The substantia nigra, also known as the black nucleus, is a large gray matter structure of the mesencephalon or midbrain (middle brain).
It gets its name from its dark appearance on the cross-sections of the brain (Latin for 'Black substance'). The dark coloration comes from the high number of neurons that contain the substance neuromelanin.
The main function of the substantia nigra is to produce the neurotransmitter dopamine which is crucial for motor and movement control, cognitive executive functions, and emotional limbic activity. Moreover, the substantia nigra also participates in the reward functions as part of the reward system circuitry.
The degeneration of the substantia nigra neurons leads to the clinical picture of Parkinson's syndrome.
English: Substantia nigra
English synonyms: Soemmering's substance, black matter, nucleus of basis pedunculi, black nucleus
Latin: Substantia nigra
Latin synonyms: Nucleus pigmentosus subthalamo-peduncularis, nucleus niger
|Definition||The dopaminergic nucleus located in the midbrain.|
|Parts||Pars compacta, pars reticulatis|
|Function||Movement control, cognitive functions, emotional limbic regulation, reward circuitry|
The substantia nigra is a paired structure of the midbrain. More specifically, it is located posterior to the crus cerebri.
Based on its structure and function, the substantia nigra can be divided into two functionally independent areas:
- Pars compacta
- Pars reticulata
The pars compacta is a larger, dorsomedial portion of the substantia nigra. This part appears particularly dark in color due to its high intracellular neuromelanin content (precursor in dopamine synthesis). The pars compacta contains mainly densely packed dopaminergic neurons.
The pars compacta receives afferent projections mainly from the pedunculopontine nucleus and the ascending reticular activating system (ARAS). It sends efferent dopaminergic projections to the striatum (nigrostriatal pathway), or more specifically the putamen and caudate nuclei. Due to these connections, the pars compacta is considered a critical part of the basal ganglia pathways and thus it has a prominent function in motor control.
The pars reticulata is a ventrolateral portion of the substantia nigra. This part faces the crura cerebri and appears rather reddish in color due to increased iron content. It is mainly composed of the inhibitory GABAergic neurons.
The pars reticulata mainly receives afferent fibers from the striatum and subthalamic nucleus. It sends efferent projection mainly to the motor thalamus. Due to its connections, the pars reticulata is also an important processing center in the basal ganglia.
Being one of the main sources of dopamine in the brain and a crucial part of the basal ganglia pathways, the prime function of the substantia nigra is in the initiation and control of movement. Movement is controlled in the basal ganglia loop via a direct pathway and an indirect inhibitory pathway. Both pathways are significantly influenced by dopamine released from the fibers of the pars compacta of the substantia nigra. The degeneration of dopaminergic cells in pars compacta leads to a condition known as Parkinson's disease.
Moreover, the substantia nigra is part of the reward system circuitry of the brain. This system reinforces behaviors associated with rewards and prevents behaviors that lead to punishment. The changes in the reward system are connected to a number of psychiatric conditions such as substance abuse, alcoholism, impulse disregulation and agression.
Learn more about the structure and function of the basal ganglia with the following study unit:
Degeneration of dopamine-producing cells in the pars compacta of the substantia nigra leads to a common clinical condition called Parkinson's disease. In Parkinson's disease, an individual has a lack of dopamine in the striatum which results in a lack of movement. Clinically, Parkinson's syndrome is expressed in the three classic main symptoms:
- Rigor - increased muscle tone
- Muscle tremor
- Akinesia - lack of movement
The treatment is drug-based by administering L-DOPA (dihydroxyphenylalanine), a precursor of dopamine. L-DOPA can then be converted into dopamine by the cells of the substantia nigra and act on the striatum.
Moreover, there are characteristic histologic changes in the substantia nigra that can be detected in patients with Parkinson's disease. They present in the form of small, round, eosinophilic inclusions called Lewy bodies. These structures are composed primarily of protein aggregates such as ɑ-synuclein.
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