The substantia nigra lies in the midbrain immediately dorsal to the cerebral peduncles. This nucleus is an important motor center that will be discussed at greater length later in the course. Right now you need to know that some of the cells project to the caudate and putamen, two nuclei of the basal ganglia that together comprise what is called the STRIATUM. These NIGROSTRIATAL cells utilize the neurotransmitter DOPAMINE.
The substantia nigra is thought to be the lesion site in PARKINSONS disease or paralysis agitans. In this disease there is muscular rigidity, a fine tremor at rest (resting tremor), akinesia or bradykinesia and a slow and shuffling gait and postural instability. You do not have to worry about the laterality (right or left) of these deficits at this time. The most consistent pathological finding in Parkinsons disease is degeneration of the melanin-containing cells in the pars compacta (another part is called the pars reticulata) of the substantia nigra (melanin is an inert by-product of the synthesis of dopamine). As mentioned above, cells within the nigra produce dopamine normally. This substance passesvia axoplasmic flowto the nerve terminals in the striatum (caudate nucleus and putamen), where it is released as a transmitter. It is the absence of this transmitter that produces the crippling disorder called Parkinsons disease.