Cerebellum Anatomy

Cerebellum Anatomy

• The cerebellum is located inferior to the cerebrum and posterior to the brainstem.
• Cerebellar peduncles connect the cerebellum to the pons.
• The surface features of the cerebellum include a cortex with folia (ridges).
• The lobes of the cerebellum are the anterior, posterior, and flocculonodular lobes.
• The flocculonodular lobe is involved in balance and eye movements.
• The deep cerebellar nuclei (medial to lateral) include the fastigial, globose, emboliform, and dentate nuclei.
• The fastigial nucleus is related to vestibular function and eye movements (medial zone).
• The globose and emboliform nuclei are involved in motor learning and correcting motor activity (intermediate zone).
• The dentate nucleus is the largest nucleus and is involved in planning, initiating, and controlling voluntary movements (lateral zone).
• The superior cerebellar peduncle primarily contains efferent tracts to the midbrain.
• The middle cerebellar peduncle contains afferent tracts from the pons (cerebrum).
• The inferior cerebellar peduncle contains afferent tracts from the spinal cord and medulla (vestibular) and efferent tracts.
• The cerebellum primarily controls and receives information from the same side of the body (ipsilateral control).

General Flow of Information in Cerebellum

• Afferent input comes from the cerebrum (via pons) and peripheral receptors (via spinal cord and brainstem).
• Information processes within the cerebellar cortex and deep nuclei.
• Efferent output goes to the brainstem, thalamus, and influences motor pathways.

Afferent Pathways to the Cerebellum

• Spinocerebellar tracts carry unconscious proprioception.
• The posterior spinocerebellar tract (DSCT) carries unconscious proprioception from the lower body ipsilaterally.
• The anterior spinocerebellar tract (VSCT) carries information about spinal reflexes from the lower body and is double crossed.
• The cuneocerebellar tract carries unconscious proprioception from the upper body ipsilaterally.
• Olivocerebellar fibers arise from the inferior olivary nucleus in the medulla and are involved in motor learning and coordination.
• Damage to the olivocerebellar fibers can contribute to dystonia.

Cellular Anatomy of the Cerebellum

• The cerebellar cortex layers are the molecular layer, Purkinje cell layer, and granular layer.
• Mossy fibers originate from the brainstem and spinal cord, carrying somatosensory information, arousal, and balance cues and synapse on granule cells.
• Climbing fibers originate from the inferior olivary nucleus; provide information about movement errors and directly synapse on Purkinje cells.
• Purkinje cells are the sole efferent neurons of the cerebellar cortex.
• Purkinje cells are inhibitory (using GABA) and project to the deep cerebellar nuclei.
• Deep cerebellar nuclei receive input from Purkinje cells and mossy/climbing fibers and relay cerebellar output.

Functional Divisions of the Cerebellum

• The vestibulocerebellum includes the flocculonodular lobe and fastigial nucleus.
• The vestibulocerebellum receives input from vestibular nuclei and CN VIII.
• The vestibulocerebellum sends output to vestibular nuclei and the vestibulospinal tract.
• The vestibulocerebellum functions in balance, posture, and eye movements.
• Lesions in the vestibulocerebellum cause imbalance, nystagmus, and gait ataxia.
• The spinocerebellum includes the vermis and intermediate zones and the emboliform and globose nuclei.
• The spinocerebellum receives input from the spinal cord (proprioception).
• The spinocerebellum sends output to vestibular nuclei, the red nucleus, and the reticular formation.
• The spinocerebellum functions in goal-directed movements and motor execution.
• Lesions in the spinocerebellum cause proprioceptive deficits, hypotonia, dysmetria, and ataxia.
• The cerebrocerebellum includes the lateral cerebellar hemispheres and dentate nucleus.
• The cerebrocerebellum receives input from the cerebral cortex (via pontine nuclei).
• The cerebrocerebellum sends output to the thalamus (VL nucleus) and then the motor cortex.
• The cerebrocerebellum functions in planning, timing, and cognitive aspects of movement.
• Lesions in the cerebrocerebellum cause difficulties with rapid alternating movements, movement fractionation, and motor planning.

Functions of the Cerebellum (Summary)

• Coordinates movement, motor learning, timing of movements, error correction (feedforward and feedback), balance, posture, eye movements, and some cognitive functions.

Clinical Application: Cerebellar Ataxia

• Cerebellar ataxia is uncoordinated movement due to cerebellar dysfunction.
• Symptoms include impaired balance, dizziness, and difficulties with coordination.
• Causes include stroke, tumor, and degenerative diseases.
• Lesions typically result in ipsilateral deficits.

