Substance nigra: what it is, functions and related disorders

The Parkinson's disease and other brain disorders are associated with alterations in the transmission of dopamine, that depends of the region of the brain that we know as the substantia nigra.

But what exactly is the black substance? In this article we will review the functions of this brain structure, its anatomical characteristics and the diseases in which it is involved.

• Related article: "Parts of the human brain (and functions)"

What is the black substance?

The black substance is part of the basal ganglia, a cerebral system composed of the neostriatum, the globus pallidus and the subthalamic nucleus, in addition to the substantia nigra.

It is located in the part of the brain known as the "midbrain". This region is considered part of the brainstem and is related to movement, consciousness, alertness, vision, and hearing.

In the human brain we find substantia nigra on both sides of the midline. That is, there is a black substance in each hemisphere of the brain.

Neuromelanin, a pigment in dopaminergic neurons

(abundant in this region), gives its name to the black substance since it gives it its characteristic dark tone.

• Related article: "Basal ganglia: anatomy and functions"

Compact part and reticulated part

The black substance is divided into two sections: the compact part and the reticulated part. Each of them has its own functions and connects the midbrain with different regions of the nervous system.

The compact part transmits signals to the rest of the basal ganglia. It projects dopaminergic neurons to the neostriatum, which is why it plays a fundamental role in the initiation and regulation of fine motor skills.

The reticulated part sends outflows from the basal ganglia to other areas of the brain, such as the thalamus, a key nucleus for information exchange between subcortical structures and the cortex cerebral.

This section is closely related to the globe pallidus and your neurons use the neurotransmitter GABA, which has an inhibitory function in the nervous system, including in the dopaminergic activity of the compact substantia nigra.

The compact part is darker than the reticulated one, since, as we have said, neuromelanin is found in dopaminergic neurons, more numerous in the compact part than in the reticulated one.

Its duties?

Most of the functions in which the substantia nigra is involved have to do with its compact part and with dopamine. However, the reticulated part also influences other processes, especially neuronal inhibition (through GABA) and eye movements.

1. Reward

Dopamine, which has a very noticeable presence in the compact part of the substantia nigra, is secreted by the body when we obtain a reward and causes pleasant sensations, in a way that helps us predict which behaviors will lead to reinforcement.

In this way, thanks to the substantia nigra, an integration between the stimuli and the reactions, making certain external data more likely to repeat a certain pattern of behaviour.

The effects of dopamine and the brain's reward system partly explain the motivation to seek reinforcement, sexual pleasure, or the development of addictions. In other words, it affects both the adaptive use of learning and its degeneration due to addictions.

2. Fine motor

The neurons of the compact part of the substantia nigra regulate the action of the neostriatum, directly involved in performing movements. In this way, the basal ganglia as a whole influence motor skills in general, while the substantia nigra is more specifically related to the control and initiation of movements fine.

The nigrostriatal tract, made up of neurons whose bodies are located in the substantia nigra, is dependent on dopamine. Damage to this dopaminergic pathway is the cause of Parkinson's disease.

3. Learning

The compact part of the substantia nigra plays a key role in learning the brain's response to stimuli. This region of the brain is especially important for spatial learning.

The learning facilitation function of the substantia nigra is also related to dopamine and its reinforcing effects; specifically, dopaminergic neurons seem to fire to a greater extent when new or surprising stimuli appear.

4. Temporary processing

Lesions in the compact part of the substantia nigra have been shown to cause deficits in the perception of time, particularly in the detection of intervals between stimuli. In this way, a notion of the temporal distribution is created in which both the stimuli and the actions taken take place.

5. Eye movements

The connections of the reticulated part of the substantia nigra with the thalamus are involved in the control of the saccades of the eyes, necessary for visual processing. They also contribute to the stabilization of the gaze, regardless of changes in the position of the head or face.

6. Sleep regulation

Studies with rats suggest that dopaminergic neurons in the compact part of the substantia nigra are essential for regulating the sleep-wake cycle. Its role is especially important in REM sleep or REM (of rapid eye movements).

This function could explain the sleep problems that occur frequently in Parkinson's disease, related to lesions in the substantia nigra.

Related disorders

Parkinson's disease is due to the degeneration of dopaminergic neurons in the compact part of the substantia nigra. In fact, the discoloration of the substantia nigra that occurs in this disorder It is due to the reduction in the density of this type of neurons, which contain neuromelanin.

Many of the characteristic symptoms of Parkinson's are related to a deficit in the functions of the substantia nigra: tremors at rest, slowness of movement, rigidity, low mood, disturbances of the sleep, etc.

Abnormal activation of substantia nigra neurons has been associated with both Parkinson's disease symptoms and seizures.

Dopamine and the substantia nigra are also implicated in schizophrenia. Dopamine pathways are disrupted in this disorder, and dopamine levels are usually very high. Likewise, in schizophrenia there are structural changes in the substantia nigra.

• You may be interested: "The 6 types of schizophrenia and associated characteristics"

Bibliographic references:

• Deransart, C., Hellwig, B., Heupel-Reuter, M., Léger, J. F., Heck, D. & Lücking, C. H. (2003). Single-unit analysis of substantia nigra pars reticulata neurons in freely behaving rats with genetic absence epilepsy. Epilepsy, 44 (12), 1513-20.

• Lima, M. M. S., Andersen, M. L., Reksidler, A. B., Vital, M. TO. B. F. & Tufik, S. (2007). The role of the substantia nigra pars compacta in regulating sleep patterns in rats. Public Library of Science, 2 (6), e513.

• Matell, M. S. & Heck, W. H. (2000). Neuropsychological mechanisms of interval timing behavior. BioEssays, 22 (1), 94-103.