42: The Cerebellum

Introduction

• Cerebellar Diseases Have Distinctive Symptoms and Signs

• The Cerebellum Has Several Functionally Distinct Regions

• The Cerebellar Microcircuit Has a Distinct and Regular Organization

  • Neurons in the Cerebellar Cortex Are Organized into Three Layers

  • Two Afferent Fiber Systems Encode Information Differently

  • Parallel Pathways Compare Excitatory and Inhibitory Signals

  • Recurrent Loops Occur at Several Levels

• The Vestibulocerebellum Regulates Balance and Eye Movements

• The Spinocerebellum Regulates Body and Limb Movements

  • Somatosensory Information Reaches the Spinocerebellum Through Direct and Indirect Mossy Fiber Pathways

  • The Spinocerebellum Modulates the Descending Motor Systems

  • The Vermis Controls Saccadic and Smooth-Pursuit Eye Movements

  • Spinocerebellar Regulation of Movement Follows Three Organizational Principles

  • Are the Parallel Fibers a Mechanism for Motor Coordination?

• The Cerebrocerebellum Is Involved in Planning Movement

  • The Cerebrocerebellum Is Part of a High-Level Internal Feedback Circuit That Plans Movement and Regulates Cortical Motor Programs

  • Lesions of the Cerebrocerebellum Disrupt Motor Planning and Prolong Reaction Time

  • The Cerebrocerebellum May Have Cognitive Functions Unconnected with Motor Control

• The Cerebellum Participates in Motor Learning

  • Climbing-Fiber Activity Produces Long-Lasting Effects on the Synaptic Efficacy of Parallel Fibers

  • Learning Occurs at Multiple Sites in the Cerebellar Microcircuit

• An Overall View

The cerebellum constitutes only 10% of the total volume of the brain but contains more than one-half of its neurons. The structure comprises a series of highly regular, repeating units, each of which contains the same basic microcircuit. Different regions of the cerebellum receive projections from different parts of the brain and spinal cord and project to different motor systems. Nonetheless, the similarity of the architecture and physiology in all regions of the cerebellum implies that different regions of the cerebellum perform similar computational operations on different inputs.

The symptoms of cerebellar damage in humans and experimental animals give the clear impression that the cerebellum participates in the control of movement. Thus we describe these symptoms because knowledge of them, in addition to being critical for the clinician, constrains conjecture about the exact role of the cerebellum in controlling behavior. The goal of cerebellar research is to understand how the connections and physiology of cerebellar neurons define the function of the cerebellum. Thus a major part of this chapter covers the fundamentals of cerebellar physiology and anatomy.

Finally, there is a relationship between cerebellar operation and more theoretical concepts of "internal models" in motor control (see Chapter 33). A fundamental precept of modern cerebellar research is that these internal representations of the external world are implemented in the cerebellum. The cerebellum could adjust motor performance by using its learning capabilities to alter the internal models to match any changes in the motor effectors of the external world. Thus at the conclusion of this chapter we discuss cerebellar learning and its possible relationship to internal models.

Cerebellar Diseases Have Distinctive Symptoms and Signs

Disorders of the human cerebellum result in disruptions of normal movement, described originally ...