Basal ganglia: anatomy and functions.

We explain the importance of this brain region.

The basal ganglia are among 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 performance of movements, although, as with all parts of the brain, they work in coordination with the other parts of the 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.

What are the basal ganglia?

The cerebral cortex is the most visible part of the human brain, as well as the best known. Its division into two hemispheres and four lobes (frontal, parietal, temporal and occipital) has been known and studied since ancient times.

However, the human brain is a complex organ, possessing in its interior 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.

The basal ganglia are 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.

They are clusters of gray matterThese are areas where 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 afferent and efferent (they receive information from other brain areas).

Thus, the gray matter indicates the presence of neuronal somas, 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 other areas of the brain in which gray matter is predominant information processing tasks carried by neurons are carried out in the basal ganglia as well as in the other areas of the brain where gray matter predominates, unlike what happens in the basal ganglia of the brain.This is in contrast to what happens in the white matter areas, which indicate the presence of axons.

The basal ganglia are distributed between areas near the center of the brain, under the cerebral cortex and around the diencephalon, and because of 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 (i.e., white matter) that communicates the cerebral cortex with subcortical areas.

The main neurotransmitters acting in these brain areas are dopamine as excitatory and GABA as inhibitory component, having different effects depending on the nucleus and the nervous circuits in which they act.

Main components of the basal ganglia

Despite being considered as a group of nuclei due to their interconnection, the basal ganglia consist of a set of nuclei, the basal ganglia consist of several distinct substructures and, in fact, physically 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 of reception of information from the basal ganglia.. That is, it is an area that receives numerous projections from various brain areas, integrating the information and acting on it. It is composed of the neostriate nucleus (composed of caudate nucleus and putamen), which is mainly responsible for receiving afferences from the nigrostriatal tract, and the lenticular nucleus (composed of putamen and globus pallidus), more focused on sending messages to other brain nuclei.

2. Caudate nucleus

Located below the frontal lobe and in connection with the occipital nucleusThis structure is linked to the sensation of alarm, warning that something is not working properly, as well as to motivation. This is due to its connections with the frontal lobe, especially with the orbitofrontal cortex.

3. Putamen

A fundamental element in the control of automated movements. Located below the caudate nucleus and joined 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 which are used to transmit information to the thalamus and the substantia nigra.

5. Substancea nigra

Located under the thalamus, in the brainstem, this structure is known to be one of the main sources of dopamine in the brain. It is actively involved in the brain's reward system. In its connection with the striatum (forming part of the nigrostriatal tract) it is also of great importance in the control of fine movement, both of limbs and eyes.

6. Nucleus accumbens

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 drug-reinforced behaviors and habituation, possessing connections with the limbic system.

7. Subthalamic nucleus

Located at the junction between the midbrain and the thalamusThe main function of the subthalamic nucleus is to regulate motor functions.

8. Red substance

This structure maintains important connections with the cerebellum and spinal cord****lIt is especially linked to motor coordination. Specifically, it has special relevance in the control of arms and shoulder.

Functions of the basal ganglia

As it has been observed in the explanation of its components, the functions of the basal ganglia are varied, participating in numerous important aspects of our life.. Making a general review of the aspects in which they participate, we can say that some of their main functions are the following:

Planning, integration and control of voluntary movement 2.

One of the functions for which the basal ganglia are best known is the regulation and management of voluntary motor actions. Acting through the corticostriatal circuit, they act specifically as an unconscious filter that inhibits or selects the movements to be performed, specifically by helping to control posture.The basal ganglia help control body posture and coordinate fine/precise movement of the limbs.

The basal ganglia allow marking the end of a movement, planning sequences and correcting them if necessary.

2. Procedural learning

Another aspect in which the basal ganglia play a predominant role is in procedural learning. is in procedural learning and in the automatization of behavior.. This type of learning is what allows us to become accustomed to performing sequences of actions such as those necessary for driving, playing an instrument, shaving or sewing, 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 the maintenance of processing speed, cognitive planning and the development of strategies to solve problems. Similarly, the connections of the basal ganglia with the orbitofrontal cortex cause them to be involved in the capacity for behavioral inhibition.

4. Involvement in emotional and motivational behavior

As mentioned above, some basal ganglia, such as the orbitofrontal cortex, are involved in emotional and motivational behavior, some basal ganglia such as the nucleus accumbens have connections with the limbic system and the brain's reward system.given their importance in the management of dopamine. Thus, the basal ganglia can be considered to be involved in emotional behavior and in reinforcement produced by drugs or stimulation.

For example, the basal ganglia play a very important role in both classical 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 causes these nuclei not to coordinate or act as they should? In that case we can find some of the following problems and disorders, generally being problems of hypo or hyperkinetic type, that is to say, linked to the movement.

Parkinson's disease

The most common and best known disorder resulting from a malfunction of the basal ganglia is Parkinson's disease. The most recognizable symptoms of this disorder are Parkinsonian or resting tremors. Muscle rigidity and loss of spontaneous movements also appear, together with marked bradykinesia or loss of motor speed and gait disturbances.

This disorder occurs especially with degeneration and death of the dopaminergic cells of the nigrostriatum, which causes the transmission of the dopaminergic response in the nigrostriatum to be impaired.which causes that the transmission of dopamine is lost to a great extent, not arriving the information to the motor cortex.

2. Huntington's chorea

It is a neurodegenerative genetic disorder caused by an alteration of a dominant gene on chromosome number four, with complete penetrance.with complete penetrance. It produces hyperkinetic symptoms: the disease causes uncontrolled dance-like movements (hence the name chorea), and significant loss of executive functions and memory. The deficits are produced mainly by the death of neurons of the caudate nucleus, especially of GABAergic and cholinergic neurons.

3. Loss of psychic self-activation syndrome

As mentioned above, the basal ganglia are related to personal motivation. A lesion in these areas can therefore have serious implications in this aspect, as in the case of psychic self-activation loss syndrome.This is the case 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 an extreme passivity in the sufferer, losing the capacity for interest, spontaneity and motivation. Sufferers are able to recognize their deficits, so that anosognosia is not present, but they show great indifference to them.

4. Disorders by tics and Tourette's syndrome

In these disorders, as the syndrome of Tourette, which are characterized by the presentation of movements, vocalizations, gestures or movements, vocalizations, gestures or very stereotyped behaviors that are carried out unconsciously, a severe affectation is presented.In these disorders, there is severe involvement of the basal ganglia. More specifically, it is believed that these disorders may be related to problems in the putamen.

5. Other related disorders

Apart from these disorders, alterations of the basal ganglia often occur in a large number of psychological problems. For example, in obsessive-compulsive disorder or ADHD, alterations of these brain systems can and appear to occur.

Bibliographical references:

• Alexander, G.E.; DeLong, M.R. & Strick, P.L. (1986). Parallel organization of functionally segregated circuits linking basal ganglia and cortex. Annu Rev Neurosci.; 9:357 - 381.
• Kandel, E. R. (2001). Principios de Neurociencia. 1º edición. McGraw-Hill.
• Melnick, M.E. (2013). Basal ganglia disorders. In: Umphred DA, Burton GU, Lazaro RT, Roller ML, eds. Umphred's Neurological Rehabilitation. 6th ed. Philadelphia, PA: Elsevier Mosby; chap 20.
• Morris, M.E.; Iansek, R., Matyas, T.A. & Summers, J.J. (1996). Stride length regulation in Parkinson's disease. Normalization strategies and underlying mechanisms. Brain. 119:551 - 68.
• Snell, R. S. (2007). Neuroanatomía clínica. Madrid: Ed. Médica Panamericana.