Substantia Nigra: What It Is, Functions And Related Disorders
Parkinson’s disease and other brain disorders are related to alterations in dopamine transmission, which depends 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.
The black substance 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, consciousness, alertness, vision and hearing.
In the human brain we find substantia nigra on both sides of the midline. That is, there is a substantia nigra in each hemisphere of the brain.
Neuromelanin, a pigment in dopaminergic neurons (abundant in this region), gives its name to the substantia nigra since it gives it its characteristic dark tone.
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 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 has a fundamental role in the initiation and regulation of fine motor skills.
The pars reticulata sends output 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 It is closely related to the globus pallidus and its neurons use the neurotransmitter GABA, which has an inhibitory function in the nervous system, including in the dopaminergic activity of the substantia nigra compacta.
The compact part is darker than the reticulated part, since, as we have said, neuromelanin is found in dopaminergic neurons, which are more numerous in the compact part 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 dopamine. However, the reticulate part also influences other processes, especially neuronal inhibition (through 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 body when we obtain a reward and causes pleasurable sensations so that it helps us predict which behaviors will lead to reinforcement.
In this way, thanks to the substantia nigra, an integration is carried out between stimuli and reactions, making certain data from the outside more likely to repeat a certain pattern of behavior.
The effects of dopamine and the brain reward 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.
The neurons of the substantia nigra pars compacta 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 fine movements.
The nigrostriatal tract, formed by neurons whose somata are located in the substantia nigra, depends on dopamine. Damage to this dopaminergic pathway is the cause of Parkinson’s disease.
3. Learning
The substantia nigra pars compacta 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 facilitating 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. Temporal processing
Lesions in the substantia nigra pars compacta 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 undertaken occur.
5. Eye movements
The connections of the pars reticulata of the substantia nigra with the thalamus are involved in the control of the saccadic eye movements, necessary for visual processing. They also contribute to the stabilization of gaze, regardless of changes in the position of the head or face.
6. Sleep regulation
Studies with rats suggest that dopaminergic neurons in the substantia nigra pars compacta are essential for regulating the sleep-wake cycle. Its role is especially important in REM sleep or REM (rapid eye movement).
This function could explain the 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 dopaminergic neurons in the substantia nigra pars compacta. 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: resting tremors, slowness of movement, rigidity, low mood, sleep disturbances, etc.
Abnormal activation of neurons in the substantia nigra has been related to both the symptoms of Parkinson’s disease and the appearance of epileptic seizures.
Dopamine and the substantia nigra They are also involved in schizophrenia Dopaminergic pathways are altered in this disorder, and dopamine levels are usually very high. Likewise, in schizophrenia structural changes occur in the substantia nigra.
Matell, M.S. & Heck, W.H. (2000). Neuropsychological mechanisms of interval timing behavior. BioEssays, 22(1), 94-103.
