How do parallel fibers in the cerebellum control the duration of a response? ? by determining the number of purkinje cells that fire in sequence by altering the velocity of action potentials from purkinje cells? by determining which one of all the available purkinje cells becomes active? by passing information back and forth between one purkinje cell and another?

The appropriate response is the second one. Parallel fibers emerge from granule cells in the cerebellar cortex. Granule cells are little and exceptionally various. They are thought to make up the same number of as half of the neurons in the cerebrum. Granule cells have axons which ascend and afterward fan out into parallel filaments. These filaments meet the Purkinje cell dendrites.