Cerebellum

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The Cerebellum: Anatomy & Structure

Location and General Appearance

The cerebellum is the largest structure in the posterior fossa, situated dorsal to the pons and rostral medulla, forming the roof of the fourth ventricle. On midsagittal section, the branching pattern of central white matter surrounded by cortical gray matter resembles a tree, historically called the arbor vitae ("tree of life"). Instead of gyri, the surface ridges are called folia ("leaves").

Figure 15.1 - Cerebellum and Posterior Fossa, Sagittal View. The cerebellum and brainstem lie within the posterior fossa, bounded by the occipital bones and clivus inferiorly and the tentorium cerebelli superiorly. (Neuroanatomy through Clinical Cases, 3rd Ed.)

Lobes

The cerebellum is divided by fissures into three lobes:

Lobe	Boundary	Key Structures
Anterior lobe	Anterior to primary fissure	Superior vermis
Posterior lobe	Between primary and posterolateral fissures	Lateral hemispheres, most of vermis
Flocculonodular lobe	Posterior to posterolateral fissure	Flocculi + nodulus (most phylogenetically ancient)

The primary fissure (deepest) separates the anterior from the posterior lobe on the dorsal surface. On the ventral inferior surface, the posterolateral fissure separates the posterior lobe from the flocculonodular lobe. The two flocculi connect to the midline nodulus (the most inferior part of the vermis) by thin pedicles.

The cerebellar tonsils sit on the inferior surface - clinically important because mass lesions or severe intracranial hypertension can force them through the foramen magnum (tonsillar herniation), compressing the medulla and its respiratory centers, causing death.

Functional Regions (Medial to Lateral)

Three functional divisions run in parallel, from medial to lateral:

Region	Function	Motor pathways influenced
Vermis + flocculonodular lobe	Proximal limb/trunk coordination; balance; vestibulo-ocular reflexes	Anterior corticospinal, reticulospinal, vestibulospinal, tectospinal tracts; MLF
Intermediate hemispheres	Distal limb coordination	Lateral corticospinal tract, rubrospinal tract
Lateral hemispheres	Motor planning for extremities; cognition	Lateral corticospinal tract

Clinically: midline lesions cause truncal ataxia and eye movement abnormalities; lateral lesions cause ipsilateral limb (appendicular) ataxia.

Peduncles

The cerebellum attaches to the brainstem by three white matter peduncles that form the walls of the fourth ventricle:

Figure 15.2 - Lateral view showing the cerebellar peduncles and vascular supply (SCA, AICA, PICA). (Neuroanatomy through Clinical Cases, 3rd Ed.)

Peduncle	Alternative name	Main direction	Key connections
Superior cerebellar peduncle	Brachium conjunctivum	Output	Decussates in midbrain at level of inferior colliculi; carries output to thalamus (and then motor cortex)
Middle cerebellar peduncle	Brachium pontis	Input	Massive input from pontine nuclei (corticopontine fibers)
Inferior cerebellar peduncle	Restiform body	Input	Spinal cord (spinocerebellar tracts), vestibular nuclei, inferior olivary nucleus

Deep Cerebellar Nuclei

All cerebellar output is relayed through the deep cerebellar nuclei (also called the roof nuclei). From lateral to medial:

Mnemonic: "Don't Eat Greasy Foods"

Dentate nucleus - largest; receives from lateral hemispheres; output via superior cerebellar peduncle to VL thalamus → motor cortex (motor planning)
Emboliform nucleus - receives from intermediate hemispheres
Globose nucleus - receives from intermediate hemispheres
Fastigial nucleus - most medial; receives from vermis and flocculonodular lobe; projects to vestibular and reticular nuclei

(The emboliform + globose together are sometimes called the interposed nuclei.)

Cerebellar Cortex: Three Layers

The cortex is uniform throughout the cerebellum, organized into repeating microcircuit units across three layers:

1. Molecular Layer (outermost)

Contains Purkinje cell dendrites, parallel fibers, stellate cells, and basket cells
Parallel fibers (axons of granule cells) run perpendicular to Purkinje cell dendritic trees

2. Purkinje Cell Layer (middle)

Single row of large Purkinje cells - the sole output neurons of the cerebellar cortex
Purkinje cells are GABAergic (inhibitory); their axons project to the deep cerebellar nuclei

3. Granule Cell Layer (innermost)

Packed with tiny granule cells - the most numerous neurons in the entire brain
Contains cerebellar glomeruli: specialized synaptic complexes where mossy fiber terminals, granule cell dendrites, and Golgi cell axon terminals interact

Afferent Fiber Systems (Inputs)

Two major input systems:

Mossy Fibers

Arise from pontine nuclei (corticopontine pathway), spinal cord, reticular formation, vestibular nuclei
Synapse on granule cell dendrites in glomeruli
Each mossy fiber contacts ~450 granule cells
Granule cells → parallel fibers → excite Purkinje cells

Climbing Fibers

Arise exclusively from the inferior olivary nucleus (medulla)
Each climbing fiber synapses on just one Purkinje cell, but makes ~300 synaptic contacts on its dendrites
Powerful modulator of Purkinje cell responsiveness; key for motor learning (long-term depression of parallel fiber synapses)

Cerebellar Microcircuit

Microcircuit of the cerebellar cortex. Mossy fibers excite granule cells; their parallel fibers excite Purkinje cells. Climbing fibers (from inferior olivary nucleus) powerfully modulate Purkinje cells. Inhibitory interneurons (basket, stellate, Golgi cells) refine spatial and temporal signal resolution. Purkinje cell axons are the sole output. (Neuroanatomy through Clinical Cases, 3rd Ed.)

Interneurons:

Basket cells (molecular layer): form "basket" synapses around Purkinje cell bodies - lateral inhibition
Stellate cells (molecular layer): inhibit Purkinje cell dendrites - lateral inhibition
Golgi cells (granule cell layer): receive parallel fiber excitation → feedback inhibition onto granule cells (sharpens temporal resolution of input signals)

Vascular Supply

Three paired arteries supply the cerebellum:

Artery	Origin	Territory
SCA (superior cerebellar artery)	Basilar artery, near junction with posterior cerebral artery	Superior cerebellar surface, deep nuclei, superior peduncle
AICA (anterior inferior cerebellar artery)	Basilar artery	Anterior inferior surface, middle peduncle, inner ear
PICA (posterior inferior cerebellar artery)	Vertebral artery	Posterior inferior surface, inferior peduncle, dorsolateral medulla

Summary: Key Structural Principles

No direct connections to lower motor neurons - the cerebellum acts exclusively via connections to motor cortex and brainstem
Ipsilateral organization - cerebellar lesions cause ipsilateral deficits (double crossing: cortex → pons crosses → cerebellar output crosses back)
Uniform cortical microcircuit - the same circuit repeats throughout; different regions perform similar computations on different inputs
Purkinje cells are the sole output of the cortex, always inhibitory, always targeting deep nuclei
The cerebellum contains >50% of all neurons in the brain despite being only ~10% of total brain volume

Sources: Neuroanatomy through Clinical Cases, 3rd Ed. | Neuroscience: Exploring the Brain, 5th Ed. | Principles of Neural Science (Kandel), 6th Ed.

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