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Crus cerebri

The crus cerebri are paired, compact bundles of descending white-matter fibers located on the ventral aspect of the midbrain. They contain major motor and associative pathways that connect the cerebral cortex with the brainstem and spinal cord. Each crus cerebri forms the anterior portion of a cerebral peduncle.

Because of their dense fiber content and strategic position, the crus cerebri are frequently involved in vascular, demyelinating, degenerative, and compressive pathologies and are routinely assessed in neuroimaging.

Synonyms

  • Cerebral crus

  • Crus of the cerebral peduncle

Location

  • Situated on the anterior (ventral) surface of the midbrain

  • Extend from the pontomesencephalic junction inferiorly to the level of the diencephalon superiorly

  • Separated medially by the interpeduncular fossa

  • Lie anterior to the midbrain tegmentum

  • Posterior to the suprasellar and interpeduncular cisterns

Anatomical components

  • Descending cortical fibers:

    • Corticospinal fibers

    • Corticobulbar fibers

    • Corticopontine fibers

  • Topographic organization:

    • Medial portion: frontopontine fibers

    • Central portion: corticospinal and corticobulbar fibers

    • Lateral portion: temporopontine and parietopontine fibers

Relations

Anteriorly:

  • Interpeduncular cistern

  • Posterior cerebral arteries and perforators

Posteriorly:

  • Substantia nigra

  • Midbrain tegmentum

Medially:

  • Interpeduncular fossa

  • Exit zone of the oculomotor nerve (CN III)

Laterally:

  • Optic tract (superolateral)

  • Cerebral peduncular surface adjacent to temporal lobe

Superiorly:

  • Thalamus and subthalamic region

Inferiorly:

  • Upper pons

Nerves related to the crus cerebri

  • Oculomotor nerve (CN III):

    • Emerges from the medial surface of the crus cerebri within the interpeduncular fossa

Function

  • Voluntary motor transmission: Conducts corticospinal fibers to spinal motor neurons

  • Cranial motor control: Conveys corticobulbar fibers to cranial nerve nuclei

  • Cerebellar communication: Corticopontine fibers relay information to pontine nuclei

  • Coordination of complex motor activity

Clinical significance

  • Midbrain infarction: Lesions cause contralateral hemiparesis due to corticospinal tract involvement

  • Weber syndrome: Ipsilateral oculomotor nerve palsy with contralateral hemiplegia

  • Tumors: Compression or infiltration by midbrain or parasellar masses

  • Demyelinating disease: Multiple sclerosis may involve the crus cerebri

  • Neurodegeneration: Affected in disorders impacting descending motor pathways

MRI appearance

T1-weighted images:

  • Normal crus cerebri: Low-to-intermediate signal intensity consistent with dense white matter

  • Well-defined margins relative to adjacent gray matter structures

T2-weighted images (including 3T MRI):

  • Normal: Low-to-intermediate signal intensity  

  • Fiber-dense architecture: Compact, homogeneous appearance

  • Pathology: Hyperintensity in infarction, demyelination, or gliosis

FLAIR:

  • Normal: Uniform low-to-intermediate signal

  • Pathology: Hyperintensity improves detection of chronic ischemic or demyelinating lesions

Diffusion-weighted imaging (DWI):

  • Normal: No diffusion restriction

  • Acute infarction: Bright restricted diffusion with low ADC values

Post-contrast T1-weighted images:

  • Normal: No enhancement

  • Pathology: Enhancement in tumors, inflammation, or blood–brain barrier disruption

CT appearance

Non-contrast CT:

  • Normal crus cerebri: Poorly distinguished from surrounding midbrain parenchyma

Post-contrast CT:

  • Normal: No enhancement

MRI images

MRI Crus cerebri axial anatomy image -img-00000-00000_00001

MRI images

MRI Crus cerebri coronal anatomy image -img-00000-00000

MRI images

MRI Crus cerebri sag anatomy image -img-00000-00000