2. Neostriatum

: Normally, the neostriatum is connected to the substantia nigra by neurons that secrete the inhibitory transmitter GABA at their termini in the substantia nigra. In turn, cells of the substantia nigra send neurons back to the neostriatum, secreting the inhibitory transmitter dopamine at their termini. This mutual inhibitory pathway normally maintains a degree of inhibition of the two separate areas. In Parkinson disease, destruction of cells in the substantia nigra results in the degeneration of the nerve terminals responsible for secreting dopamine in the neostriatum. Thus, the normal modulating inhibitory influence of dopamine on cholinergic neurons in the neostriatum is significantly diminished, resulting in overproduction or a relative overactivity of acetylcholine by the stimulatory neurons. This triggers a chain of abnormal signaling, resulting in loss of the control of muscle movements.