A 66-year-old man comes to the clinic for progressive slowness of movement and resting tremor for 1 year. Neurologic examination shows cogwheel rigidity, bradykinesia, and a 4–5 Hz resting tremor. MRI is unremarkable. The clinician explains that degeneration of a pigmented nucleus in the rostral midbrain that normally facilitates movement contributes to his symptoms. Which of the following best explains why this nucleus has a black to dark brown appearance on gross pathology?