Climbing fibs.

Mossy fibs.








Now, what about branches of the DSCT, CCT, VCT and PCT that are destined for the cerebellar cortex. The axons of these inputs are called MOSSY fibers. Like climbing fibers, the information carried by mossy fibers is heading for the Purkinje cells. However, unlike climbing fibers, mossy fibers DO NOT go directly to the Purkinje cell. Each mossy fiber branches profusely in the white matter. Each of these branches has multiple (up to 50) swellings (that resembled moss to the old time neuroanatomists) that contain round vesicles and synaptic thickenings. Each swelling, called a "rosette", is a synapse of the mossy fiber onto the dendrite of a granule cell. In the detail above you can see two rosettes contacting two different dendrites of the same granule cell. These are excitatory synapses. A rosette can also occur where the dendrites of several (up to 15) granule cells are contacted. Each mossy fiber can have up to 50 rosettes. You can see that there is considerable divergence of the mossy fiber signal.

Granule cells have long axons that pass dorsally through the granule and Purkinje, cell layers to reach the molecular layer of the cerebellar cortex, where they bifurcate and run PARALLEL to the long axis of the folium. These fibers, which are called parallel fibers, travel at right angles to the dendrites of the Purkinje cells (think of telephone lines running through a row of (flattened) trees in the fall after peak color). Each parallel fiber synapses upon and excites the dendritic spines of numerous Purkinje cells, but the synaptic effect of a single parallel fiber upon a Purkinje cell is extremely weak (contrast this with a climbing fiber). How then can the mossy fiber input fire the Purkinje cells? Well, what is needed is for many mossy fibers to fire rapidly and together, which causes many granule cells to fire together, which turns on lots of parallel fibers which then excite enough of the spines on a Purkinje cell to result in an action potential. When this occurs, the Purkinje cell exhibits what is called a simple spike. Such a spike in a Purkinje cell is shown below. In contrast to those lazy climbing fibers that fire about 1 per second (yet have a very powerful effect upon the Purkinje cell resulting in the complex spike), mossy fibers are really "gunners" (I use the term affectionately) in that they are always working. Thus they fire spontaneously and rapidly (50-100 per second) and cause (via the granule cells and parallel fibers of course) Purkinje cells to fire simple spikes at the same frequency.