However, when the Purkinje cell also receives excitatory input over all its parallel fibers simultaneously, it will subsequently inhibit the deep nucleus which cannot fire anymore. Weakened synapses from the pf to the inhibitory cell no longer fire cell, and the excitatory pathway remains active. Thus, weakening an inhibitory input onto an excitatory circuit facilitates the excitatory pathway. Now, let's see how the elements of the cerebellum work together. Unless firing in bursts, parallel fiber EPSPs do not fire Purkinje cells .
Parallel fibers provide excitation to all of the Purkinje cells they encounter.
Thus, granule cell activity results in a strip of activated Purkinje cells . Mossy fiber input goes to both cerebellar cortex and deep nuclei . When the Purkinje cell fires, it inhibits output from the deep nuclei . This is called long-term depression . Weakened parallel fiber synapses result in less Purkinje cell activity and less inhibition to the deep nuclei , resulting in facilitated deep nuclei output.