Where is the substantia nigra?
The substantia nigra is a nucleus in the midbrain that is considered part of the basal ganglia. It looks like a darkened streak in unstained brain tissue; this is where it gets its name, which is Latin for "black substance." Although it is often referred to as one structure, the substantia nigra is actually made up of two anatomically and functionally distinct portions: the substantia nigra pars compacta and the substantia nigra pars reticulata. Neurons in the pars compacta are much more densely packed together (or compact) than those in the pars reticulata.
What is the substantia nigra and what does it do?
Most of the dopamine neurons of the brain originate in the midbrain and are found in either the substantia nigra or the ventral tegmental area, which is located adjacent to the substantia nigra. The dopamine neurons in the substantia nigra express high levels of a pigment called neuromelanin, which accounts for their dark color. These dopamine neurons, however, are found predominantly in the substantia nigra pars compacta. The pars reticulata is instead populated largely by GABA neurons.
Many of the dopamine neurons of the substantia nigra project to the striatum, another part of the basal ganglia that is made up of the caudate and putamen. In doing so they form a pathway called the nigrostriatal dopamine pathway that is thought to be crucial in the facilitation of movement.
The influence of the substantia nigra on movement is made apparent by observing the symptoms of Parkinson's disease, which are associated with the death of dopamine neurons in the substantia nigra pars compacta. Although it still isn't clear what exactly causes neurodegeneration in Parkinson's disease, when a significant number of these neurons have died, the individual will likely start to experience movement-related problems like tremors, rigidity, slowness of movement, and postural instability--all hallmark symptoms of Parkinson's disease.
Being one of the major dopamine-producing areas of the brain, however, the substantia nigra has functions that extend beyond just motor control. It is also thought to play important roles in a number of other functions and behaviors, including learning, drug addiction, and emotion.