One of the principal roles of the basal ganglia in motor control is to function in association with the corticospinal system to control complex patterns of motor activity. An example is the writing of letters of the alphabet. When the basal ganglia sustain serious damage, the cortical system of motor control can no longer provide these pat terns. Instead, one’s writing becomes crude, as if one were learning how to write for the first time.
Other patterns that require the basal ganglia are cutting paper with scissors, hammering nails, shooting a basketball through a hoop, passing a football, throwing a baseball, the movements of shoveling dirt, most aspects of vocalization, controlled movements of the eyes, and virtually any other of our skilled movements, most of them performed subconsciously.
Neural Pathways of the Putamen Circuit. Figure 1 shows the principal pathways through the basal ganglia for executing learned patterns of movement. They begin mainly in the premotor and supplementary areas of the motor cortex and in the somatosensory areas of the sensory cortex. Next they pass to the putamen (mainly bypassing the caudate nucleus), then to the internal portion of the globus pallidus, and next to the ventroanterior and ventrolateral relay nuclei of the thalamus, and they finally return to the cerebral primary motor cortex and to portions of the premotor and supplementary cerebral areas closely associated with the primary motor cortex. Thus, the putamen circuit has its inputs mainly from the parts of the brain adjacent to the primary motor cortex but not much from the primary motor cortex itself. Then its outputs do go mainly back to the primary motor cortex or closely associated premotor and supplementary cortex. Functioning in close association with this primary putamen circuit are ancillary circuits that pass from the putamen through the external globus pallidus, the subthalamus, and the substantia nigra—finally returning to the motor cortex by way of the thalamus.
Fig1. Putamen circuit through the basal ganglia for subconscious execution of learned patterns of movement.
Abnormal Function in the Putamen Circuit: Athetosis, Hemiballismus, and Chorea. How does the putamen circuit function to help execute patterns of movement? Little is known about this function. However, when a portion of the circuit is damaged or blocked, certain patterns of movement become severely abnormal. For instance, lesions in the globus pallidus frequently lead to spontaneous and often continuous writhing movements of a hand, an arm, the neck, or the face. These movements are called athetosis.
A lesion in the subthalamus often leads to sudden flailing movements of an entire limb, a condition called hemiballismus.
Multiple small lesions in the putamen lead to flicking movements in the hands, face, and other parts of the body, called chorea.
Lesions of the substantia nigra lead to the common and extremely severe disease of rigidity, akinesia, and tremors known as Parkinson’s disease, which we discuss in more detail later in this chapter.
