Black substance: what it is, functions and related disorders

April 13, 2024

Parkinson's disease and other brain disorders are related to alterations in the transmission of dopamine, which depends on the region of the brain that we know as a substantia nigra .

But what exactly is the substantia nigra? 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 substantia nigra?

The black substance part of the basal ganglia , a cerebral system composed of the neoestriate, the pale globe and the subthalamic nucleus, as well as the substantia nigra.

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

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

Neuromelanin, a pigment of dopaminergic neurons (abundant in this region), gives its name to the black substance as it gives it its characteristic dark tone.

Related article: "Basal ganglia: anatomy and functions"

Compact part and reticulated part

The substantia nigra is divided into Two sections: the compact part and the reticulated part . Each of them has its own functions and connects the mesencephalon 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 neo-extrastriate, which is why it plays a fundamental role in the initiation and regulation of fine motor skills.

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

This section is intimately related to the pale globe and their 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 part, since, as we have said, neuromelanin is found in dopaminergic neurons, more numerous in the compact than in the reticulated part.

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 ocular movements.

1. Reward

Dopamine, which has a very noticeable presence in the compact part of the substantia nigra, is secreted by the organism when we obtain a reward and causes pleasurable sensations , so that it helps us predict which behaviors will entail reinforcement.

In this way, thanks to the black substance, an integration between the stimuli and the reactions is carried out, making certain data from the outside make it more possible for a certain pattern of behavior to be repeated.

The effects of dopamine and the reward brain system partly explain the motivation to seek reinforcements, 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 skills

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

The nigrostriatal tract, formed by neurons whose somas are located in the substantia nigra, depends on dopamine. Damages in this dopaminergic pathway are the cause of Parkinson's disease.

3. Learning

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

The facilitating function of learning of the substantia nigra is also related to dopamine and its reinforcing effects; in particular, dopaminergic neurons seem to shoot up more in the face of new or surprising stimuli.

4. Temporary processing

It has been shown that lesions in the compact part of the substantia nigra 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 undertaken are happening.

5. Eye movements

The connections of the reticulated part of the substantia nigra with the thalamus are involved in the control of the saccadic movements 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. Regulation of sleep

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

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

Related disorders

Parkinson's disease is due to the degeneration of the dopaminergic neurons of 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 of rest, slowness of movement, rigidity, low mood, sleep disturbances, etc.

The abnormal activation of the neurons of the substantia nigra has been related both to the symptoms of Parkinson's disease and to the appearance of epileptic seizures.

Dopamine and the substantia nigra they are also involved in schizophrenia . The dopaminergic pathways are altered in this disorder, and dopamine levels are usually very high. Likewise, schizophrenia produces 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., Leger, 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.A. B.F. & Tufik, S. (2007). The role of the substantia nigra pars compact in regulating sleep patterns in rats. Public Library of Science, 2 (6), e513.