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Lateral pallidum

The lateral (external) pallidum, also known as the external segment of the globus pallidus, is a major nucleus of the basal ganglia involved in the indirect pathway of motor control. It plays a crucial role in modulation of voluntary movement by regulating inhibitory output within basal ganglia circuits.

The lateral pallidum functions primarily as a relay and regulatory nucleus, influencing thalamocortical motor activity through its connections with other basal ganglia components.

Synonyms

  • External segment of globus pallidus

  • Globus pallidus externus (GPe)

Location

  • Located within the lentiform nucleus of the basal ganglia

  • Situated lateral to the medial (internal) pallidum

  • Medial to the putamen

  • Lateral to the internal capsule

  • Inferior to the caudate nucleus

  • Superior to the subthalamic nucleus

Anatomical components

  • GABAergic projection neurons

  • Dense intrinsic connections with basal ganglia nuclei

  • Part of the indirect motor pathway

Relations

Laterally:

  • Putamen

Medially:

  • Medial (internal) pallidum

Anteriorly:

  • Head of the caudate nucleus (indirect relation)

Posteriorly:

  • Thalamus (via pallidothalamic pathways)

Superiorly:

  • Internal capsule (posterior limb)

Inferiorly:

  • Subthalamic nucleus

Connections

  • Afferent connections:

    • From the striatum (putamen) via inhibitory GABAergic fibers

  • Efferent connections:

    • To the subthalamic nucleus

    • To the medial (internal) pallidum

  • Intrinsic connections:

    • Extensive reciprocal connections within basal ganglia

Function

  • Motor modulation: Regulates movement by inhibiting excessive motor activity

  • Indirect pathway regulation: Acts as a key inhibitory relay controlling subthalamic nucleus activity

  • Movement refinement: Contributes to smooth, coordinated voluntary motion

  • Basal ganglia balance: Maintains equilibrium between facilitation and inhibition of movement

MRI appearance (normal)

T1-weighted images:

  • Lateral pallidum: Intermediate signal intensity

  • Iron content: Slightly lower signal compared with putamen

  • Internal medullary lamina: Thin low-signal line separating it from medial pallidum

T2-weighted images:

  • Normal: Relatively low signal intensity due to iron deposition

  • Clear differentiation: Appears darker than putamen and caudate nucleus

  • Symmetry: Bilateral symmetric appearance

FLAIR:

  • Normal: Low-to-intermediate signal

Diffusion-weighted imaging (DWI):

  • Normal: No diffusion restriction

Post-contrast T1-weighted images:

  • Normal: No enhancement

CT appearance (normal)

Non-contrast CT:

  • Lateral pallidum: Slightly hyperdense relative to surrounding white matter

Post-contrast CT:

  • Normal: No enhancement

MRI image

MRI Lateral (external) pallidum axial anatomy image -img-00000-00000