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Elimination of redundant climbing fibers to PC synapses in the developing cerebellum is also regulated by retrograde signals from postsynaptic cells, such as semaphorin3A and semaphorin7A [66, 67].
In addition, 5-H[T.sub.3]R knockout animals show delayed CF elimination from P7 to P24, whereas no difference in the number of climbing fibers innervating one Purkinje cell was observed at P5 and P6 compared to controls.
The main afferent pathway of the normal cerebellar circuit is through the mossy and the climbing fibers. The mossy fibers forms glutamatergic synapses on the Granular cells, and these cells in turn project their axons to the molecular layer, to synapse on the dendritic spines of Purkinje cells.
(2007) Multiple climbing fibers signal to molecular layer interneurons exclusively via glutamate spillover.
A Purkinje cell receives another type of excitatory synaptic inputs from a neuron in an inferior olivary nucleus through a climbing fiber. A single climbing fiber forms >300 synapses on a Purkinje cell, and provides powerful excitatory drive.
This use-dependent change in synaptic efficacy is induced by conjunctive stimulation of parallel and climbing fibers (Crepel et al.,).
In addition, however, Purkinje cells receive a unique, strong synaptic input from climbing fibers. In many current models of cerebellar function, climbing fibers signal error in motor control (7, 8), and their activity drives learning at the parallel fiber to Purkinje cell synapses in such a way as to reduce that error in a manner not unlike the learning that takes place in cerebellar-like sensory structures.
The Scripps group finds that in normal rats an intoxicating dose of alcohol increases the activity of one major source of input to Purkinje cells, the nerve processes called climbing fibers. This is a particularly important input, explains Bloom's co-worker Steven Henrisken, because it preempts the Purkinje cell, interrupting the cell's other activities.
Kano, "Long-lasting depression of parallel fiber: purkinje cell transmission induced by conjunctive stimulation of parallel fibers and climbing fibers in the cerebellar cortex, " Neuroscience Letters, vol.
Sulcus PC arbors are also more frequently innervated by multiple climbing fibers relative to PC arbors of the apex and bank, further underscoring the uniqueness of the sulcus [4].