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These neurons normally transmit dopamine to another BG nucleus, the striatum, but their degeneration leads to dysfunction of these neuronal circuits that include the BG and motor cortical areas. Within the brain, the major pathological change is progressive degeneration of neurons in the pars compacta of the substantia nigra, one of the nuclei that constitute the basal ganglia (BG). The effects of Parkinson’s disease (PD) can be described at different levels.

In this article, we review the concept of motor control and describe a few motor symptoms that illustrate the challenges in understanding such symptoms as motor control abnormalities. This type of understanding, however, remains limited, partly because of limitations in our knowledge of normal motor control. Two long-term benefits of understanding the motor control basis of motor symptoms include the future design of neural prostheses to replace the function of damaged basal ganglia circuits, and the rational design of rehabilitation strategies.

In the case of Parkinson’s disease, movement slowness, for example, would be explained by a disruption of the control processes that determine normal movement speed. Understanding motor symptoms as motor control abnormalities means to identify how the disease disrupts normal control processes. Motor symptoms can also be described in terms of motor control, a level of description that explains how movement variables, such as a limb’s position and speed, are controlled and coordinated. We know a considerable amount about the abnormalities of neuronal and muscle activity that correlate with these symptoms. The primary manifestations of Parkinson’s disease are abnormalities of movement, including movement slowness, difficulties with gait and balance, and tremor.