Cerebellum

Macroscopic and internal structure

Anatomically, the organ consists of paired hemispheres and an unpaired median structure — the vermis. The surface relief is formed by narrow gyri (folia) separated by deep fissures. This architecture enormously increases the surface area of the cortex.

The basis of the internal structure is a clear demarcation of white and gray matter. On a median cross-section, their alternation forms a recognizable branching pattern — the «Arbor vitae» (arbor vitae).

Gray matter (substantia grisea) is represented by the cortex covering the gyri and paired subcortical nuclei (e.g., dentate nuclei) located deep within. White matter (substantia alba) forms narrow layers inside the gyri, which then merge into massive trunks in the center of the organ. An important clinical nuance: unlike the brainstem and medulla oblongata, isolated damage to the cerebellum does not lead to the cessation of vital functions (respiration or heartbeat).

System of cerebellar peduncles

Connection with the rest of the central nervous system is provided by three pairs of peduncles, which represent continuations of the white matter. Afferent (to the cerebellum) and efferent (from it) pathways pass through them.

• Inferior cerebellar peduncles (restiform bodies, corpora restiformia): predominantly afferent. They convey impulses from the spinal cord (posterior spinocerebellar tract), medulla oblongata (olivocerebellar tract), and pons (vestibulocerebellar tract). Efferent fibers run back to the olives and vestibular nuclei.
• Middle cerebellar peduncles: the most massive ones. They transmit signals from the cerebral cortex via the pontine nuclei (the second link of the cerebro-cerebellar pathway).
• Superior cerebellar peduncles: receive the anterior spinocerebellar tract (input) and send efferent fibers to the midbrain, specifically to the red nuclei.

Cortical cytoarchitecture

The cerebellar cortex has a strict three-layered organization. Common glial elements (microglia, astrocytes, oligodendrocytes) are present in each layer, but the neuronal composition is strictly specific:

1. Molecular layer (superficial): the widest, but poor in cell bodies. Contains interneuronal inhibitory neurons: small stellate cells in the upper part and large basket cells in the lower part.
2. Ganglionic layer (middle): represented by a single row of giant flask-shaped neurons — Purkinje cells. Their densely branching dendrites extend into the molecular layer, while their axons descend into the white matter.
3. Granular layer (deep): densely packed with small granule cells, and also contains Golgi cells and spindle-shaped neurons.

Conducting fibers and neuronal networks

Two types of afferent fibers enter the cortex:

• Climbing fibers: originate from the inferior olives and form synapses directly on the dendrites of Purkinje cells, forming the "shortest" reflex arc.
• Mossy fibers: terminate in the granular layer on the dendrites of granule cells. Their contact zone is called the cerebellar glomerulus.

Granule cells — the only excitatory neurons of the cortex. Their axons ascend into the molecular layer, branch in a T-shape (parallel fibers), and activate other cells. Quantitatively they dominate: there are about 1,600 granule cells per single Purkinje cell. The ratio of excitatory to inhibitory neurons is approximately 1000:1.

However, all efferent output from the cortex is carried out exclusively through the axons of Purkinje cells, which exert an inhibitory effect on the deep cerebellar nuclei.

Local inhibitory neurons provide fine-tuning of the entire system: the axons of basket cells entwine the bodies of Purkinje cells (forming "baskets") and limit the output signal, while Golgi cells create negative feedback, inhibiting the transmission of impulses from mossy fibers to granule cells and thereby limiting the input signal.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The mechanism of operation of the "short" and "long" reflex arcs of the cortex.
• Negative feedback involving Golgi cells.
• Morphology of basket cells and the formation of "baskets".
• The general principle of local inhibition organization in the central nervous system using the cerebellar cortex as an example.

Other topics in the section «Nervous system: brain»

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