lifetimes. Tremor A tremor can be defined as a rhythmic, back and forth or oscillating involuntary movement about a joint axis. Tremors are symmetric about a midpoint within the movement, and both portions of the movement occur at the same speed. Unlike the other hyperkinetic movements, tremors lack both the jerking associated movements and posturing. Essential tremor (ET), also known as benign essential tremor, or familial tremor, is the most common movement disorder. It is estimated that 5 percent of people worldwide have this condition, affecting those of all ages but typically staying within families. ET typically affects the hands and arms but can also affect the head, voice, chin, trunk and legs. Both sides of the body tend to be equally affected. The tremor is called an action tremor, becoming noticeable in the arms when they are being used. Patients often report that alcohol helps lessen the symptoms. Primary medical treatments for ET are usually beta-blockers. For patients who fail to respond sufficiently to medication, deep brain stimulation and thalamotomy can be highly effective. A "flapping tremor," or asterixis, is characterized by irregular flapping-hand movement, which appears most often with outstretched arms and wrist extension. Individuals with this condition resemble birds flapping their wings. Volitional hyperkinesia Volitional hyperkinesia refers to any type of involuntary movement described above that interrupts an intended voluntary muscular movement. These movements tend to be jolts that present suddenly during an otherwise smoothly coordinated action of skeletal muscle. Pathophysiology The causes of the majority of the above hyperkinetic movements can be traced to improper modulation of the basal ganglia by the subthalamic nucleus. In many cases, the excitatory output of the subthalamic nucleus is reduced, leading to a reduced inhibitory outflow of the basal ganglia. Without the normal restraining influence of the basal ganglia, upper motor neurons of the circuit tend to become more readily activated by inappropriate signals, resulting in the characteristic abnormal movements. There are two pathways involving basal ganglia-thalamocortical circuitry, both of which originate in the neostriatum. The direct pathway projects to the internal globus pallidus (GPi) and to the substantia nigra pars reticulata (SNr). These projections are inhibitory and have been found to utilize both GABA and substance P. The indirect pathway, which projects to the globus pallidus external (GPe), is also inhibitory and uses GABA and enkephalin. The GPe projects to the subthalamic nucleus (STN), which then projects back to