The globus pallidus is a subcortical brain structure that regulates voluntary movement. It functions as a component of the basal ganglia, a group of nuclei that helps coordinate movements at a subconscious level. The globus pallidus contributes to the smooth and controlled execution of physical activities, ensuring intended movements occur without disruption.
Unveiling Its Location and Structure
The globus pallidus is a triangular-shaped structure situated deep within the cerebral hemispheres, beneath the cerebral cortex. It resides medial to the putamen, another key brain structure, and together they form the lentiform nucleus. Its name, Latin for “pale globe,” reflects its pale appearance due to the myelinated axons that compose it.
This structure is divided into two distinct segments: the globus pallidus interna (GPi), or internal segment, and the globus pallidus externa (GPe), or external segment. A thin sheet of white matter, known as the medial medullary lamina, separates these two segments. Both the GPi and GPe receive their primary inputs from the striatum, which consists of the caudate nucleus and the putamen.
Its Role in Movement
The globus pallidus generates inhibitory signals, largely through the neurotransmitter GABA, which helps fine-tune movements and suppress unwanted motions. This inhibitory output is continuous, allowing the basal ganglia to both increase and decrease activity as needed for motor control.
This brain region is particularly involved in two main pathways that modulate motor activity: the direct and indirect pathways. The direct pathway facilitates movement by ultimately reducing inhibition on the thalamus, a brain region that relays signals to the motor cortex. When activated, the striatum inhibits the GPi, which then disinhibits the thalamus, thereby promoting movement.
Conversely, the indirect pathway works to inhibit movement. In this pathway, the striatum inhibits the GPe, which in turn leads to less inhibition of the subthalamic nucleus. The subthalamic nucleus then excites the GPi, increasing its inhibitory output to the thalamus, which results in a suppression of motor activity. This delicate balance between the direct and indirect pathways, modulated by the globus pallidus, allows for precise and coordinated control of movement.
When It Doesn’t Function Properly
Dysfunction within the globus pallidus can lead to various movement disorders. In Parkinson’s disease, for example, the degeneration of dopamine-producing cells elsewhere in the brain leads to imbalances in the basal ganglia, affecting the globus pallidus. This disruption contributes to common Parkinson’s symptoms such as rigidity, slow movement (bradykinesia), and resting tremors. Studies indicate a significant loss of dopamine in both the GPe and GPi in Parkinson’s patients.
The globus pallidus is also implicated in conditions like dystonia, characterized by sustained muscle contractions causing twisting and repetitive movements or abnormal postures. Huntington’s disease, another neurological disorder, is also associated with issues in the globus pallidus, leading to involuntary movements. The abnormal activity patterns within the globus pallidus contribute to the uncontrolled movements seen in these conditions.
Understanding the globus pallidus’s involvement in these disorders has paved the way for therapeutic interventions. Deep brain stimulation (DBS) targeting the GPi has become an established treatment option for advanced Parkinson’s disease and dystonia. By delivering electrical impulses to the GPi, DBS can modulate its activity, helping to alleviate motor symptoms and improve motor function, restoring a degree of normal motor control.