Simple spike activity predicts occurrence of complex spikes in cerebellar Purkinje cells

Purkinje cells are the only output neurons in the cerebellar cortex, which plays a vital if poorly understood role in the coordination of movement. Purkinje cells show two forms of activity. Complex spikes (CS) occur at a low frequency and are driven by excitatory input from a single climbing fiber, whereas simple spikes (SS) occur at much higher frequency and are driven by the inputs from a much larger number of parallel fibers (Fig. 1). It is well established that the parallel fiber-Purkinje cell synapse can undergo long-term depression (LTD) in response to the coincident firing of both parallel and climbing fibers. There is also some evidence that repetitive firing of parallel fibers alone can induce long-term potentiation (LTP) at the same synapses. Presumably, these opposing tendencies must be balanced in order to maintain an appropriate distribution of synaptic strengths, and the climbing fibers are likely to play an important role in controlling this balance.