Anatomical Location and Relations
The putamen lies deep within the white matter of the telencephalon and is related to several adjacent structures of the diencephalon and telencephalon:
- Globus pallidus — located medial to the putamen. The putamen partially covers it like an eggshell and extends beyond it rostrally and caudally. Together, the putamen and globus pallidus form the lentiform nucleus (nucleus lentiformis), with the putamen serving as its lateral part. They are separated by a thin band of white matter known as the medial medullary lamina (lamina medullaris medialis).
- Internal capsule (capsula interna) — a major bundle of projection fibers whose anterior limb separates the head of the caudate nucleus from the putamen.
- External capsule (capsula externa) — separates the putamen from the claustrum, a thin sheet of gray matter situated lateral to the putamen.
- Thalamus — a large gray matter mass located medial to the putamen and internal capsule.
Structure and Phylogenesis
During embryogenesis, the putamen develops from the ganglionic eminence, a derivative of the ventral part of the telencephalic vesicle.
- Corpus striatum — the anatomical grouping of the putamen and the caudate nucleus (nucleus caudatus). These nuclei are interconnected by numerous fine bridges of gray matter. The striated appearance is formed during embryonic development when fibers of the internal capsule grow through the initially uniform basal ganglia.
- Neostriatum — the phylogenetically younger part of the basal ganglia, comprising the putamen and the caudate nucleus. During evolution, the neostriatum expanded in parallel with the cerebral cortex, reaching its maximal development in higher mammals and humans.
Neural Connections and Pathways
The putamen contains D1 and D2 dopamine receptors.
- Afferent pathways: Excitatory signals arrive at the putamen from the frontal cortex using glutamate as a neurotransmitter. Modulatory signals arrive via the nigrostriatal pathway from the substantia nigra using dopamine.
- Efferent pathways: Inhibitory pathways originating from the D1 receptor zone project to the internal segment of the globus pallidus via GABA. Inhibitory pathways originating from the D2 receptor zone project to the external segment of the globus pallidus via GABA.
Functions and Clinical Significance
As part of the striopallidal system, the putamen participates in the regulatory loop interconnecting the cerebral cortex and the basal ganglia.
- Functions: The basal ganglia modulate the activity of motor cortical areas by facilitating or inhibiting movement. They participate in the initiation and coordination of movements, regulation of muscle tone, and the control of complex motor patterns and automated movements.
- Pathology: Damage to the basal ganglia is evaluated alongside functionally related nuclei, including the substantia nigra and subthalamic nucleus. Dysfunction leads to an excess or deficit of motor impulses and pathological alterations in muscle tone. The most common pathology affecting the basal ganglia is Parkinson's disease, the clinical triad of which includes rigidity, akinesia, and resting tremor.
Differential Note: Biological Egg Shell
In biology and embryology, the term "shell" (putamen or eggshell) also refers to the hard calcareous or leathery outer covering of bird and reptile eggs. This protective envelope evolved as an adaptation for terrestrial development, safeguarding the embryo against desiccation and mechanical trauma while facilitating gas exchange.