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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Understand how the basal ganglia, through the thalamus, control motor sequences by selecting a particular motor sequence pathway from abstract goal representation in prefrontal cortex to specific muscle output in primary motor cortex.

  • Relay how the basal ganglia are organized, with the caudate and putamen forming the striatum input and the globus pallidus as the main output to the thalamus.

  • See how the substantia nigra (SN) and subthalamic nucleus (STN) interact with the globus pallidus in the basal ganglia complex.

  • Understand the direct and indirect pathways through the basal ganglia to the globus pallidus.

  • Comprehend how disorders of the basal ganglia are associated with the important neurologic disorders of Parkinson disease and Huntington disease.

  • Understand how the cerebellum mediates feedforward control and refinement of movement sequences.

  • Relay the 3 major functional subdivisions of the cerebellum: neocerebellum, spinocerebellum, and vestibulocerebellum.

  • Understand the projection pathways into the cerebellum, the circuit processing within it, and the projections from it.

BASAL GANGLIA INITIATE MOVEMENT

What structures in the brain cause movement? Bacteria and plants exhibit phototaxis, movement toward or away from the sun. Virtually all animals execute reflex movements such as moving a limb away from a painful stimulus. Vertebrate animals clearly have higher, limbic system types of goals such as pursuit of food, water, and mates and avoidance of predators and temperature extremes. Humans are thought to consciously make movement decisions based on high-level goals. The likely locus of these high-level goals and their translation into motor sequences were postulated in Chapter 16 to be the prefrontal cortex. However, as any Parkinson disease sufferer can tell you, an intact prefrontal cortex alone is not sufficient to initiate and control movement.

The initiation and modulation of movement in vertebrates generally depends on 2 phylogenetically old systems: the basal ganglia and cerebellum. In mammals, the basal ganglia system interacts with the frontal lobe to produce motor behavior (Figure 17–1). The large, complex frontal neocortex provides the basal ganglia with a wealth of behavioral options. The basal ganglia, acting through the thalamus, act as a central program that calls frontal and cerebellar subroutines that contain the expertise for specific, complex motor sequences. Much of this high-level control relies on lower level motor expertise in the brainstem and spinal cord that controls balance and limb alteration and other basic locomotion procedures.

FIGURE 17–1

Hierarchical organization of the motor control system with the basal ganglia–frontal cortex subsystem in green and the cerebellar modulation systems in blue. The basal ganglia receive inputs from virtually the entire frontal lobe and project back to it through the thalamus. The cerebellum consists of 3 main substructures: the vestibulocerebellum, spinocerebellum, and neocerebellum.

Organization of the Basal Ganglia: Nuclei

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