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Substantia nigra

from the latin: niger - black
German: Substantia nigra

1 Definition

The term substantia nigra refers to a nuclear complex in the mesencephalon area which appears to be coloured dark due to a high intracellular concentration of melanin and iron.

2 Anatomy

The substantia nigra is located in the mesencephalon and envelopes the cerebral crus dorsally. Morphologically one can distinguish:

Some authors also distinguish an additional third region: The pars lateralis, which is mostly attributed to the pars reticulata.

The substantia nigra can be understood as being a functional part of the basal ganglia. It receives afferent nerve pathways from the precentral cortex, the caudate nucleus and the putamen. The efferent nerve pathways go to the striatium and to the thalamus. Histochemical investigation can discriminate the different neurotransmitters of the neurons in the substantia nigra. For the pars compacta an especially high dopamine content is typical.

3 Physiology

The substantia nigra is part of various neural circuits of the extrapyramidal motor system and through that is in contact with the cortex, striatum, subthalamic nucleus, thalamus and other brain structures. It is an important section of the basal ganglia loops. Its dopaminergic neurons communicate signals that are especially important for initiation of motion and its planning ("starting function").

4 Pathophysiology

A failure of the dopaminergic system gives rise to a loss of disinhibition of other parts of the neuronal circuits and presents as the typical symptoms associated with Parkinon’s disease. In contrast to that, the failure of the subthalamic nucleus or other external parts of the globus pallidus gives rise to a relative hyperactivity of the nigral neurons and presents as diseases like Huntington’s (pallidum) or ballism (subthalamic nucleus).

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