the cord can initiate.
Therefore, these circuits are part of the basis for
antagonist turn-off at the onset of movement and then
turn-on at termination of movement, mirroring (doing
the same) whatever occurs in the agonist muscles.

It’s speculated that the other inhibitory cells in the
cerebellum could play roles in the initial inhibition of
the antagonist muscles at onset of a movement and
subsequent excitation at the end of a movement.

4. THE PURKINJE CELLS “LEARN” TO CORRECT MOTOR ERRORS – Role of the climbing fibers.

The degree to which the cerebellum supports onset of offset of muscle contractions, as well as timing
of contractions, must be learned by the cerebellum every time that a person performs a movement.
 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 contraction,
and the timing of these are almost always incorrect.
 When the act has been performed many times, the control becomes progressively more
precise.

How does this learning or these adjustments come about?

It’s not exactly know, but it’s true that the sensitivity levels of cerebellar circuits themselves
progressively adapt during the training process, specially the sensitivity of the Purkinje cells to respond
to granule cell excitation.

The sensitivity change of the Purkinje cells is due to signals from the climbing fibers that enter the
cerebellum from the inferior olivary complex. Under resting conditions, the climbing fibers fire about
one time per second. But each time that they fire, they cause extreme depolarization of the entire
dendritic tree of the Purkinje cell, lasting for up to a second. The action potential is the complex spike.

 When a movement is performed for the first time, feedback signals from the proprioceptors
(muscle spindles and Golgi tendon organs) tell to the cerebellum how much the movement
have failed from the intended.
 The climbing fiber signals alter long-term sensitivity of the Purkinje cells, maybe along with
other possible "learning" functions of the cerebellum: it is believed they improve timing and
other aspects of cerebellar control of movements.
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