Substantia Nigra

The substantia nigra is a darkly pigmented nucleus located in the midbrain, between the cerebral peduncle and tegmentum. Its characteristic colour is produced by neuromelanin within dopaminergic neurons. It is divided into the pars compacta and pars reticulata and forms an important component of the basal ganglia motor circuit. Dopamine released from the pars compacta reaches the striatum through the nigrostriatal pathway, helping regulate the initiation and smooth control of voluntary movement. Progressive loss of dopaminergic neurons and depigmentation of the substantia nigra are characteristic of Parkinson disease, producing bradykinesia, rigidity, resting tremor and postural instability. Microscopically, surviving neurons may contain Lewy bodies composed predominantly of α-synuclein.