Climbing fibs.

Mossy fibs.








In addition to the climbing fiber and mossy fiber-granule cell-parallel fiber input to the Purkinje cells, which are EXCITATORY, there are other inputs to the Purkinje cell that are INHIBITORY. The first cell that we will talk about lies in the granule cell layer and is called a Golgi cell (sorry there is no Golgi cell layer!!). Compared to the many millions of small granule cells (hence the name granule cell layer), there are relatively few Golgi cells and they are much larger than granule cells. The dendrites of the Golgi cells lie in the molecular layer (in no particular plane) and are EXCITED by the parallel fibers of granule cells. The axon of the Golgi cell enters into a complex arrangement with the mossy fiber terminal-granule cell dendrite (shown above) such that the Golgi cell axon INHIBITS (via GABA) the mossy fiber-granule cell relay. This is called feedback inhibition, because the Golgi cell inhibits information that is coming into the cerebellar circuitry.

The parallel fibers of granule cells (which travel in the molecular layer) also excite the dendrites of basket cells. Both the dendrites and somas of the basket cells lie in the molecular layer. These dendrites, like those of Purkinje cells, lie in a plane that is transverse to the long axes of the folia. The axons of basket cells also run in this plane (transverse to the long axis of the folia) and terminate on the somas of the Purkinje cells. The inhibition of Purkinje cells by the basket cell axons is called feedforward inhibition. Remember, inhibition of the input=feedback (Golgi cell axon-mossy fiber-granule cell relay) while inhibition of the output=feedforward (basket cell axon-Purkinje cell initial segment). Another way to look at these types of inhibition is whether the inhibiting cell is acting on an "earlier" or "later" cell in the cerebellar circuitry.