Macroscopic and Internal Structure
Anatomically, the organ consists of paired hemispheres and an unpaired midline structure called the vermis. The surface is covered by narrow folds (folia) separated by deep fissures, an architecture that massively increases cortical surface area.
The internal structure features a clear separation of white and gray matter. On a midsagittal section, their alternation forms a characteristic branching pattern known as the arbor vitae.
Gray matter (substantia grisea) comprises the surface cortex and the deep cerebellar nuclei (such as the dentate nucleus) embedded deep within. White matter (substantia alba) forms narrow inner cores within the folia that converge into major tracts at the center of the organ. Clinical note: unlike the brainstem and medulla oblongata, isolated damage to the cerebellum does not lead to immediate cessation of vital functions (such as respiration or heart rate).
Cerebellar Peduncles
Connections with the rest of the central nervous system are maintained via three pairs of peduncles, which are bundles of white matter containing afferent (input) and efferent (output) pathways.
- Inferior cerebellar peduncles (restiform bodies, corpora restiformia): predominantly afferent. They carry impulses from the spinal cord (posterior spinocerebellar tract), medulla oblongata (olivocerebellar tract), and pons (vestibulocerebellar tract). Efferent fibers project back to the olivary nuclei and vestibular nuclei.
- Middle cerebellar peduncles (brachia pontis): the largest peduncles. They transmit signals from the cerebral cortex via the pontine nuclei (the second relay of the corticopontocerebellar pathway).
- Superior cerebellar peduncles (brachia conjunctiva): convey the anterior spinocerebellar tract (input) and project efferent fibers to the midbrain, specifically the red nuclei.
Cortical Cytoarchitecture
The cerebellar cortex has a strict three-layered organization. While general glial cells (microglia, astrocytes, oligodendrocytes) are present throughout, the neuronal composition is highly specific:
- Molecular layer (superficial): the widest layer, but cell-sparse. It contains local inhibitory interneurons: small stellate cells in the outer portion and larger basket cells in the inner portion.
- Ganglionic layer (Purkinje cell layer, middle): a single monolayer of giant pear-shaped neurons—Purkinje cells. Their densely branching dendrites extend into the molecular layer, while their axons project down into the white matter.
- Granular layer (deep): densely packed with small granule cells, and also contains Golgi cells and unipolar brush cells.
Afferent Fibers and Neuronal Circuits
Two main types of afferent fibers enter the cortex:
- Climbing fibers: originate from the inferior olivary nucleus and synapse directly onto the dendrites of Purkinje cells, forming a powerful, direct excitatory drive.
- Mossy fibers: terminate in the granular layer on the dendrites of granule cells within structures known as cerebellar glomeruli.
Granule cells are the only excitatory neurons in the cortex. Their axons ascend to the molecular layer, bifurcate in a T-shape to form parallel fibers, and excite overlying neurons. They vastly outnumber other cells: about 1,600 granule cells exist for every single Purkinje cell, yielding an excitatory-to-inhibitory neuron ratio of roughly 1000:1.
Despite this, all cortical output is funneled exclusively through the axons of Purkinje cells, which exert an inhibitory effect on the deep cerebellar nuclei.
Local inhibitory interneurons fine-tune the circuit: basket cell axons wrap around Purkinje cell somas (forming 'baskets') to gate output, whereas Golgi cells provide negative feedback by inhibiting transmission from mossy fibers to granule cells, thereby regulating incoming signals.