Topics

Topic

design image
Medial pallidum

The medial (internal) pallidum, also known as the internal segment of the globus pallidus (GPi), is a major output nucleus of the basal ganglia. It plays a central role in motor control by regulating thalamocortical activity and ensuring smooth, coordinated voluntary movement.

The GPi is a key structure in functional neuroanatomy and neuroimaging, particularly in movement disorder evaluation and surgical planning.

Synonyms

  • Globus pallidus internus (GPi)

  • Internal segment of the globus pallidus

Location

  • Situated within the lentiform nucleus of the basal ganglia

  • Medial to the lateral (external) pallidum (GPe)

  • Lateral to the posterior limb of the internal capsule

  • Inferior to the body of the lateral ventricle

  • Anterior to the thalamus

  • Separated from the putamen by the lateral pallidal segment

Anatomical components

  • Gray matter nucleus:

    • Composed predominantly of GABAergic neurons

  • Pallidal laminae:

    • Medial and lateral medullary laminae containing myelinated fibers

  • Functional divisions:

    • Motor territory (posteroventral GPi)

    • Associative and limbic territories (anterodorsal GPi)

Relations

Laterally:

  • External globus pallidus (GPe)

Medially:

  • Posterior limb of the internal capsule

Anteriorly:

  • Anterior limb of the internal capsule

Posteriorly:

  • Thalamus

Superiorly:

  • Corona radiata

Inferiorly:

  • Substantia innominata

Connections

  • Afferent inputs:

    • Striatum (putamen) via direct pathway

    • Subthalamic nucleus

  • Efferent outputs:

    • Ventral anterior (VA) and ventral lateral (VL) nuclei of the thalamus

    • Brainstem motor nuclei

Function

  • Motor inhibition: Provides tonic inhibitory output to the thalamus

  • Movement modulation: Regulates initiation and amplitude of voluntary movement

  • Motor precision: Prevents excessive or unwanted movements

  • Basal ganglia output: Principal output nucleus of the basal ganglia motor circuit

MRI appearance (normal)

T1-weighted images:

  • Medial pallidum: Intermediate-to-low signal intensity

  • Pallidal boundaries: Defined by internal capsule laterally and medullary laminae

T2-weighted images:

  • Normal : Characteristically low signal intensity due to high iron content

  • Clear distinction: Darker than putamen and caudate nucleus

FLAIR:

  • Normal: Low signal with sharp margins

  • Symmetry: Bilateral and symmetric appearance

Diffusion-weighted imaging (DWI):

  • Normal: No diffusion restriction

Post-contrast T1-weighted images:

  • Normal: No enhancement

CT appearance (normal)

Non-contrast CT:

  • Medial pallidum: Slightly hyperdense compared to surrounding basal ganglia

Post-contrast CT:

  • Normal: No enhancement

MRI images

MRI Medial (internal) pallidum axial anatomy image -img-00000-00000

MRI images

MRI Medial (internal) pallidum axial anatomy image -img-00000-00000_00001