Basal Ganglia and its function

The basal ganglia or basal nuclei are clumps of gray mass located below the cortex in the depth of both cerebral hemispheres (1). These nuclei can have different shapes and are involved in the control of movement.

The basal ganglia are surrounded by a white mass of the cerebral hemisphere, and the individual nuclei that enter into their composition build the walls of the lateral cerebral chambers.

In this article, we will talk about
the types and functions of each basal ganglia.

Types of the basal ganglia

The basal ganglia include:

• corpus striatum
• claustrum
• the amygdala
• substantia nigra
• subthalamic sails (1).

We can classify these nuclei into
the following groups:

• Input nuclei: corpus striatum
  (caudate, putamen),
• Intermediary nuclei: Globus Pallidus
  Externa, Nigra substance, and Subthalamic Nucleus.
• Output nuclei: Substantia Nigra and
  Globus Pallidus (1).

Functions of the basal ganglia

In order to understand the functions of the basal ganglia, we must mention the extrapyramidal system. This system is the part of the brain and brain stem that participates in motor control except for the corticospinal (pyramid) system. It includes:

• Basal ganglia and their pathways
• Portions of the cerebral cortex that give projections to the basal ganglia
• Parts of the cerebellum that give projections to the basal ganglia
• Parts of the reticular formation that are connected to the basal ganglia and cerebral cortex
• Thalamus nuclei associated with the basal ganglia and reticular formation.

The role of the extrapyramidal system is to control automatic movements, skeletal muscle tone, and maintenance of postural reflexes.

The basal ganglia exert their role in motor control through constant interaction with the cerebral cortex and the corticospinal pathway (1). They get information mainly from the cerebral cortex and send out information.

Almost all the motor and sensory nerve fibers that connect the cerebral cortex to the spinal cord pass between the major masses of the basal ganglia (nucleus caudatus and putamen) and are called the internal brain capsule.

The connections of the motor cortex, the thalamus and the joint circuits of the brain stem and cerebellum are very important. Namely, the main circuit of the basal ganglia system involves a huge number of connections between the basal ganglia themselves, as well as numerous entry and exit pathways between the motor regions of the brain and the basal ganglia.

The most prominent functions of the
basal ganglia include:

• Represents the accessory motor system. Mediates between neocortical motor centers and the "elderly" motor areas of the brainstem Selects the purposeful and desired motor activity and suppresses unwanted movements.
• Acts by modifying ongoing neural activity in motor projections
• Delivers an inhibitory role in motor control
• Inhibits muscle tone (balance of excitatory and inbound input signals according to PMN terminating on skeletal muscle)
• Monitor and adjust slow and continuous contractions (equilibrium, body position, etc.)
• Regulates attention and individual cognitive processes
• Participates in motor planning and learning
• Assisting the cerebral cortex in making subconscious, learned movements
• Temporal pattern of movement and gradation of the intensity of movement (2).

One of the major roles of the basal ganglia is to participate in the control of complex patterns of motor activity such as: letter writing, cutting paper with scissors, throwing a ball into a basket, adding the ball in football, many aspects of vocalization, controlled eye movements, or literally all our other skilled movements.

Cognitive control of motor activity in which the nucleus caudatus plays a major role is another important function of the basal ganglia. Likewise, planning which movement patterns will be used together, or in what order in order to achieve a complex goal, is another role of the basal ganglia.

Basal Ganglia Neurotransmitter Systems

This advanced system consists of several important segments or systems. Those are:

• A system of dopamine neurons located in the substantia nigra, and give projections to the nucleus caudatus and putamen.
• A system of GABA-containing neurons located in nucleus caudatus and putamen, and give projections in substance nigra.
• A system of acetylcholine neurons located in the cerebral cortex, and they give the projections to the nucleus caudatus and the putamen.
• Noradrenergic, serotonin and other neuronal systems are located outside the basal ganglia system, and yield projections into this system.

Striatum

Corpus striatum builds up the lentiformis and caudatus cores/nuclei. Nucleus lentiformis consists of putamen and globus pallidus. The globus palidus is the inner layer, while the putamen is located on the outside.

The striped body is divided into dorsal and ventral parts. The dorsal part is associated with the somatosensory and somatomotoric systems, and the ventral one is associated with the limbic system. All information aimed at the basal ganglia goes through the striatum.

