Basal ganglia: anatomy and functions

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 performing movements, although, as with all parts of the brain, they work in coordination with the rest of the parts of the system highly strung.

In this article we will see what the basal ganglia are, the functions they carry out, 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.

• Related article: "Parts of the Nervous System: anatomical structures and functions"

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, as well as into 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 within it. fundamental for the functioning and maintenance of bodily and cognitive functions, participating in a large number of scopes. Examples of these

parts of the brain They are the hippocampus, the limbic system or the set of nuclei that this article deals with, 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 clumps of gray matter; that is, areas in which the parts of the neurons that are not myelinated) 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 afferences as of efferences (They receive information from other brain areas).

Thus, gray matter indicates the presence of neuronal bodies, which are the parts of neurons in which we find the cell nucleus. This implies that both in the basal ganglia and in the rest of the brain areas in which gray matter predominates information processing tasks carried by neurons are carried out, unlike what happens in the areas of white matter, 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 due to their location, they have a role in actions that are between the automatic and the 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 and the GABA as an 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 a as a set of nuclei given their interconnection, the basal ganglia consist of several distinct substructures and, in fact, it is physically easy to repair the spaces between them. Below you can find the main structures of this set of brain structures:

1. Striated body

The striatum is considered the main information reception area of ​​the basal ganglia. In other words, it is an area that receives numerous projections from various brain areas, integrating the information and acting with it. It is composed of the neo-striated nucleus (composed of the caudate nucleus and the putamen), which is mainly responsible for receiving input 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 occiputThis 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 with the orbitofrontal cortex.

3. Putamen

Fundamental element in the control of automated movements, Located below the caudate nucleus and joined by the anterior zone. Linked to movement of the face and limbs.

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. Black substance

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 participate in the brain's 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 extremities and of the eyes.

6. Nucleus accumbens

Located under the globus pallidus, it receives signals from the ventral tegmental area and sends signals to the globe 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 shoulder.

Basal ganglia functions

As has been observed in the explanation of its components, the functions of the basal ganglia are varied, participating in numerous and 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:

1. Planning, integration and control of voluntary movement

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

The basal ganglia allow you to mark the end of a movement, plan sequences and correct them if you need to.

2. Procedural learning

Another aspect in which the basal ganglia have a predominant performance is in procedural learning and in the automation of behavior. This type of learning is what allows you to get used to performing sequences of actions such as those necessary to drive, touch an instrument, shaving or sewing, allowing the management of attentional resources so that they can be directed towards others necessities.

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. In the same way, the connections of the basal ganglia with the orbitofrontal cortex cause them to be 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 to the limbic system and the brain reward system, given its importance in the dopamine management. Thus, it can be considered that the basal ganglia 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 the classical conditioning as in the operant conditioning.

Disorders Linked to Basal Ganglia Problems

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 treating hypo or hyperkinetic problems, that is, related to movement.

1. Parkinson's

The most common and best known disorder derived from a malfunction of the basal ganglia is Parkinson's disease. The most recognizable symptoms of this disorder are parkinsonian or resting tremors. Likewise, muscle stiffness and loss of spontaneous movements also appear. along with marked bradykinesia or loss of motor speed and gait disturbances.

This disorder occurs especially before the degeneration and death of the dopaminergic cells of the nigrostriatal, which causes the transmission of dopamine to be lost to a great extent, with the information not reaching the motor cortex.

• Related article: "Parkinson's: causes, symptoms, treatment and prevention"

2. Huntington's chorea

It is a neurodegenerative genetic disorder caused by an alteration of a dominant gene on chromosome number four, with full penetrance. 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 mainly caused by the death of neurons in the caudate nucleus, especially GABAergic and cholinergic neurons.

3. Loss of psychic self-activation syndrome

As mentioned above, the basal ganglia are related to personal motivation. An injury in these areas can therefore have serious implications in this regard., as 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 it are able to recognize their deficits, so that it does not appear anosognosia, but they show great disregard for them.

4. Tic Disorders and Tourette Syndrome

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

5. Other linked disorders

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

Bibliographic 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). Principles of Neuroscience. 1st edition. 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. (nineteen ninety six). Stride length regulation in Parkinson's disease. Normalization strategies and underlying mechanisms. Brain. 119:551 - 68.
• Snell, R. S. (2007). Clinical neuroanatomy. Madrid: Panamerican Medical Ed.