The basal ganglia They are part of the most primitive structures of the human brain.
These groups of nerve cells are responsible for carrying out processes related to implicit learning, the incentive system and the execution of movements, although, as with all parts of the brain, they work in coordination with the rest of the parts of the brain. nervous system.
In this article we will see what the basal ganglia are, the functions they perform, their anatomical and functional characteristics, and some of the main neurological diseases that affect them, summarizing and explaining it in a simple way so that it is easy to understand.
The cerebral cortex is the most visible part of the human brain, as well as the best known. Its division into two hemispheres, as well as four lobes (frontal, parietal, temporal and occipital) has been known and studied since ancient times.
However, the human brain is a complex organ, possessing different structures and substructures that are fundamental for the functioning and maintenance of bodily and cognitive functions, participating in a large number of areas. Examples of these parts of the brain are the hippocampus, the limbic system or the set of nuclei discussed in this article, the basal ganglia
We call the basal ganglia a set of interconnected subcortical nuclei located around the limbic system and the third ventricle. They are located on both sides of the thalamus, at the level of the temporal lobe.
These are clusters of gray matter ; that is, areas in which the parts of the neurons that are not myelinated are concentrated) that have a large number of connections with other areas of the brain, such as the cortex or the thalamus, both at the level of afferents and efferents (they receive information from other brain areas).
Thus, gray matter indicates the presence of neuronal somata, which are the parts of the neurons in which we find the cell nucleus. This implies that both in the basal ganglia and in the rest of the areas of the brain in which gray matter predominates tasks of processing the information carried by neurons are carried out unlike what happens in the white matter areas, which indicate the presence of axons.
The basal ganglia are distributed among areas near the center of the brain, under the cerebral cortex and around the diencephalon, and due to their location they play a role in actions that are between automatic and voluntary. Between them passes the internal capsule, which is a set of axons (that is, white matter) that communicates the cerebral cortex with subcortical areas.
The main neurotransmitters that act in these brain areas are dopamine as an excitatory component and GABA as an inhibitory component, having different effects depending on the nucleus and nervous circuits in which they act.
Main components of the basal ganglia
Despite being considered a set of nuclei given their interconnection, The basal ganglia consist of various differentiated substructures and, in fact, physically it is easy to notice the spaces between them. Below you can find the main structures of this set of brain structures:
1. Striatum
The striatum is considered the main area for receiving information from the basal ganglia That is, it is an area that receives numerous projections from various brain areas, integrating the information and acting with it. It is composed of the neostriatal nucleus (composed of the caudate nucleus and putamen), which is mainly responsible for receiving afferents from the nigrostriatal tract, and the lenticular nucleus (made up of putamen and globus pallidus), more focused on sending messages to other nuclei. cerebral.
2. Caudate Nucleus
Located below the frontal lobe and in connection with the occipital, this structure is linked to the feeling of alarm, warning that something is not working correctly, as well as motivation. This is due to its connections with the frontal lobe, especially the orbitofrontal cortex.
3. Putamen
Fundamental element in the control of automated movements, Located below the caudate nucleus and united by the anterior area. It is linked to the movement of the face and extremities
4. Globus pallidus
Derived from the diencephalon, it is located between the putamen and the internal capsule In this case it has myelinated neurons that are used to transmit information to the thalamus and the substantia nigra.
5. Substantia nigra
Located under the thalamus, in the brain stem, this structure is known to be one of the main sources of dopamine in the brain. Actively participates in the brain reward system. In its connection with the striatum (forming part of the nigrostriatal tract) it also has great importance in the control of fine movement, both of the limbs and the eyes.
Located under the globus pallidus, it receives signals from the ventral tegmental area and sends signals to the globus pallidus This nucleus participates in the maintenance of behaviors reinforced by drugs and habituation, having connections with the limbic system.
7. Subthalamic nucleus
Located at the junction between the midbrain and thalamus the main function given to the subthalamic nucleus is to regulate motor functions.
