Corpus: Substantia nigra

The term substantia nigra refers to a nuclear complex in the area of the mesencephalon that appears dark in colour due to a high intracellular content of melanin and iron.

The substantia nigra is located in the mesencephalon and attaches to the dorsal side of the cerebral hemispheres. A morphological distinction is made between

• Pars compacta: densely packed, melanin-containing neurones
• Pars reticulata: nerve cells with a particularly high iron content

Some authors differentiate between a third region, the pars lateralis, which is usually categorised as part of the pars reticulata.

The substantia nigra can be seen as a functional component of the basal ganglia and receives afferents from the precentral cortex, the caudate nucleus and the putamen. Its efferents travel to the striatum and the thalamus. Histochemically, various neurotransmitters can be detected in the neurones of the substantia nigra; the pars compacta is characterised by a particularly high dopamine content, while the pars reticulata mainly contains GABAergic neurones.

The substantia nigra is integrated into various circuits of the extrapyramidal motor system and is thus connected to the cortex, striatum, subthalamic nucleus, thalamus and other brain structures. It is an important section of the basal ganglia loop. Its dopaminergic neurones transmit signals that have a particular effect on movement initiation and planning ("starter function").

Failure of the dopaminergic system leads to the loss of disinhibition of other components of the circuit and thus to the symptoms of Parkinson's disease. In contrast, a failure of the subthalamic nucleus or the external parts of the globus pallidus leads to a relative hyperfunction of the nigra neurones and thus to diseases such as Huntington's disease (pallidum) or hemiballism (subthalamic nucleus).