In addition to motor reflexes, there are many aspects of the control of skeletal muscle contraction that are unconscious or only under partial conscious control. These motor activities include tone, posture, initiation of movement, performance of certain semivoluntary movements such as walking, and the suppression of many types of abnormal involuntary movements, such as tremors. The collection of nuclei called the basal ganglia, which are located deep in the cerebrum and the upper brainstem, appear to be important for controlling these involuntary and semivoluntary motor activities. The basal ganglia receives information from many areas of the brain, performs complex processing of that information, and then sends information to the ipsilateral motor cortices. In addition to the motor cortices, the basal ganglia is connected with many other areas of the brain, and parts of it also play roles in cognition and emotion.
Basal ganglia dysfunction may cause many motor abnormalities, that are often referred to collectively as movement disorders; some of these may also sometimes occur with lesions in other parts of the brain. A type of hypertonia called rigidity may occur, which is velocity-independent, meaning that there is the same amount of resistance to passive joint movement regardless of speed. Difficulty initiating movements may occur. The cause of this is unclear, but it is likely that there are motor programs that the basal ganglia activate to start movements, so that association motor cortices do not have to completely recreate a new motor plan every time for frequent movements, such as walking. Akinesia may occur, which means there are few overall movements, such as the frequent small postural changes that normally occur; these patients may sit or stand “like a statue”. Bradykinesia may occur, which means that movements may occur at slower speeds than normal. Dyskinesias may occur, which are involuntary abnormal movements or postures without weakness. Dyskinesias were previously called extrapyramidal signs, to distinguish them from the signs of corticospinal (which is also called pyramidal) tract dysfunction, but this term is used inconsistently.