The caudate and putamen receive input from the motor areas of the frontal lobe and then project fibers to the globus pallidus. There is also cortical input to the substantia nigra and subthalamic nucleus. The substantia nigra is reciprocally connected to the caudate/putamen; the subthalamic nucleus is reciprocally connected to the globus pallidus. Major efferent flow from the basal ganglia is from the globus pallidus, passing under and through the posterior limb of the internal capsule, to the thalamus (ventral anterior and ventral lateral nuclei) as the thalamic fasciculus. Deep cerebellar nuclei also project to these thalamic nuclei which are reciprocally connected to motor areas of the frontal lobe.
The main task of the basal ganglia is to organized the activities initiated in the ipsilateral motor and premotor cortex for the smooth execution of automatic motor acts. These extrapyramidal functions of the basal ganglia and the ipsilateral motor cortex with which it interacts is integrated with areas of the cortex which give rise to the corticospinal tract, as well as corticostriate, corticonigral, and corticoreticular pathways. Most of these fiber systems pass within the internal capsule before crossing to the opposite side of the body where the role of the basal ganglia is realized. Basal ganglia disease (unilateral) results in contralateral disorders of movement which are characterized by hypokinesia, rigidity, and dyskinesia (abnormal movements e.g., tremor at rest, choreoathetoid and ballistic movements).