Purposes of the Vestibular System

• Detects head motion.
• Processes multimodal sensory information for balance.
• Drives motor output for balance and gaze stabilization.

Anatomy of the Peripheral Vestibular System

• The labyrinth includes the bony labyrinth (containing perilymph) and the membranous labyrinth (containing endolymph) within the temporal bone.
• Semicircular canals (3) detect angular acceleration (rotational movements): anterior, posterior, and horizontal canals.
• The horizontal canal is oriented ~30° upward.
• The anterior/posterior canals are oriented ~45° relative to the nose.
• The ampulla is a swelling at the end of each canal containing the cupula (gelatinous structure) and hair cells (stereocilia and kinocilium).
• Otolith organs (2) detect linear acceleration and head tilt relative to gravity.
• The utricle is primarily sensitive to horizontal movement and tilt.
• The saccule is primarily sensitive to vertical movement and tilt.
• Maculae are sensory epithelium in the utricle and saccule containing hair cells topped with the otolithic membrane embedded with otoconia (calcium carbonate crystals).

Push-Pull Relationship

• During head movement, one vestibular organ is excited while the corresponding one on the other side is inhibited.
• This provides sensory redundancy and allows the brain to interpret the direction of movement.
• Disruption can lead to vertigo and imbalance.

Vascular Supply of the Peripheral Vestibular System

• The Labyrinthine artery (branch of Anterior Inferior Cerebellar Artery - AICA) supplies the peripheral vestibular system.

Vestibulo-Ocular Reflex (VOR)

• Stabilizes gaze during head movement by producing compensatory eye movements in the opposite direction.
• The relationship is Eye movement (degrees) ≈ - Head movement (degrees).
• Operates effectively at head movement frequencies > 0.8 Hz.

Benign Paroxysmal Positional Vertigo (BPPV)

• Otoconia dislodge from the utricle or saccule and migrate into the semicircular canals.
• This causes transient vertigo with specific head positions.

Central Vestibular System

• Vestibular nuclei in the brainstem (pons/medulla) receive input from CN VIII, the cerebellum, and other sensory systems.
• The nuclei process information and contribute to VOR and VSR.

Auditory System

• Sound waves are processed by the outer and middle ear and transduced into neural signals in the cochlea (inner ear).
• Signals travel via the cochlear nerve (CN VIII) to the cochlear nuclei in the brainstem and then to the primary auditory cortex in the temporal lobe via the medial geniculate body of the thalamus.

Sensorineural Hearing Loss

• Caused by damage to the inner ear (cochlea or hair cells) or the auditory nerve pathway.

How Vision Works

• Light is refracted by the cornea and lens and focused on the retina.
• Photoreceptors (rods and cones) transduce light into neural signals.
• Signals are processed through retinal layers and transmitted via the optic nerve to the brain.

Key Structures of the Eye

• Cornea
• Iris
• Lens
• Retina
• Fovea
• Optic disc
• Optic nerve

Photoreceptors

• Rods are sensitive to low light, responsible for black and white vision, and located in the periphery of the retina.
• Cones are responsible for color vision and high visual acuity, concentrated in the fovea, and active in bright light.

Visual Pathways

• Retina -> Optic Nerve (CN II) -> Optic Chiasm (nasal fibers cross) -> Optic Tract -> Lateral Geniculate Body (LGB) of the Thalamus -> Optic Radiations -> Primary Visual Cortex (V1) in the Occipital Lobe.
• The temporal visual field projects to the nasal retina, and the nasal visual field projects to the temporal retina.

Visual Field Cuts

• Optic Nerve Lesion: Ipsilateral blindness.
• Optic Chiasm Lesion: Bitemporal hemianopsia.
• Optic Tract Lesion: Contralateral homonymous hemianopsia.
• Optic Radiation/Visual Cortex Lesion: Contralateral homonymous hemianopsia.

Processing Streams in the Visual Cortex

• Ventral Stream ("What"): Temporal lobe; object recognition and identification.
• Dorsal Stream ("Where"): Parietal lobe; spatial processing, motion detection.

Clinical Application: Stroke

• Unilateral Spatial Neglect: Often due to right parietal lobe stroke; failure to attend to the contralateral (usually left) side of space, not solely a visual field deficit.
• Homonymous Hemianopsia: Loss of the same half of the visual field in both eyes, contralateral to the stroke.

Clinical Application: Diabetic Retinopathy

• Damage to retinal blood vessels due to diabetes, leading to vision changes.

Clinical Application: Visual Motion Sensitivity/Motion Sickness

• Conflict between visual and vestibular systems.

Testing Visual Function

• Visual acuity tests
• Assessment of eye movements (cranial nerves III, IV, VI)
• Visual field testing (perimetry)