Climbing fibs.

Mossy fibs.








Lets now schematize the basics of the internal cerebellar circuitry. First, we can look at an old friend who we first met at level 3 in the brain stem-THE INFERIOR OLIVE. Remember, I told you that cells in the olive have axons that pass to the contralateral cerebellum as climbing fibers. These fibers go to all parts of the cerebellum, that is, they are not restricted to a particular zone. The drawing above shows that a climbing fiber sends a collateral to the deep cerebellar nuclei, which is excitatory, and then "climbs"up and like ivy, entwines and synapses all over the dendrites of the Purkinje cell. Each Purkinje cell receives input from only 1 climbing fiber axon, but each climbing fiber axon can split to innervate several Purkinje cells. These climbing fiber-Purkinje cell synapses are excitatory.

Since climbing fibers have synapses all over the dendritic tree of a Purkinje cell, their total excitatory action is extremely strong. In fact, the synaptic connection between the climbing fiber and the Purkinje cell is one of the most powerful in the nervous system. A single action potential in a climbing fiber elicits a burst of action potentials in the Purkinje cells that it contacts. This burst of action potentials exhibited by a Purkinje cell is called a complex spike. Climbing fibers are "lazy" (but strong), thus Purkinje cells exhibit complex spikes at a rate of about 1 per second. The illustration above depicts an intracellular recording from a Purkinje cell that has just been turned on by stimulating the climbing fiber a single time. This single climbing fiber stimulus has a powerful effect in that it results in 4 action potentials (i.e., complex spike) of varying amplitudes in the Purkinje cell.