Parkinson’s disease and other brain disorders are related to alterations in the transmission of dopamine, which depends on the region of the brain known 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 features and the diseases in which it is involved.
What is the substantia nigra?
The substantia nigra is part of the basal ganglia , a brain system composed of the neostriatum, globus pallidus and subthalamic nucleus, as well as 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, awareness, alertness, vision, and hearing.
In the human brain we found black substance on both sides of the midline. This means that there is a substantia nigra in each hemisphere of the brain.
Neuromelanin, a pigment of the dopaminergic neurons (abundant in this region), gives the black substance its name as it gives it its characteristic dark tone.
Compact part and cross-linked part
The black substance is divided into two sections: the compact part and the cross-linked part . Each section has its own function and connects the midbrain to different regions of the nervous system.
The compact part transmits signals to the other 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 basal ganglia efferences 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 closely related to the globus pallidus and its neurons use the neurotransmitter GABA, which has an inhibitory function on the nervous system, including on 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 part than in the reticulated one.
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 reticulated part also influences other processes, especially neuronal inhibition (via GABA) and eye movements.
Dopamine, which has a very notable presence in the compact part of the black substance, is secreted by the organism when we obtain a reward and causes pleasant sensations , so that it helps us to predict which behaviours will lead to a reinforcement.
In this way, thanks to the black substance, an integration between the stimuli and the reactions takes place, making certain data from the outside more likely to repeat a certain pattern of behaviour.
The effects of dopamine and the brain’s reward system explain in part 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 skills
The neurons of the compact part of the substantia nigra control the action of the neostriatum, directly involved in the performance of movements . Thus the basal ganglia as a whole influence motility 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. Damage to this dopaminergic pathway is the cause of Parkinson’s disease.
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 role of the substantia nigra is also related to dopamine and its reinforcing effects; in particular, dopaminergic neurons seem to be triggered to a greater extent by new or surprising stimuli.
4. Temporary processing
It has been shown that lesions in the compact part of the substantia nigra cause deficits in time perception , particularly in the detection of intervals between stimuli. In this way, a notion of the time distribution is created in which both the stimuli and the actions taken follow each other.
5. Eye movements
The connections of the cross-linked part of the black substance with the thalamus are involved in controlling the saccadic movements of the eyes , which are necessary for visual processing. They also contribute to the stabilization of the gaze, regardless of changes in head or face position.
6. Sleep regulation
Studies with rats suggest that the 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 or REM sleep (rapid eye movement).
This function may explain the sleep problems that frequently occur in Parkinson’s disease, which is related to lesions in the substantia nigra.
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 is due to the reduction in density of this type of neuron, which contains the 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 neurons in the substantia nigra has been linked to both the symptoms of Parkinson’s disease and the occurrence of seizures.
Dopamine and the substance nigger are also involved in schizophrenia . Dopaminergic pathways are altered in this disorder, and dopamine levels are often very high. Structural changes in the substantia nigra also occur in schizophrenia.
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