Substantia nigra: what is it, functions and related disorders?

This brain structure belonging to the basal ganglia is related to movement.

Parkinson's disease and other brain disorders are related to alterations in the transmission of dopamine, which is dependent on the region of the brain known as the 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.

What is the substantia nigra?

The substantia nigra is part of the basal gangliaa brain 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 brain stem and is related to movement, consciousness, alertness, vision and hearing.

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

Neuromelanin, a pigment of the dopaminergic neurons (which are abundant in this region) (abundant in this region), confers its name to the substantia nigra as it gives it its characteristic dark hue.

Compact part and reticulate part

The substantia nigra is divided into two sections: the compact part and the reticulate part.. Each 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, thus playing a fundamental role in the initiation and regulation of fine motor skills.

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

This section is intimately related to the globus pallidus, and its neurons use and its neurons utilize the neurotransmitter GABA, which has an inhibitory function in the nervous system, including on the dopaminergic activity of the substantia nigra pars compacta.

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

What are its functions?

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

1. Reward

Dopamine, which has a very notable presence in the compact part of the substantia nigra, is secreted by the organism when we obtain a reward and provokes pleasurable sensationsIt helps us to predict which behaviors will lead to reinforcement.

Thus, thanks to the substantia nigra, an integration between stimuli and reactions takes place, making certain external data more likely to lead to the repetition of a certain behavioral pattern.

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.

Fine motor skills

The neurons of the compact part of the substantia nigra regulate the action of the neostriatum, directly involved in the performance of movements.. Thus 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 fine movements.

The nigrostriatal tract, consisting of neurons whose somas 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 especially important for spatial learning.

The learning facilitating function of the substantia nigra is also related to dopamine and its reinforcing effects; in particular, dopaminergic neurons seem to fire to a greater extent upon the appearance of new or surprising stimuli.

4. Temporal processing

Lesions in the compact part of the substantia nigra have been shown to cause deficits in time perception. in the perception of timeparticularly in the detection of intervals between stimuli. In this way, a notion of temporal distribution is created in which both stimuli and actions undertaken follow one after the other.

5. Eye movements

The connections of the reticulate part of the substantia nigra with the thalamus are involved in the control of saccadic eye movements. saccadic movements of the eyesnecessary for visual processing. They also contribute to the stabilization of gaze, independently of changes in head or face position.

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. Their role is especially important in REM sleep or REM (rapid eye movement) sleep.

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

Related disorders

Parkinson's disease is due to 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 is due to a reduction in the density of these neurons, which contain neuromelanin. is due to a reduction in the density of these neurons, which contain neuromelanin.

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

Abnormal activation of substantia nigra neurons has been linked both to the symptoms of Parkinson's disease and to the occurrence of epileptic seizures.

Dopamine and substantia nigra are also implicated in schizophrenia.. Dopaminergic pathways are altered in this disorder, and dopamine levels are often very high. Also, in schizophrenia, structural changes occur in the substantia nigra.

Referencias bibliográficas:

• 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. Epilepsia, 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 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.