Parallel fibers are the myelinated axonal projections that extend from granule cells in the cerebellar cortex. The axons from granule cells grow toward Purkinje cells in the molecular layer of the cerebellum, which is where the dendritic fields of Purkinje cells arborize. Purkinje cells are the output cells of the cerebellum. The thousands of synapses formed between parallel fibers from granule cells and dendritic arborizations of Purkinje cells are excitatory, using glutamate as the neurotransmitter.
The cerebellum is the area of the brain that controls motor movement. Although this region makes up a relatively small part of the volume of the entire brain, the granule cells that are found in the cerebellum are thought to comprise about half the cells in the brain. These small cells, averaging about 10 microns in diameter, are found in other brain areas but are most numerous in the cerebellum. The granule cells from which parallel fibers project receive information from mossy fibers, which are projections from cells found in the pontine nuclei.
As the name of these fibers would suggest, parallel fibers run perpendicular to the long axis of the dendritic field of a Purkinje cell, and parallel relative to the surface of the cerebellar cortex. The extension of projections from granule cells first rise before branching in a parallel orientation to the dendritic fields of Purkinje cells. This is a unique characteristic since for most neurons in the brain, the orientation of the main neurite axis, with the apical dendrite and oppositely positioned axon, is perpendicular to the nearest cortical surface. As there are so many sulci and fissures on the cortex of the mammalian brain, many cells then change orientation based on which cortical form they are most near. For example, on one side of a fissure, a layer four pyramidal cell will lie sideways relative to a pyramidal cell at the top of a sulcus.
Long-term depression occurs when parallel fibers are repeatedly activated at the same time, which has a direct bearing on the function of these cells, especially since they are innervated by inhibitory cerebellar Golgi cells, pushing the circuit toward a default synchronous firing pattern. The fact that it's possible to induce long-term depression indicates that these fibers, or the granule cells from which they extend, can change their firing properties based on previously received and transmitted information. In other words, these cells can "learn" on a miniscule, autonomous level.