Location and Borders
The globus pallidus is located within the subcortical white matter of the telencephalon (telencephalon), although embryologically part of it derives from the diencephalon.
Topographic relationships:
- Lateral border: Externally, the globus pallidus is covered by the putamen (putamen). They are separated by a thin band of white matter, the medial medullary lamina (lamina medullaris medialis).
- Medial border: Separated from the diencephalon by a massive sheet of projection fibers, the internal capsule (capsula interna).
- Posterior border: Posteriorly, the structure borders the rostral portion of the substantia nigra (substantia nigra).
On a horizontal brain slice, the globus pallidus is visualized as the inner part of the lentiform nucleus, lying medial to the putamen.
Structure and Anatomical Associations
The nucleus got its name from its relatively pale color in anatomical specimens. Structurally, the globus pallidus consists of two segments: the internal and external segments (globus pallidus medialis et lateralis).
- Lentiform nucleus (nucleus lentiformis): Includes the globus pallidus and the putamen.
- Corpus striatum (corpus striatum): The globus pallidus and caudate nucleus are connected by numerous fine gray matter "bridges." Internal capsule fibers, growing through the originally homogeneous basal ganglia during embryogenesis, create the characteristic striated appearance.
Connections and Pathways
The basal ganglia are interconnected with each other and with the motor areas of the cerebral cortex via feedback regulatory loops.
- Connections with the thalamus: The central median nucleus of the thalamus receives information from the internal segment of the globus pallidus. This nucleus is the largest within the intralaminar complex. The thalamus serves as a relay between the basal ganglia and the cortex; the basal ganglia exert inhibitory or disinhibitory influences on the thalamus, thereby "permitting" or "forbidding" the initiation of motor programs in the cortex.
- Connections with the subthalamus: The globus pallidus maintains reciprocal relationships with the subthalamic nucleus (nucleus subthalamicus).
Phylogeny and Role in Motor Control
During evolution, the globus pallidus formed earlier than the caudate nucleus and putamen (which belong to the neostriatum); therefore, it is termed the paleostriatum (paleostriatum).
In phylogenetically older species, the paleostriatum plays a major role in maintaining muscle tone and automated movement control. In humans, the newer motor (pyramidal) system of the cerebral cortex has subordinated these older centers. Older neural centers are unable to compensate for the dysfunction of newer centers: when the motor cortex is damaged, humans lose the ability to walk.
Clinical Significance and Radiology
Damage to the basal ganglia leads to an excess or deficit of motor impulses and pathological alterations in muscle tone. Parkinson's disease is characterized by the classic triad: rigidity, akinesia, and resting tremor.
- Syncinesias: During the development of spastic paralysis, involuntary associated movements may appear in the paralyzed limbs, generated by phylogenetically older motor centers.
- MRI Diagnosis: Localized deposition of minerals (such as iron or copper) within the inner part of the globus pallidus manifests as signal hypointensity—making the tissue appear abnormally dark.