Cerebellum, A Description
The cerebellum has been the object of detailed study since the latter part of the nineteenth century. A concise description of its anatomy and physiology has been given by Llinás (1975). For details, see Eccles et al. 1967 and Palay and Chan-Palay 1974. A summary of Eccles et al. has been given by Marr (1969).
The principal cell types of the cerebellar cortex are excitatory granule cells, inhibitory Purkinje cells, inhibitory basket cells, inhibitory stellate cells, and inhibitory Golgi cells. Input to the cerebellar cortext is by mossy fibers and climbing fibers, both of which are excitatory. The Purkinje cells provide the sole output of the cortex. Figure 9.2 shows the neurons of the cerebellar cortex.
The cortex is organized in two layers; the inner or granular layer and the outer or molecular layer. The cell bodies of the Purkinje cells define the border between the two. The Purkinje-cell dendrites are in the outer layer, and the axons leave the cortex through the inner layer.
The granule cells occupy the inner layer. They are relatively simple and are abundant. Over hanlf of the neurons in the nervous system (possibly as many as 1011) are granule cells of the cerebellum. A granule cell has 3-5 dendrites, and the cell's axon rises to the outer layer, where it divides into two branches in the shape of the letter T. The tops of the T's form the parallel fibers, which synapse with the other four cell types of the cerebellar cortex.
The granule cells receive input from the mossy fibers (which are axons of neurons outside the cerebellar cortex and from the Golgi cells. A single mossy fiber forms synapses with a great number of granule cells. Of all axons of the nervous system, a mossy fiber forms synapses with the greatest number of neurons.
The Purkinje cells are alighed, almost in rows, perpendicular to the parallel fibers. The board and flat dendrite systems of a row form a plane perpendicular to the parallel fibers. The dendrite planes are stacked side by side like books on a shelf, and a single parallel fiber passes through many (200-450) such planes, forming synapses in some if not all of them (estimates range from 45 to 450). The arragement is ideal for bringing many parallel fibers into contact with many Purkinje cells. The dendrite system of one Purkinje cell intersects as many as 400,000 parallel fibers and forms synapses with some or all of them. No other neuron in the nervous system receives input from as many neurons. Other inputs to the Purkinje cells come from the stellate cells, the basket cells, and the climbing fibers. Besides leaving the cortex, branches of the Purkinje-cell axon go to basket cells and Golgi cells.
Pu = Purkinje cell (black) St = stellate cell Go = Golgi cell (dotted) Ba = basket cell Gr = granule cell Cl = climbing fiber Pa = parallel fiber Mo = mossy fiber (black)
Neurons on the cerebellar cortex. (Source Llinás 1975. Copyright 1975 by Scientific American, Inc. All rights reserved.)
Stellate and basket cells reside in the outer layer. A cell of either kind receives input from the parallel fibers, and its output goes, by way of inhibitory synapses, to a row of Purkinje cells. The basket cells also receive input from the climbing fibers and the Purkinje cells.
Golgi-cell input is about as varied as Purkinje-cell input; it comes mostly from the parallel fibers but also from the mossy fibers, the climbing fibers, and the Purkinje cells. Golgi-cell output goes to the granule cells. Golgi-cell axons and mossy fibers form special double synapses with the dendrites of the granule cells.
The climbing fibers pair off, one for one, with the Purkinje cells. A climbing fiber follows the dendrites of its Purkinje cell, branching where they branch (hence the name), and forms many synpases with the dendrites. The firing of a climbing fiber guarantees the firing of the Purkinje cell. Climbing fibers provide input also to basket cells and Golgi cells.
Eccles, J. C., M. Iro, and J. Szentagothai. 1967. The Cerebellum as a Neuronal Machine. Berlin: Springer-Verlag.
Llinás, R. R. 1975. The cortex of the cerebellum. Scientific American 232(1):56-71
Marr, D. 1969. A theory of cerebellar cortex. Journal of Psychology 202:437-470.
Palay, S. L., and V. Chan-Palay. 1974. Cerebellar Cortex: Cytology and Organization. New York: Springer-Verlag.
from "Sparse Distributed Memory"
Quoted on Mon Apr 15th, 2013