The dorsal part of the striatum is built by a repatriate and lenticular nucleus. The lens juxtaposes a layer of white mass on the outside - a putamen and the globus palidus that is inside.

These two elements are separated by fibers of the inner capsule, but in some places, they are connected by cellular bridges. The putamen, together with the nucleus caudatus, builds the neostriatum, and the globus palidus forms paleostriatum (2).

Nucleus caudatus

The nucleus caudatus has a shape of letter C. It consists of the head, body and tail. With its structures, it participates in the construction of the lateral brain chamber. On the inside of the nucleus caudatus, there is the thalamus.

Between this nucleus and the thalamus, is the stria terminalis, and above it is positioned the vein of the thalamostriata. The corpus callosum is positioned above the nucleus caudatus.

At the tip of it, we can locate a very important brain element, the amygdala. The anterior limb of the inner medulla separates the nucleus caudatus from the nucleus lentiformis (2).

Nucleus Lentiformis

Nucleus lentiformis is located medially from the insula cortex (2). Between these two segments, from the outside to the inside, we can locate the capsula extrema, claustrum, and the element called capsula externa.

As mentioned earlier, the lentiformis nucleus builds the inner part of the globe pallidus and the outer putamen. They are separated by a layer of white mass. The globe palidus consists of an inner and an outer part.

The ventral portion of the striatum is smaller. It is located below the front commissure. It consists of a basal nucleus, nucleus accumbens, and segment called tuberculum olfactorium. A significant loss of large neurons in the basal nucleus was observed in patients with Alzheimer's disease.

Nucleus accumbens is located at the junction of the putamen and the tail of the nucleus caudatus. The nucleus accumbens integrates and reconciles information from the extrapyramidal and limbic systems.

The role of the scent element scientifically called tuberculum olfactorium has not yet been fully clarified. However, these basal ganglia play a very important role in the central nervous system.

Claustrum (rampart) Basal Ganglia

The claustrum is a layer of gray
mass that lies between the end capsule (capsula extrema) and the outer capsule
(capsula externa). It is separated from the putamen by the outer capsule. The
function of the ramparts has not yet been sufficiently researched.

The Amygdala

Some authors describe the amygdala
solely within the limbic system. As the human brain is still incompletely
explored, we do not have the full insight into the interconnectedness of
certain brain structures. The amygdala is probably part of the limbic system
and belongs to the basal ganglia.

Nigral complex (substantia nigra)

This complex is composed of the so-called "black substance" and the ventral tegmental nuclei. The black substance is found in the midbrain. It consists of a reticular and compact part. The reticular part contains cells that are similar to cells of the globe palidus.

These two structures are separated by an inner capsule. The compact section contains dark-colored dopaminergic neurons. The loss of these neurons occurs in Parkinson's disease.

Sub-thalamic nucleus

This nucleus is located below the thalamus. It's a cigarette-shaped brain segment. The subthalamic nucleus receives information from the globus palidus, the cerebral cortex, the substantia nigra, and the mesh substance of the pons.

Conclusion

The basal ganglia are a set of brain structures located beneath the cerebral cortex that receive information from the cortex, transmit it to the motor centers, and return it to the part of the cerebral cortex that is in charge of motion planning.

The basal ganglia represent a
second, auxiliary motor system that functions independently, just like the
cerebellum, and is closely related to the cerebral cortex and the
cortico-spinal motor system. The basal ganglia receive most of their input
signals from the cortex itself and also return almost all of their output
signals to the cortex.

The set of the basal ganglia consist of nucleus caudatus, putamen, globus palidus, nigra substance, and subtalamus nucleus. They are located mainly lateral to the thalamus, surrounding it so that they occupy large portions of the internal regions of both cerebral hemispheres.

References

1. Young CB, Sonne J. Neuroanatomy, Basal Ganglia. [Updated 2018 Dec 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Found online at: https://www.ncbi.nlm.nih.gov/books/NBK537141/
2. Lanciego JL, Luquin N, Obeso JA. Functional neuroanatomy of the basal ganglia. Cold Spring Harb Perspect Med. 2012 Dec 1;2(12):a009621. doi: 10.1101/cshperspect.a009621. PMID: 23071379; PMCID: PMC3543080.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3543080/