Cerebellum and Motor Function

• The cerebellum and basal ganglia are essential brain structures for normal motor function
• They function in association with other motor control systems

Cerebellum Function

• The cerebellum is responsible for timing motor activities
• Enables rapid and smooth transitions between muscle movements
• Regulates muscle contraction intensity when load changes
• Controls interplay between agonist and antagonist muscle groups
• Aids the cerebral cortex in planning the subsequent sequential movement
• Learns from mistakes to adjust future movements, improving correspondence with intended actions

Cerebellum Input

• Receives continuously updated information about the desired sequence of muscle contractions from brain motor control areas
• Gets continuous sensory information from the body's periphery
• Specific information includes position, rate of movement, and forces acting on each body part

Cerebellum Output

• If disparities exist between intended and actual movements, instantaneous subconscious corrective signals adjust specific muscle activation levels

Consequences of Removal

• Body movements become highly abnormal
• Coordination is drastically impacted, especially during rapid activities like running, typing, playing piano, or talking
• Impairment occurs without causing muscle paralysis

Cerebellum Anatomy

• Divided into three lobes: anterior, posterior, and flocculonodular
• Anatomical divisions include:
  • Anterior lobe
  • Posterior lobe
  • Flocculonodular lobe
• Flocculonodular lobe is the oldest part of the cerebellum
• This lobe is responsible for controlling body equilibrium functions with the vestibular system

Longitudinal Functional Divisions

• Anterior and posterior lobes are functionally organized along a longitudinal axis
• The vermis controls muscle movements of the axial body, neck, shoulders, and hips
• Lateral protrusions form cerebellar hemispheres divided into intermediate and lateral zones
• Intermediate zone controls muscle contractions in distal upper and lower limbs, particularly hands, fingers, feet, and toes
• Lateral zone collaborates with the cerebral cortex in planning sequential motor movements
• Lacking the lateral zone, discrete motor activities lose appropriate timing and sequencing, resulting in uncoordinated movements

Somatosensory Projection

• Vermis and intermediate zones show topographical representations
• Axial body portions map to the vermis
• Limbs and facial regions map to intermediate zones
• The cerebellar hemispheres lack topographical representations; they receive input from the cerebral cortex, especially the premotor and somatosensory association areas
• Connectivity between areas allows contribution to planning and coordinating rapid sequential muscular activities

Cerebellar Cortex

• A large folded sheet
• Roughly 17 cm wide by 120 cm long

Folium

• Each fold in the Cerebellar Cortex

Cerebellar Nuclei

• Located deep beneath the cortex

Afferent Pathways from the Brain

• Critical input route is the corticopontocerebellar pathway
• It originates in the cerebral motor and premotor cortices and the somatosensory cortex
• Passes through the pontile nuclei and pontocerebellar tracts to the lateral cerebellar hemisphere divisions on the brain's opposite side

Other Afferent Tracts Originating in Brain Stem

• Olivocerebellar tract passes from the inferior olive to all cerebellar regions
  • Excited by fibers from cerebral motor cortex, basal ganglia, reticular formation, and spinal cord
• Vestibulocerebellar fibers originating in the vestibular apparatus and brain stem vestibular nuclei
  • Terminate in flocculonodular lobe and fastigial nucleus
• Reticulocerebellar fibers originate in different parts of the brain stem reticular formation
  • Terminate in midline cerebellar areas (mainly the vermis)

Afferent Pathways from the Periphery

• Receives sensory signals directly from the body through pairs of dorsal and ventral tracts
• Dorsal spinocerebellar tract enters through the inferior cerebellar peduncle
  • Terminates in the vermis and intermediate zones
  • Relays muscle contraction status, tendon tension, body part position, and exerted forces
• Ventral spinocerebellar tract enters through the superior cerebellar peduncle
  • Terminates in both sides of the cerebellum
  • Conveys motor signals arriving in the anterior horns of the spinal cord
  • Provides a copy of the anterior horn motor drive

Spinocerebellar Tracts

• Capable of impulse speeds of up to 120 m/sec
• Important for instantaneous peripheral muscle action updates

Other Signal Pathways

• Signals transmitted from the body periphery through the spinal dorsal columns to medulla nuclei then cerebellum
• Signals transmitted through the spinoreticular pathway to the brain stem reticular formation, then the cerebellum
• Signals are transmitted through the spino-olivary pathway to the inferior olivary nucleus, then the cerebellum
• The result is comprehensive information collection about body movements and positions at a subconscious level

