Could many connections in the brain's movement-control circuitry be too weak to produce a detectable electrical response?

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The cerebellar cortex contains huge numbers of synapses between granule cells and Purkinje cells. These synapses are thought to be a major storage site for information needed to execute coordinated movements.

To describe this connection quantitatively, the researchers conducted an electrophysiological experiment, recording unitary synaptic responses between pairs of granule cells and Purkinje cells in adult rat cerebellar slices. The results were consistent with parallel fiber–Purkinje cell synapses having high release probabilities and modest paired-pulse facilitation. However, response amplitudes varied widely.

Parallel fiber connections were detected for only 7% of the granule cells screened, compared with an expected 54%. This led the researchers to suggest that up to 85% of parallel fiber–Purkinje cell synapses do not generate detectable electrical responses.

The researchers also investigated the possible role of granule cell ascending axons by recording from granule cells near the Purkinje cell. A high proportion of these local granule cells—up to 50%—generated detectable synaptic responses. However, most of these connections were indistinguishable from parallel fiber connections, suggesting that powerful ascending axon connections are rare.

The existence of many very weak synapses would provide a mechanism for Purkinje cells to selectively extract information from the mass of information supplied by parallel fibers.