Answer to Question 1
The cerebellum receives input from the spinal cord, from each of the sensory systems by way of the cranial nerve nuclei, and from the cerebral cortex. That information eventually reaches the cerebellar cortex, the surface of the cerebellum.
The neurons are arranged in a precise geometrical pattern, with multiple repetitions of the same units.
The Purkinje cells are flat (two-dimensional) cells in sequential planes, parallel to one another.
The parallel fibers are axons parallel to one another and perpendicular to the planes of the Purkinje cells.
Action potentials in parallel fibers excite one Purkinje cell after another. Each Purkinje cell then transmits an inhibitory message to cells in the nuclei of the cerebellum (clusters of cell bodies in the interior of the cerebellum) and the vestibular nuclei in the brainstem, which in turn send information to the midbrain and the thalamus.
Depending on which and how many parallel fibers are active, they might stimulate only the first few Purkinje cells or a long series of them. Because the parallel fibers' messages reach different Purkinje cells one after another, the greater the number of excited Purkinje cells, the greater their collective duration of response. That is, if the parallel fibers stimulate only the first few Purkinje cells, the result is a brief message to the target cells; if they stimulate more Purkinje cells, the message lasts longer. The output of Purkinje cells controls the timing of a movement, including both its onset and offset.
Answer to Question 2
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