Climbing threads

Climbing fibers

Olivo-cerebellar nerve fibers, Complex spike, Complex nerve impulse, Long term depression, LTD

The so-called climbing wires start from nerve cells in the lower olive kernel complex on one side of the midline, cross over to the opposite side and enter the cerebellum via the lower cerebellar arm. Together with the moss threads, the climbing threads convey the large influx of excitatory/driving nerve signals to the cerebellum. They use glutamate as a transmitter substance.

The olivo-cerbelar axons divide into two main stems when they reach into the cerebellum.

1/ One main stem enters one of the cerebellar nuclei. Note that the outwardly conducting nerve cells of the cerebellar nuclei are of two types: excitatory and inhibitory!

2/ The second main trunk finds its way to the cerebellar cortex and divides into about ten final branches, so-called climbing threads. As a result, each olivo-cerebellar axon with its climbing threads will contact a small group of Purkinje cells (< 10 st.) på sådant sätt att var och en av Purkinjeneuronen inom gruppen endast kontaktas av a single climbing thread! The climbing wire then produces several offshoots that wrap around the proximal dendrite section of the Purkinje cell. It forms a large number of highly excitatory synapses.

The olivo-cerebellar neurons have a spontaneous eigenfrequency of 1 action potential/second. In excitation, the frequency is increased to 4-5/second.

A nerve signal in a climbing wire depolarizes the dendrite tree and soma of the contacted Purkinje cell and triggers a complex spike in its axon. The axons of the activated Purkinje cell group contact the same neurons in the cerebellar nucleus that were activated moments earlier by the branch of the olivo-cerebellar axon in question into the nucleus.

Should a situation occur that a Purkinje cell is depolarized by its climbing wire while one or more of its dendrite tags are depolisized due to parallel wire activation, the dendrite tag synapses in question will significantly reduce their sensitivity, even become insensitive, to further parallel-wire stimulation. The synapses in question then exhibit a phenomenon called LTD (Long Term Depression). The LTD effect lingers for anywhere from a few hours up to several days. The LTD effect thus constitutes a kind of memory fragment; the memory of that particular Pukinje cell and that parallel wire being activated at one point at the same time!

The olivo-cerebellar branches into the cerebellar nuclei are thought to have only a minor effect on the neurons in the nuclei; mainly as a result of the very low signaling frequency of the olivo-cerebellar wires.

The importance of the olivo-cerebellar branches (climbing threads) to the cerebellar cortex, on the other hand, cannot be overestimated. Here, the climbing wire activity brings about lasting changes in the Purkinje cells' way of reacting to incoming moss wire signals and influences and develops stored movement programs!!

The accessory lower olive kernels contact the Purkinje cells of the cortex of the vestibulo and spinocerebellum. The large lower olive nucleus (nucleus olivaris inferior) contacts the Purkinje cells in the bark of the cerebrocerebellum.