The Cerebellum and Basal Ganglia

Goals

• Explore the anatomy, physiology, and function of the cerebellum and basal ganglia.
• Discuss how these systems support motor control and learning.
• Introduce the concept of “neural machines” to understand these structures’ computational roles.

Neural Machines

• Both the cerebellum and basal ganglia are modulatory systems influencing motor control and possibly cognition.
• The concept of neural machines:
  • Stereotypical neural circuits repeated across the structures suggest algorithmic operations.
  • Principal neurons:
    • Purkinje cells in the cerebellum.
    • Medium spiny neurons in the basal ganglia.
  • Topographically organized input-output patterns maintain closed-loop functionality.
• Neither the cerebellum nor the basal ganglia directly project to alpha motor neurons:
  • Damage does not result in paralysis but leads to deficits in movement coordination and initiation.

The Cerebellum

Structure and Anatomy

• Cerebellum means “little brain”:
  • Accounts for ~10% of brain volume but contains ~80% of its neurons.
  • Key components:
    • Inputs: Mossy fibers (granule cells) and climbing fibers (inferior olive).
    • Outputs: Deep cerebellar nuclei (DCN): fastigial, interposed, and dentate nuclei.
  • Divided into three subsystems:
    1. Vestibulocerebellum:
       • Inputs: Vestibular nuclei and visual regions.
       • Role: Balance and eye movement coordination.
    2. Spinocerebellum:
       • Inputs: Proprioceptive and somatosensory information.
       • Role: Smooth movements, coordination, and balance.
    3. Cerebrocerebellum:
       • Inputs: Motor cortex (efference copy) and other cortical areas.
       • Role: Planning, executing, and learning movements.

Functions

1. Motor Coordination:
   • Integrates sensory input (e.g., proprioception) with motor commands.
   • Smoothens movement through error correction and anticipatory adjustments.
2. Forward Model Hypothesis:
   • Predicts the outcome of motor commands using efference copy and sensory feedback.
   • Compares predicted outcomes with actual sensory feedback to generate corrective signals.
3. Timing Hypothesis:
   • Critical for precise timing of muscle group activation.
4. Learning Hypothesis:
   • Essential for motor learning, particularly in tasks like the vestibulo-ocular reflex (VOR).

Disorders of the Cerebellum

• Ataxia: Uncoordinated movement and balance issues.
• Intention Tremor: Oscillation during goal-directed movement.
• Alcohol Sensitivity: Cerebellum is highly susceptible to alcohol, mimicking ataxic symptoms.
• Cognitive Deficits:
  • Emerging evidence links the cerebellum to tasks like working memory, sequencing, and language.

The Basal Ganglia

Structure and Anatomy

• Located deep in the cerebral hemispheres with complex 3D geometry.
• Key components:
  • Striatum (input): Caudate nucleus, putamen, and nucleus accumbens.
  • Pallidum (output): Globus pallidus internus (GPi) and substantia nigra pars reticulata (SNr).
  • Modulatory structures: Subthalamic nucleus (STN) and substantia nigra pars compacta (SNc).

Functions

1. Motor Control:
   • Direct Pathway:
     • Facilitates desired movements via disinhibition of the motor cortex.
   • Indirect Pathway:
     • Suppresses unwanted movements.
   • Dopamine modulates these pathways through D1 (excitatory) and D2 (inhibitory) receptors.
2. Reward Processing:
   • Involved in selecting behaviors based on reward predictions.

Disorders of the Basal Ganglia

• Parkinson’s Disease:
  • Symptoms: Bradykinesia, rigidity, and masked facial expressions.
  • Cause: Degeneration of dopamine-producing neurons in the SNc.
• Huntington’s Disease:
  • Symptoms: Chorea (involuntary movements) due to loss of inhibitory control.
  • Cause: Genetic mutation leading to neurodegeneration in the striatum.

Comparative Insights and Evolutionary Context

1. Cerebellar Expansion in Primates:
   • Rapid evolution in apes and humans linked to technical intelligence and cognitive flexibility.
2. Electric Fish Model:
   • Highlights the cerebellum’s role in forward models for sensory-motor integration.

Key Concepts

• Efference Copy: Internal copy of motor commands for predictive modeling.
• Corollary Discharge: Inhibits sensory responses to self-generated movements.
• Error Correction: Long-term depression (LTD) in Purkinje cells adjusts motor plans.
• Direct and Indirect Pathways: Basal ganglia pathways modulating motor output.

This lecture delved into the intricate roles of the cerebellum and basal ganglia as neural machines, emphasizing their contributions to motor control, learning, and cognition. It underscored their distinct yet complementary functions in smooth and adaptive behavior.