Cerebellum: Anatomy and Function

Cerebellum Overview

• The cerebellum is located within the posterior cranial fossa of the skull.
• The tentorium cerebelli, a dural septum, separates it from the cerebrum.
• Folia (folds) increase the cerebellar surface area.
• The outer gray matter comprises unmyelinated cell bodies and dendrites.
• The inner part contains the arbor vitae, myelinated axons connecting the cerebellum to the brainstem, cerebrum, spinal cord, and inner ear.

Structural Lobes and Fissures

• The primary and posterolateral fissures divide the cerebellum into lobes.
• The flocculonodular lobe is considered a primitive area of the cerebellum.
• The posterior lobe is a newer part of the cerebellum.
• The anterior lobe is a distinct anatomical division.

Cerebellum Functions

• Balance and equilibrium arise from inner ear connections.
• The cerebellum contributes to muscle tone and movement coordination.
• Motor learning is an important function.
• Proprioceptors, Golgi tendon organs, muscle spindles, and joint capsules send proprioceptive information to the cerebellar cortex.
• The cerebral cortex sends motor plans to the cerebellum.
• Upper motor neurons from the cerebral cortex activate lower motor neurons, causing muscle contraction.
• The cerebellum coordinates the motor plan and sends instructions back to the cortex.

Functional Anatomy - Posterior View

• The anterior lobe corresponds to the spinocerebellum.
• The posterior lobe corresponds to the cerebrocerebellum.
• The flocculonodular lobe corresponds to the vestibulocerebellum.
• The primary fissure separates the anterior lobe from the posterior lobe
• The posterior lateral fissure separates the posterior lobe from the flocculonodular lobe

Functional Zones

• The spinocerebellum occupies the vermal and paravermal (intermediate) zones.
• The sensory homunculus in the vermis represents the trunk, neck, and head.
• The sensory homunculus in the paravermal area represents the upper and lower extremities.
• The cerebrocerebellum occupies the lateral hemispheres.
• The vestibulocerebellum receives information from the vestibular system.

Deep Cerebellar Nuclei

• Located within the white matter of the cerebellum.
• From lateral to medial, they are the dentate, interposed (emboliform and globose), and fastigial nuclei.
• Mnemonic: "Don't Eat Greasy Food" (Dentate, Emboliform, Globose, Fastigial).
• The dentate nucleus is associated with the lateral hemispheres.
• The interposed nucleus is associated with the vermal and paravermal areas.
• The fastigial nucleus is associated with the flocculonodular lobe and vermis.

Internal Circuitry and Neural Sharpening

• The cerebellar cortex has three layers: molecular, Purkinje, and granular.
• The molecular layer contains stellate cells, basket cells, and parallel fibers of granule cells.
• The Purkinje layer contains Purkinje neurons/cells.
• The granular layer contains granule cells.
• Neural sharpening is a key function, ensuring appropriate motor responses.

Climbing Fibers

• Originate from the inferior olivary nucleus.
• They send axons directly to the deep cerebellar nuclei.
• Ascend and send axons to deep cerebellar nuclei, granular layer, and Purkinje fibers.
• Release aspartate, a stimulatory neurotransmitter, to stimulate deep cerebellar nuclei and Purkinje fibers.
• Stimulated Purkinje fibers release GABA (inhibitory neurotransmitter).

Mossy Fibers

• Arise from various sensory pathways.
• Release glutamate, a stimulatory neurotransmitter that activates the deep cerebellar nuclei.
• Axons are given to granular cells in the granular layer which then ascend through the Purkinje layer into the molecule layer.
• Fibers connect with granule cells in the granular layer within glomeruli.
• Granule cells stimulate stellate and basket cells in the molecular layer, which release inhibitory neurotransmitters to inhibit Purkinje fibers.
• Granule cells also send axons directly to the Purkinje fibers.
• Stimulated by mossy fibers, Golgi cells inhibit granule cells.

Cerebellar Peduncles - Superior Cerebellar Peduncles (SCP)

• Contain both afferent (incoming) and efferent (outgoing) connections.
• Efferent connections primarily involve the dentate nucleus (cerebrocerebellum).
• The dentate nucleus sends axons to the contralateral red nucleus or thalamus.
• The dentothalamic pathway transmits stimulus from the dentate nucleus to the ipsilateral thalamus, then to the primary somatosensory cortex, primary motor cortex, premotor cortex, and supplementary motor area.
• The dentorubrothalamic pathway travels from the dentate nucleus to the red nucleus and then to the thalamus.
• The red nucleus, via the rubrospinal pathway, influences flexor muscles, decussates, and descends down the spinal cord.
• The interposed nucleus (globose and emboliform) also sends axons to the contralateral red nucleus.

Cerebellar Vestibular Pathway

• Purkinje fibers directly stimulate the vestibular nuclei, bypassing the deep cerebellar nuclei.
• Vestibular nuclei activate the vestibulospinal tract, influencing extensor muscles.
• This pathway activates the medial longitudinal fasciculus, which controls extraocular movements.
• The vestibulospinal tract influences extensor (anti-gravity/postural) muscles.

Afferent Connections via SCP

• The ventral spinocerebellar tract carries proprioceptive information from below the L2/L3 level.
• The rostral cerebellar tract carries information from the cervical region and upper extremities.
• The tectocerebellar tract carries visual and auditory information from the superior and inferior colliculi within the tectum.

Middle Cerebellar Peduncles (MCP)

• These are all afferent connections.
• Corticopontocerebellar fibers carry information from the primary somatosensory, premotor, primary motor, and somatosensory cortex.
• Motor plans are sent to the pontine nuclei, which cross over to the other side of the cerebellar cortex.

Inferior Cerebellar Peduncles (ICP)

• The dorsal spinocerebellar tract carries proprioceptive information from C8 to L2/L3.
• The cuneocerebellar tract carries proprioceptive information from above C8.
• Vestibular tracts carry information about balance and equilibrium.
• Olivocerebellar tracts carry information from the inferior olives to the contralateral side.
• Reticulocerebellar tracts carry sensory information from throughout the entire body.

Efferent Connections via ICP

• Cerebellar reticular fibers project from the vestigial nucleus to the reticular formation.
• Cerebellar vestibular fibers send information from Purkinje fibers directly to the vestibular nuclei.