The Substantia Nigra

The substantia nigra, a midbrain structure, is considered part of the basal ganglia complex due to its close ties with the striatum. Classically it has been divided into two components: the pars compacta (SNc), and the pars reticulata (SNr). The pars compacta is a cell--rich region that in humans is composed of large pigmented neurons. In some animals (for example, humans and squirrel monkeys) the large nigral neurons exhibit a characteristic black pigmentation; hence the origin of the structure's name ("black substance").

Cytology
Cells in both the pars compacta and pars reticulata exhibit a triangular (fusiform) shape. These cells have traditionally been divided into two types. Small neurons (10-12µm soma diameter) with short axons are considered to the major nigral interneurons (Parent 1986). These (in rodents) average at 10% of the total neurons in the SNc and 40% in the SNr (Carpenter 1981). The second type of neuron is larger (15-20µm soma diameter) and constitute the nigral output neurons.

The projection neurons display three to five long smooth dendrites orientated in two main directions and with few branches (see above). A close similarity has been drawn with the globus pallidus (external segment) output neurons. Neurons in the pars compacta and pars reticulata share many morphological features, yet one of the important distinctions found in rat is in their dendritic branching pattern. Most neurons in the pars compacta send 1--2 dendrites deep into the pars reticulata (Juraska et al. 1977).

References

Carpenter, M. (1981). Anatomy of the corpus striatum and brainstem integrating systems. Handbook of Physiology --- The Nervous System 2. 947-995

Juraska, J., Wilson, C. and Groves, P. (1977). The substantia nigra of the rat: A Golgi study. J. Comp. Neurol. 172. 585-600

Parent, A. (1986). Comparative Neurobiology of the Basal Ganglia. Wiley. New York.