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genu of internal capsule

The genu of the internal capsule is the sharply curved portion of the internal capsule, a compact band of white matter deep within each cerebral hemisphere. The genu represents the flexure between the anterior and posterior limbs of the internal capsule, situated between the head of the caudate nucleus and the thalamus. This critical structure transmits important projection fibers, most notably those of the corticobulbar tract, allowing communication between the cerebral cortex and the brainstem. Damage to the genu can result in profound neurological deficits, highlighting its clinical significance.

Anatomical Description

  • The genu forms the medial bend between the anterior and posterior limbs of the internal capsule.

  • It is located at the level of the foramen of Monro, medial to the lentiform nucleus, and lateral to the thalamus.

  • Composed of dense, myelinated white matter tracts.

Function

  • Contains corticobulbar fibers, which relay voluntary motor commands from the motor cortex to cranial nerve nuclei in the brainstem (controlling facial, head, and neck muscles).

  • Facilitates communication between the cerebral cortex and subcortical structures, including the thalamus and brainstem.

  • Plays a role in integrating motor, sensory, and higher-order functions.

  • Lesions here can result in contralateral lower facial weakness, dysarthria, and pseudobulbar palsy.

Arterial Supply

  • Mainly supplied by the superior (lateral) lenticulostriate arteries, which are deep penetrating branches of the middle cerebral artery (MCA).

  • The recurrent artery of Heubner (branch of the anterior cerebral artery, ACA) may also contribute to the blood supply, especially to the anterior limb and genu.

  • Occasionally, the anterior choroidal artery (from the internal carotid artery) can supply part of the genu.

Venous Drainage

  • Venous blood from the genu drains into the deep cerebral veins.

  • Key veins include the internal cerebral vein and the basal vein of Rosenthal.

  • Drainage eventually reaches the great cerebral vein (vein of Galen).

MRI Appearance

  • T1-weighted MRI:

    • Genu appears hyperintense (bright) relative to gray matter due to its high myelin content.

    • Stands out clearly from neighboring structures.

  • T2-weighted MRI:

    • Genu appears hypointense (dark) compared to gray matter, again due to tightly packed myelinated fibers.

    • Lesions (e.g., infarction, demyelination) will appear hyperintense (bright) in the affected area.

  • FLAIR (Fluid Attenuated Inversion Recovery):

    • Normal genu is intermediate to low signal.

    • Abnormalities (e.g., ischemia, demyelination, edema) appear hyperintense (bright) against the relatively darker background of normal white matter.

CT Appearance

  • On non-contrast CT, the genu appears as a region of lower density (darker) compared to gray matter because of its high myelin (fat) content.

  • Genu is best visualized at the level of the basal ganglia, appearing as part of the V-shaped internal capsule.

  • Pathological changes (such as infarction or hemorrhage) in the genu will manifest as focal hypodensity (for ischemia) or hyperdensity (for acute hemorrhage).

MRI image

genu of internal capsule mri 3t coronal image