What is Parkinson Disease?

Parkinson disease (PD) is a disease of the nervous sys-tem. Initially, PD affects a region of the brain called the basal ganglia, a region that regulates movement, posture, and balance. In time, PD may affect the cortex, the thinking and remembering part of the brain. And, in time, PD may affect the autonomic nervous system (ANS), which regulates blood pressure, the bowels and the bladder. All of these areas are affected by PD. PD is, in part, a disease of slowed movement: every-thing you do takes longer. PD is also, in part, a disease of fast movement: the tremors of PD are 4 to 6 cycles per second (CPS)—faster than the eye can count. Why are some of the movements fast and others slow?

Willed, voluntary movement begins in the cortex, the thinking part of your brain. Your cortex specifies the speed, amplitude, direction, shape, regularity, and duration of each movement you make. The cortex is like the President, or chief executive, of your brain. Once your cortex decides on a course of movement, like walking down a road or up a flight of steps, it must decide how the movement will look (speed, amplitude, duration, etc.), then it “asks” the basal ganglia (a subconscious region of your brain) to set up rules that will govern these movements and allow them to go or stop in response to changing conditions, such as a bump in the road or a cat lying on the staircase. The basal ganglia are like Congress— they formulate the commands of the cortex into repetitive ongoing movements (e.g., walking) that continue while the cortex directs your attention to other things.Just like Congress, the basal ganglia have two parts.

The first includes the substantia nigra, which is like the “Go” party that wants to implement the President’s commands. In PD, the substantia nigra is damaged, preventing the basal ganglia from correctly carrying out the cortex’s commands, which results in slowed or incomplete movements. The basal ganglia also include the globus pallidus and the subthalamic nucleus, which function as modulators of the cortex’s commands, or a “Stop” party. The interaction of the damaged “Go” and “Stop” parties results in various combinations of fast and slow movements.

The cerebellum (literally “little brain” in Latin) sits below the basal ganglia . Damage to the cerebellum results in disorders of balance. Patients with disease of the cerebellum are unsteady and they may teeter or lurch as though they are drunk. The cerebellum is like the Supreme Court of the brain: it judges whether the movements being carried out by the basal ganglia (the Congress) are faithful to the commands of the cortex (the President). Even though the cerebellum is undamaged in PD, it believes the basal ganglia are behaving incorrectly, and in trying to “correct” them, makes things worse by causing lurching or trembling.