Key Concepts:

Figure 1. The major gross anatomical features of the cerebellar cortex in the mouse. (a) The cerebellum, seen here in a dorsal view, is divided longitudinally into a medial vermis (V), more lateral hemispheres (H) and most lateral flocculi (F). Running at right angles to the boundary between the vermis and hemispheres are the folds that divide the cerebellar cortex into lobules. Visible lobules are numbered from IV to IX. (b) Cerebellar lobulation is most easily seen in a sagittal section – the vertical line in (a). Anterior is to the right and dorsal is upwards. Lobules I–V constitute the traditional anterior lobe, VI–IX the posterior lobe and X the flocculonodular lobe. (c) The underlying parasagittal organisation of the cerebellar cortex can be seen by the distribution of numerous molecules. Shown here is a transverse section through the cerebellum – the horizontal line in (a) – stained with immunoperoxidase to reveal the distribution of a typical compartmentation marker, zebrin II/aldolase C. Only certain subsets of Purkinje cells express zebrin II and these are arranged in parasagittal bands. (d) The distribution of zebrin II-immunoreactive Purkinje cells has been reconstructed (seen from the anterior) to demonstrate the parasagittal band organisation of the cerebellar cortex.

Figure 2. The laminar structure of the cerebellar cortex is seen in a section through the mouse cerebellum immunoperoxidase stained for zebrin II, a marker of Purkinje cells. The Purkinje cell somata form a monolayer in the Purkinje cell layer (pcl) and their dendrites are seen extending throughout the molecular layer (ml). Purkinje cell axons run through the granular layer (gl) and into the white matter axon tracts (wm).

Figure 3. The microcircuits of the cerebellum. (a) The cerebellum receives two primary afferent inputs. The inferior olivary complex (IOC) sends climbing fibres (cf) through the white matter (WM) to form excitatory (+) synapses on the smooth portions of the Purkinje cell dendrites (PC) in the molecular layer (ML). Several regions send mossy fibre (mf) projections to the cerebellar cortex. The main sources are the spinocerebellar tract (SCT), the pontocerebellar tract (PCT), the reticulocerebellar tract (RCT) and the vestibulocerebellar tract (VCT). Mossy fibres terminate in the granular layer (GL) in synaptic glomeruli (sg). In the glomeruli, they form excitatory synapses with the granule cell dendrites (gc). The granule cells send an ascending axon that bifurcates in the molecular layer to form parallel fibres (pf). All portions of the axon form excitatory synapses on Purkinje cell dendritic spines. The Purkinje cells send an inhibitory projection (–) to the cerebellar and vestibular nuclei (CN), which modulates the pattern of excitation. (b) The basic cerebellar circuit is modulated by three main inhibitory interneurons. Basket cell somata are located in the molecular layer close to the Purkinje cell layer; stellate cells have their somata more superficially; and Golgi cells have their somata in the granular layer. All three interneurons are excited by parallel fibre input. The stellate cells then inhibit the Purkinje cell dendrites, the basket cells inhibit the Purkinje cells at their somata and the Golgi cells feed back to the synaptic glomeruli and thus terminate mossy fibre volleys.