The anatomical organization of the cerebellar cortex has remained nearly unchanged despite its significant expansion throughout vertebrate evolution. The circuitry in this brain region is highly regular and stereotyped. Within the cerebellar cortex, there are eight different types of neurons: Purkinje cells, Golgi cells, granule cells, Lugaro cells, basket cells, stellate cells, unipolar brush cells, and candelabrum cells. These cells are present in all areas of the cerebellar cortex except for the vestibular input-receiving regions, where unipolar brush cells are predominantly found. These cell types are distributed across three layers that compose the cerebellar cortex in higher vertebrates (Fig. 9.21). The outermost layer, known as the molecular layer, contains stellate and basket cells. The deepest layer, called the granule cell layer, has the highest cellular density in the central nervous system (CNS) and consists of granule, Golgi, and unipolar brush cells. Separating the molecular and granule cell layers is the Purkinje cell layer, which is composed of a single sheet of Purkinje cell somata. Candelabrum cells are also located within this layer. Lugaro cells are slightly deeper, situated at the upper border of the granule cell layer.

The sole efferent fiber originating from the cortex is the axon of the Purkinje cell. This axon also possesses local collaterals and acts as an inhibitory GABAergic fiber. Therefore, the remaining seven cell types function as local interneurons. Among these, stellate, basket, Golgi, Lugaro, and candelabrum cells are also inhibitory GABAergic neurons, while granule and unipolar brush cells are excitatory.

The organization of dendrites, axons, and synaptic connections within the cerebellar cortex is predominantly based on the transverse (short) and longitudinal (long) axes of the folium (Fig. 9.21). In the vermis, where the folia run perpendicular to the sagittal plane, these axes are aligned with the sagittal and coronal planes, respectively. In the hemispheres, where the folia have various angles in relation to the sagittal plane, this correspondence is lost, and the local axes of the folia become the reference axes.

The dendritic tree of Purkinje cells is the largest in the central nervous system. It extends from the Purkinje cell layer through the molecular layer to the surface of the cerebellar cortex and covers several hundred microns along the transverse axis of the folium. However, in the longitudinal direction, it only spans 30 to 40 µm. Consequently, the Purkinje cell dendritic trees can be envisioned as a stack of pancakes aligned along the longitudinal axis of the folium.

The functional connectivity of the cellular elements within the cerebellar cortex can be determined electrophysiologically due to the geometric organization of the cortical circuits. A classic paradigm involves the activation of a beam of parallel fibers through stimulation of the cerebellar cortex (Fig. 9.22). Recordings from stellate and basket cells, as well as Purkinje cells, in line with this beam reveal their excitation by the parallel fibers. In contrast, Purkinje cells flanking the beam experience inhibition only, resulting from the perpendicular spatial relationship between the parallel fibers and the axons of stellate and basket cells.
Cellular Elements and Efferent Fibers of the Cortex
Despite its enormous expansion throughout vertebrate evolution, the basic anatomical organization of the cerebellar cortex has remained nearly invariant. The circuitry is also among the most regular and stereotyped of any brain region. The cerebellar cortex contains eight different neuronal types: Purkinje cells, Golgi cells, granule cells, Lugaro cells, basket cells, stellate cells, unipolar brush cells, and candelabrum cells. These cells are found in all regions of the cerebellar cortex, with the exception of unipolar brush cells, which are limited mainly to cerebellar areas receiving vestibular input. These eight cell types are distributed among the three layers that make up the cerebellar cortex of higher vertebrates (
Fig. 9.21 ). The outer or superficial layer is the molecular layer; stellate and basket cells are found there. The deepest layer is the granule cell layer; this layer has the highest cellular density in the CNS and contains granule, Golgi, and unipolar brush cells. Separating the molecular and granule cell layers is the Purkinje cell layer, formed by Purkinje cell somata, which are arranged as a one-cell-thick sheet of cells. Candelabrum cells are also located in this layer. Lugaro cells are situated slightly deeper at the upper border of the granule cell layer.
The sole efferent fiber from the cortex is the Purkinje cell axon, which also has local collaterals and is GABAergic and inhibitory. Thus the remaining seven cell types are local interneurons. Of these, the stellate, basket, Golgi, Lugaro, and candelabrum cells are also inhibitory GABAergic neurons, whereas the granule and unipolar brush cells are excitatory.
Microcircuitry of the Cortex
The dendrites, axons, and patterns of synaptic connections of most neurons within the cerebellar cortex are organized with regard to the transverse (short) and longitudinal (long) axes of the folium (Fig. 9.21 ). In the vermis, where the folia run perpendicular to the sagittal plane, these axes lie in the sagittal and coronal planes, respectively. In the hemispheres, where the folia are oriented at various angles with regard to the sagittal plane, this correspondence is lost, and the local axes of the folia must then serve as the reference axes.
The Purkinje cell dendritic tree is the largest in the CNS. It extends from the Purkinje cell layer through the molecular layer to the surface of the cerebellar cortex and for several hundred microns along the transverse axis of the folium but for only 30 to 40 µm in the longitudinal direction. Thus it is like a flat pancake that lies in a plane parallel to the transverse axis of the folium. Accordingly, a set of Purkinje cell dendritic trees can be thought of as a stack of pancakes, with the stack running along the longitudinal axis of the folium.
•Fig. 9.22 Functional Connectivity of the Cerebellar Cortex. Becauseofthegeometricorganizationofthecerebellarcorticalcircuits,thefunctionalconnectivityofthecellularelementscanbedeterminedelectrophysiologically.Thefiguredepictsaclassicparadigminwhichstimulationofthecerebellarcortexactivatesabeamofparallelfibers(orange). Recordingsfromthestellateandbasketcells(green cells) andPurkinjecells(PCs;orange cells)inlinewiththisbeamshowthattheyareexcitedbytheparallelfibers.Incontrast,Purkinjecellsflankingthebeamreceiveonlyinhibition(purple areas) asaresultoftheperpendicularspatialrelationshipoftheparallelfibersandthestellateandbasketcellaxons.