Deep Cerebellar Nuclei

• Located deep within the cerebellar mass on each side

Efferent Pathways

• Signals arrive in the cerebellum and divides in two
  • Signals going straight to cerebellar deep nuclei
    • Corresponding area of cortex overlying this nucleus
  • Cortex relays an inhibitory signal to the deep nucleus soon after
• All inputs to the cerebellum eventually end in the deep nuclei
  • Outputs then leave the cerebellum to go the brain

Major Efferent Pathways

• Pathway originating in the midline cerebellar structures (vermis) via the fastigial nuclei, going into the medullary and pontile brain stem regions
  • Works with the equilibrium apparatus and brain stem vestibular nuclei for equilibrium control
• Pathway originating in the intermediate cerebellar hemisphere zone, via the interposed nucleus, to the thalamus, the cerebral cortex, then to various midline thalamus structures, to the basal ganglia, to the red nucleus and reticular formation - Helps with reciprocal agonist/antagonist muscle contractions in limb peripheries
• Pathway originating in the lateral cerebellar hemisphere zone, via the dentate nucleus, to the ventrolateral/ventroanterior thalamus nuclei, and finally to the cerebral cortex
  • Coordination of sequential motor activities initiated by cerebral cortex

Cerebellar Network

• Relies on the Purkinje cells and Deep nucleur cells
• Includes 30 Million functional units, both connected
• Purkinje cells are inhibitory
• Afferent inputs are of Mossy or Climbing Fiber type
• Climbing fibers extend throughout the cortex, making 300 Synapses with each Purkinje cell
• Mossy fibre connect to deep nuclear and granule cells, sending axons which split, synapsing with the Purkinje

Operational Model

• Deep Nuclear Cells are constantly modulated by signals
• Signals originate from Motor Cortex or Brainstem, resulting in initiation excitation
• Signals are then received back from the Purkinje cells
• This process results in Damping
• In absences of correct signal motor systems oscillate

Lateral Inhibition

• Basket and Stellate cells inhibit the Purkinje cells
• Serves to sharpen the motor ouput, in a similar way as lateral inhibition in other neurone circuits

Turn On/Turn Off Signals

• The Cerebellum functions mainly to sharpen movement signals
• Agonist muscles recieve rapid turn of signals, whereas antagonist recieve the opposite
• Allows fine motor movements

Error Correction Model

• Initial attempted movements are innacurate
• Repeated movements yield better results
• Signals from climbing fibres facilitate fine tuning
• Allows more accurate, and coordinated motion

Cerebellar Control

• Functions at three levels
  • The vestibulocerebellum controls equilibrium
  • The spinocerebellum controls distal portions of limbs
  • The cerebrocerebellum works with sensorimotor system, planning body movements

Vestibulocerebellum

• Originated phylogenetically at about the same time that the vestibular apparatus in the inner ear developed
• Damage cases extreme disturbance of equilibrium

Motion Signal Problems

• Difficult to achieve balance
• Even with pathways capable of up to 120 m/sec, transmission delays
• The vestibulocerebellum works to account for delays

Spinocerebellum Feedback

• Feedback controls through intermediate cerebellar cortext
• The cerebellum recieves 2 types of signals during movement
  • Information on intention
  • Actual results from periphery

Dysmetria and Ataxia

• Cereballar motor system results in un-coordinated movements
• Results from damage to the tract

Past Pointing

• The cerrebelum normally starts signal for motor-off after an initated moment
• Movements that over-extend and miss intended mark

###Dysdiadochokinesia

• The cerebellum is unable to regulate changes in body position
• Results in lack of coordination

Dyarthria

• Speech disturbances, in which sound is not made properly

Nystagmus

• Tremor exhibited specifically by the eyeballs, while fixiated on an off-center object
• Results from damage to the semi-circular duct

Hypotonia

• Lower muscle tone, resulting from los of Cerebellar Nuclei
• Tone is reduced be decreased facilitation

Basal Ganglia

• Like the cerebellum, the basal ganglia are an accessory motor system
• Works closley with the corticospinal system

Major Componenets

• The caudate nucleus
• Putamen
• Globus pallidus
• Substantia nigra
• Subthalamic nucleus
• All connect to spinal cord fibres

Putamen Circuit

• Regulates patterned movement
• Controls:
  • Writing
  • Throwing
  • Vocalisation

Circuit Damage

• Result in abnormal movement
• Can lead to Athetosis, Hemiballismus, and Chorea

Cognitive control

• Caudate nucleus extends all over Cerebrum
• Input is recieved from most association areas
• Result in large amount of output across Cerebrum

Movements

• Basal Ganglia control movement speed
• Important coritical area is posterior parietal cortex
• damage to this region causes Agnosia