The basal ganglia are paired structures and consist of gray matter nuclei separated by white-fibre layers of the inner and outer capsule of the brain.(Function of Basal Ganglia) The basal ganglia are made up of the striatum, which contains the caudal nucleus and shell, pallidum, and fence.
From a functional point of view, sometimes the subthalamic nucleus and substantia nigra also referred to the concept of basal ganglia. The large size of these nuclei and the similarity in structure across species suggest that they make a great contribution to the organization of the brain of terrestrial vertebrates.
Function of Basal Ganglia
The basal ganglia initiate physiological and cognitive processes. They inhibit the spontaneous activity of the organism by choosing what is most important to it. Important processes are uninhabited, unimportant processes are obstructed.
The basal ganglia receive their information from all areas of the cerebral cortex and pass information through the thalamus to the premotor and frontal cortex. This pathway performs the function of rapid and coordinated regulation of the processes of unconscious, automatic, fine motor movements.
The striatum controls all response and expression movements that do not pass through the cerebral cortex. This includes everything that is related to facial expressions and gestures.
The connection to the frontal lobe via the caudate nucleus serves an abstract scheme that does not lead directly to movements. Cell actions in the globus pallidus are responsible for the expenditure of force and the direction of motion.
The nucleus accumbens is an essential part of the brain’s reward system and is therefore important for the development of addiction. There are several dopamine receptors, whose stimulation triggers feelings of pleasure through stimulating, relaxing or pain-relieving effects.
Stimulants such as opiates, cannabis (THC), cocaine or amphetamines stimulate these receptors. From there, the information is carried to the limbic system and hypothalamus, where it is processed and a vegetative response is generated.
Messenger substances of The Basal Ganglia
The most important transmitters in the basal ganglia are:
- Dopamine: an excitatory neurotransmitter; the “happiness hormone”.
- Acetylcholine: a transmitter that plays a central role in both the central and peripheral nervous systems.
- Gamma-aminobutyric acid (GABA): the most important inhibitory neurotransmitter in the central nervous system.
Functions of the striatum and how it is structured
The striatum is made up of two distinct parts, the matrix and a compartment called the striasome. These compartments have different histological features and contain different receptors. The striosome compartment receives afferents mainly from the limbic cortex and projects mainly to the pars compacta of the substantia nigra. To better understand the workings of the striatum, it is pertinent to mention how the circuitry or communication between different brain regions works.
All regions of the cerebral cortex send glutamatergic excitatory projections to specific regions of the striatum. The striatum also receives excitatory signals from the intramineral nucleus of the thalamus, dopaminergic projections from the midbrain, and serotonergic projections from the raphe nucleus.
Specifically, the striatum is composed of different types of cells, but 90–95% of the cells that make up it are composed of GABAergic projection neurons. They represent the main target of different projections from the cerebral cortex and are also the sole source of different projections. They are usually silent neurons, except during movement or after the application of peripheral stimuli.
The striatum is also composed of local inhibitory interneurons which, thanks to their developed axonal collaterals, reduce the activity of the different neurons of the striatum. Although these neurons are present in small amounts, they are responsible for most of the tonic activity of the striatum.
As for the circuitry, the striatum projects to the nucleus from which different pathways arise from two pathways, a so-called direct pathway that is excitatory and an indirect pathway of inhibitory type.
Basal Ganglia Motor Control
The basal ganglia are involved in the subconscious control of muscle tone and coordination of learned movements. Basically, the basal ganglia once initiated ensure the rhythm and correct voluntary movement patterns.
The basal ganglia receive signals from the cortex and send them back to the cortex via intermediate synaptic stations in the thalamus. One of the functions of this neuronal circuit is to allow the cortex to optimize the initiation of intentional movements by inhibiting unwanted ones.
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