ABSTRACT

This chapter situates the basal ganglia (BG) in the control hierarchy for the generation of behavior. At the lowest levels of the hierarchy, located in the spinal cord, are controllers for muscle tension and length. These receive descending projections from numerous regions, including the brainstem and the cerebral cortex, and directly regulate the activity of alpha motor neurons. The BG do not usually send direct projections to motor neurons; rather, they project to the brainstem nuclei that implement control of body configuration and orientation, as well as the ventral thalamus, which can ultimately influence the thalamocortical network. Projection neurons in BG output nuclei show activity that represents instantaneous position coordinates; different classes of neurons represent different components of the position vector. They are in a position to command the brainstem and midbrain position control systems by changing their reference signals. When BG outputs are constant, there is no volitional action. When they change their firing rates, they change reference signals to lower-level position controllers and generate volitional actions.