Anatomy of the Basal Ganglia

The basal ganglia (or basal nuclei) is a group of subcortical nuclei found deep within the brain’s cerebral hemispheres or white matter⁽¹⁾.

The subcortical structures form part of the extrapyramidal motor system and work with the pyramidal and limbic systems. Five pairs of nuclei make up the basal ganglia: caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.

The basal ganglia’s function is fine-tuning the voluntary movements by receiving, processing, and adjusting the impulses for the upcoming movement from the cerebral cortex⁽²⁾. 

The basal ganglia convey their instructions to the thalamus, which relays the information back to the cortex. The tracts of the pyramidal motor system send the fine-tuned movement instruction to the skeletal muscles. 

Components of the Basal Ganglia and their Functions

Striatum

The striatum is a complex nucleus situated deep in the forebrain’s subcortical structures, inside the insular lobe. The dorsal and ventral parts comprise the striatum⁽³⁾.

The ventral striatum is part of the limbic system. Meanwhile, the dorsal striatum is a component of the basal ganglia.

The dorsal striatum consists of two parts: the caudate nucleus and putamen. The internal capsule separates these parts of the striatum and gives it a striped appearance through the radiating myelinated fibers⁽⁴⁾. 

The striatum is the basal ganglia’s main input unit. This unit receives excitatory glutamatergic (glutamate-related) inputs from the cerebral cortex, with a synapsing pattern that reflects the cortex’s topography⁽⁵⁾.

Glutamate is the major excitatory neurotransmitter in the nervous system⁽⁶⁾.

Caudate Nucleus

The caudate nucleus is a C-shaped nucleus that lies anteriorly to the thalamus, laterally to the lateral ventricles, and medially to the internal capsule. This nucleus consists of the head, body, and tail.

The caudate nucleus’ head contributes to the lateral ventricle’s lateral wall. Meanwhile, the tail forms the roof of the lateral ventricle’s inferior horn. 

The tail arches over the thalamus’ ventral surface, enters the temporal lobe and terminates by connecting with the amygdala, the cells near the base of the brain.

The caudate nucleus’ functions include integrating sensory information about the body’s spatial position and sending the information about the stimuli’s fine-tuned motor response to the thalamus⁽⁷⁾. 

Additionally, the caudate nucleus contributes to body and limb posture, speed, and accuracy of directed movements. Aside from motor control, this nucleus is involved in memory, learning, language processing emotions, and goal pursuit⁽⁸⁾. 

Putamen

The putamen is a round structure at the forebrain’s base. This structure is the most lateral of the basal ganglia on the brain’s axial section.

The putamen lies laterally to the globus pallidus, medially to the external capsule, and encircled by the caudate nucleus. The medial medullary lamina, a thin layer of white matter, separates the putamen and globus pallidus⁽⁹⁾. 

The main functions of putamen include regulating motor functions and influencing various types of learning. The putamen also employs dopamine to perform its functions⁽¹⁰⁾.

Nucleus Accumbens and Olfactory Tubercle

The paired structures located at the base of the forebrain are nucleus accumbens and olfactory tubercles. These structures are components of the ventral striatum and input nuclei for the ventral tegmental area. 

The nucleus accumbens is situated in the rostral forebrain, where the caudate nucleus’ head and putamen meet. Meanwhile, the olfactory tubercle is situated ventrally to the nucleus accumbens⁽¹¹⁾.

Although the two structures are not involved in movement regulation, they play a vital role in the “reward circuit,” or referred to as the “limbic-motor interface⁽¹²⁾”. 

Rewarding activities, like food, drugs, or sex, activate dopamine neurons in the brain’s ventral tegmental area. When activated, these neurons project to the nucleus accumbens and the olfactory tubercle, resulting in increased dopamine levels⁽¹³⁾. 

Globus Pallidus

The globus pallidus is a paired subcortical brain structure composed of inhibitory GABAergic projection neurons. This structure consists of internal (GPi) and external (GPe) segments⁽¹⁴⁾. 

The globus pallidus’ superior and medial aspects are in contact with the internal capsule, which separates the caudate nucleus from the globus pallidus.

The inferior surface comes in contact with the subthalamic nucleus and zona incerta, which separate the globus pallidus from the thalamus. 

Both the GPi and GPe segments play an essential role in modulating the motor program in the direct and indirect pathways⁽¹⁵⁾.

Subthalamic Nucleus

Subthalamic nuclei are small biconvex paired structures situated within the subthalamus. Although not an anatomical part of the basal ganglia, the subthalamic nucleus is listed as a functional part⁽¹⁶⁾.

The subthalamic nucleus is composed of excitatory glutamatergic projection neurons. The nucleus receives excitatory inputs from the frontal cortex in a somatotopically organized manner.

Some studies suggest that the subthalamic nucleus has a crucial role in the hyperdirect pathway in modulating the planned motor program⁽¹⁷⁾. 

The subthalamic nucleus is also considered the pacemaker of the basal ganglia due to its nucleus firing pattern. 

Substantia Nigra

The substantia nigra is a motor nucleus located between the midbrain’s cerebral peduncle and tegmentum. Two parts with different functions that comprise the substantia nigra are the pars compacta (SNc) and the pars reticulata (SNr).

The pars compacta makes up the dorsal portion of the substantia nigra and consists of numerous melanin-filled neurons. These neurons give the substantia nigra its distinctive dark color. 

The pars compacta mainly serves as an output to the basal ganglia circuit and supplies the striatum with dopamine⁽¹⁸⁾.

The pars reticulata is larger and lies ventrally to pars compacta to convey signal signals from the basal ganglia to the thalamus⁽¹⁹⁾.

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