Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less. In this installment I will discuss the substantia nigra.
The substantia nigra is a structure found in the brainstem that looks like a dark streak
in unstained brain tissue. Its appearance is where it gets its name, as substantia nigra
is Latin for “black substance.” There are two substantiae nigrae, one on each side
of the midline of the midbrain.
The substantia nigra is actually made up of two distinct regions: the substantia nigra
pars compacta and the substantia nigra pars reticulata. The substantia nigra gets its
dark coloring from the large number of dopamine neurons in the substantia nigra pars compacta,
as they express high levels of a dark pigment called neuromelanin. In fact, most of the
dopamine-producing neurons of the brain are found in either the substantia nigra or an
adjacent area called the ventral tegmental area. The dopamine neurons in the substantia
nigra are primarily found in the substantia nigra pars compacta, as the substantia nigra
pars reticulata is more heavily populated with GABA neurons.
The substantia nigra is considered part of the basal ganglia, and many of the dopamine
neurons in the substantia nigra pars compacta project to two other structures of the basal
ganglia, the caudate and putamen (which together are referred to as the striatum). These dopamine
neurons make up a large bundle of fibers called the nigrostriatal pathway. The nigrostriatal
pathway is one of the major dopamine pathways in the brain, and is thought to be extremely
important in the facilitation of movement. The GABA neurons of the substantia nigra pars
reticulata also play important roles in movement, and thus movement is one of the main functions
of the substantia nigra.
The substantia nigra also has many functions beyond motor control, however the role for
the substantia nigra in movement is particularly emphasized when we observe the symptoms of
Parkinson’s disease, a neurodegenerative disorder that involves the death of dopamine
neurons in the substantia nigra. Individuals suffering from Parkinson’s disease display
severe movement-related deficits.