8. Red substance
This structure maintains important connections with the cerebellum and spinal cord****l, being especially linked to motor coordination. Specifically, it has special relevance in the control of arms and shoulders.
Functions of the basal ganglia
As has been seen in the explanation of its components, The functions of the basal ganglia are varied, participating in numerous and important aspects of our lives Making a general review of the aspects in which they participate, we can say that some of their main functions are the following:
1. Planning, integration and control of voluntary movement
One of the functions for which the basal ganglia is best known is the regulation and management of voluntary motor actions. Acting through the corticostriatal circuit, They specifically act as an unconscious filter that inhibits or selects the movements to be made specifically helping to control body posture and coordinate fine/precise movement of the extremities.
The basal ganglia allows you to mark the end of a movement, plan sequences and correct them if necessary.
2. Procedural learning
Another aspect in which the basal ganglia Their predominant performance is in procedural learning and in the automation of behavior This type of learning is what allows you to get used to carrying out sequences of actions such as those necessary to drive, play an instrument, shave or sew, allowing the management of attentional resources so that they can be directed towards other tasks.
3. Executive functions
The basal ganglia are also actively involved in executive functions Specifically, it contributes to maintaining processing speed, planning at a cognitive level and developing strategies to solve problems. Similarly, the connections of the basal ganglia with the orbitofrontal cortex mean that they are involved in the capacity for behavioral inhibition.
4. Participation in emotional and motivational behavior
As it mentioned above, some basal ganglia such as the nucleus accumbens have connections with the limbic system and the brain reward system, given its importance in the management of dopamine. Thus, the basal ganglia can be considered to participate in emotional behavior and in the reinforcement produced by drugs or stimulation.
For example, the basal ganglia play a very important role in both classical conditioning and operant conditioning.
Disorders linked to problems in the basal ganglia
As can be seen, all these elements and functions make the basal ganglia fundamental elements for the correct functionality of the organism.
But…, What happens when there is an injury or an event that causes these nuclei to not coordinate or act as they should? In that case we may encounter some of the following problems and disorders, generally dealing with hypo- or hyperkinetic problems, that is, those linked to movement.
1. Parkinson’s
The most common and best-known disorder resulting from poor functioning of the basal ganglia is Parkinson’s disease. The most recognizable symptoms of this disorder are parkinsonian or resting tremors. Likewise, muscle rigidity and loss of spontaneous movements also appear. along with marked bradykinesia or loss of motor speed and gait disorders.
This disorder occurs especially due to the degeneration and death of dopaminergic cells in the nigrostriatum which causes dopamine transmission to be largely lost, with information not reaching the motor cortex.
2. Huntington’s chorea
It is a neurodegenerative genetic disorder caused by an alteration in a dominant gene on chromosome number four, with full penetration. It produces hyperkinetic symptoms: the disease causes uncontrolled movements similar to a dance (hence the name chorea), and significant loss of executive functions and memory. The deficits are mainly produced by the death of neurons in the caudate nucleus, especially GABAergic and cholinergic neurons.
3. Syndrome of loss of psychic self-activation
As mentioned above, the basal ganglia is related to personal motivation. An injury in these areas can therefore have serious implications in this regard as happens in the syndrome of loss of psychic self-activation or PAP.
This disorder, also known as pure psychic akinesia or loss of initiation or maintenance of action, produces extreme passivity in those who suffer from it, losing the ability to be interested, spontaneity and motivation. Those who suffer from it are able to recognize their deficits, so that anosognosia does not occur, but they have great indifference towards them.
4. Tic disorders and Tourette syndrome
In these disorders, such as Tourette syndrome, which are characterized by the presentation of very stereotyped movements, vocalizations, gestures or behaviors that are performed unconsciously, there is severe involvement of the basal ganglia. More specifically, it is thought that these disorders may be related to problems in the putamen.
5. Other linked disorders
Apart from these disorders, alterations in the basal ganglia usually occur in a large number of psychological problems. For example, in obsessive-compulsive disorder or ADHD there can and seem to be alterations in these brain systems.
