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(2) Mossy fibers: these enter the cerebellum from the higher brain (thalamus, motor cortex,
basal ganglia…), brain stem and spinal cord. They send collaterals to excite the deep nuclear
cells and go to the cortex where they, too, synapse with hundreds to thousands of granule cells.
These granule cells send axons (of less than 1 micrometer in diameter) up to the molecular
layer in the outer surface of the cerebellar cortex. Here the axons divide into two branches that
extend 1 to 2 mm in each direction parallel to the foliia (conjunto de foliums). They constitute
the parallel fibers that are many millions because there are some 500 to 1000 granule cells for
every Purkinje cell. The Purkinje cells send their dendrite into this molecular layer of parallel
fibers.
Each Purkinje cell synapses with 80.000 to 200.000 of the parallel fibers. The mossy fibers make
weak synaptic connections with the Purkinje, so large numbers of mossy fibers must be
stimulated at the same time to excite it. Activation usually takes the form of a much weaker
short-duration Purkinje cell action potential, the simple spike, rather than the complex action
potential of the climbing fiber.
So, the mossy fibers stimulate the granule cells and these ones, through parallel fibers, can
stimulate: (1) the Purkinje cell in a slow signal or (2) the deep nuclei in a rapid signal.
Climbing fibers Mossy fibers
One climbing fiber for each 5 to 10 From 500 to 1000 granule cells for each
Purkinje cells. Purkinje cell.
Action potential: complex spike. Long and Action potential: simple spike. Short and
strong. weak.
2.2. Purkinje cells and deep nuclear cells fire continuously under normal resting conditions.
One characteristic of both Purkinje cells and deep nuclear cells is that normally both of them fire
continuously: the Purkinje cell fires at about 50 to 100 action potentials per second, and the deep
nuclear cells at much higher rates. Furthermore, the output activity of both these cells can be
modulated upward or downward.
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