Overview
Deep within the cerebral cortex, there lies a group of structures important in forming networks involved in movement. These structures are known as the basal ganglia, which are sometimes referred to as nuclei. Besides motor control, the basal ganglia are also engaged in motor learning, behaviours, and emotions.
History
The neuroanatomy of the basal ganglia originated from the 17th century. Many authors, like Thomas Willis and Christain Reil, made significant contributions to the morphological description of the deep gray-matter nuclei of the brain. However, the functional system of the basal ganglia was not entirely studied until recent times. That is, the understanding of the functions of the basal ganglia was strongly shaped by the clinical observations in the 20th century. From here, our comprehension of these structures even advanced the pathophysiology of Parkinson's disease, a progressive brain disorder that impacts movement.
Image from Physiopedia
Subdivisions
The basal ganglia can be divided into different structures, including:
Corpus Striatum
Globus Pallidus
Subthalamic Nucleus
Substantia Nigra
Corpus Striatum
Being the largest structure within the basal ganglia, the corpus striatum receives afferent sensory information from many subcortical structures and branches off to multiple basal ganglia nuclei. The corpus striatum can be further divided into the dorsal striatum and ventral striatum. The dorsal striatum is mainly involved in voluntary movements and executive functions. Meanwhile, the ventral striatum is primarily responsible for limbic functions of reward and aversion.
Globus Pallidus
While the corpus striatum acts as the main input nuclei of the basal ganglia, the globus pallidus is involved in the output of the basal ganglia. Within this structure, the internal and external globus pallidus that are involved in both the facilitation and inhibition of movement. Neurons would leave the globus pallidus to form a pathway that targets neurons in the motor cortex responsible for initiating movements.
Subthalamic Nucleus
The subthalamic nucleus is a small lens-shaped structure mainly involved in movement regulation. Specifically, the neurons found in the subthalamic nucleus are excitatory glutaminergic neurons that lie in the pathway between the external and internal globus pallidus. From this, the subthalamic nucleus can regulate the excitation of the thalamus and control movement.
Substantia Nigra
The substantia nigra is a long, darkened streak nucleus where most of the dopamine neurons are located. Many of the dopamine neurons extend to the corpus striatum to form the nigrostriatal dopamine pathway which is believed to play a role in the facilitation of movement. In fact, what makes the substantia nigra appear dark is due to the high levels of neuromelanin that contribute to the colour.
Functions
Besides knowing the main function of muscle movement, the complexity of how the basal ganglia executes it is still not fully understood. However, a well-known hypothesis theorizes that there are different pathways in the basal ganglia that stimulate and inhibit movement. This would involve the direct and indirect pathway to regulate movement.
Direct Pathway
During voluntary movement, a signal would first be sent from the cortex to the basal ganglia to initiate movement. From here, the signal follows the direct pathway, which leads to the inhibition of neurons in the globus pallidus and substantia nigra. This would then release the thalamus from the inhibitory effect of the basal ganglia, and thus, stimulate movement.
Indirect Pathway
Contrary to the direct pathway, the indirect pathway has an opposite effect to movement. When signals from the cerebral cortex activate the indirect pathway, GABA neurons in the corpus striatum are also activated. This would inhibit the globus pallidus external neurons from performing its function, which is to inhibit neurons in the subthalamic nucleus. From this, movement would therefore be inhibited. In fact, it is believed that the balance between the direct and indirect pathway allows for the execution of smooth movement.
For more information and diagrams, watch this video!
Linked Medical Conditions
Parkinson’s disease is one of the most well-known diseases of the basal ganglia. This disease stems from the neurodegeneration of dopaminergic neurons which leads to the deterioration of the substantia nigra. We know that there is a high concentration of pigment in the substantia nigra. Hence, degeneration of this structure would result in the loss of the dark pigmentation and dopamine. With a decreased input of dopamine, motor inhibition would then occur, which gives rise to the classical symptoms of bradykinesia, resting tremor, and postural instability.
Future Research
In November of 2021, a new device called Exablate Neuro was approved by U.S. Food and Drug Administration to treat Parkinson’s disease on one side of the brain. To explain, this device is a non-invasive ultrasound treatment that targets the globus pallidus. Interestingly, the patients are awake during the procedure, allowing for direct feedback to the physicians to monitor the immediate effects of the treatment. What makes this treatment special is the non-invasive procedure, which reduces the risks of other complications. In fact, one of the principal investigators of the study, Howard Eisenberg, even explains that “Focused ultrasound has less ominous side effects for patients since there’s no risk of infection or damage to blood vessels from the electrodes.” Therefore, this breakthrough of a new non-invasive treatment really paved the way for the pioneering of future research for Parkinson’s disease.
Image from NeuroNews International
Summary
The basal ganglia is a vital structure involved in movement. With the many structures including the corpus striatum, globus pallidus, subthalamic nucleus, and substantia nigra, pathways can be formed to facilitate or inhibit movement. However, when the basal ganglia is degenerated, conditions like Parkinson’s disease can occur. With more advanced research, the understanding of Parkinson’s disease can be further understood to develop more innovative treatments.
Term Bank
Neuromelanin | Dark pigment found in the brain which is structurally linked to melanin |
Bradykinesia | Slowness of movement |
Sources
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Know Your Brain: Globus Pallidus. (n.d.). Neuroscientifically Challenged. Retrieved
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