The brainstem is located at the top of the spinal cord. The parts affected by PSP are the pons, which controls facial nerves and the muscles that turn the eye outward, and the midbrain, the visual center. The basal ganglia are islands of nerve cells located deep within the brain. The cerebellum is located at the base of the skull. It controls balance and muscle coordination.
Vision is controlled by groups of cells called nuclei in the brainstem.
In PSP, the nuclei continue to function, but the mechanisms that control the nuclei are destroyed. At first, the difficulty only occurs in trying to look down. As the disease progresses, ability to move the eyes right and left is also affected. However, reflex or unwilled eye movements remain normal. Thus, when the patient's head is tilted upwards, the eyes move to look down.