Corticobulbar Tracts Lecture Notes

Key Goals

• Discuss origin, destination, and function of corticobulbar tracts
• Illustrate the pathway in sagittal and coronal views

Origin

• Starts in the motor cortex:
  • Primary Motor Cortex (30%): Located on the precentral gyrus
  • Motor Association Cortex (30%): Includes pre-motor (15%) and supplementary motor cortex (15%)
  • Primary Somatosensory Cortex (40%): Located on the postcentral gyrus in the parietal lobe
• Specific location on the homunculus: Lateral and inferior portion supplying head, neck, and tongue

Destination

• Corticobulbar tracts descend and synapse in multiple cranial nerve nuclei:
  • Trigeminal Nerve Nucleus (CN V): Muscles of mastication
  • Facial Nerve Nucleus (CN VII): Muscles of facial expression
  • Nucleus Ambiguus: Associated with CN IX (glossopharyngeal), CN X (vagus), and CN XI (accessory)
  • Hypoglossal Nucleus (CN XII): Muscles of the tongue
• Indirect connections to CN III, IV, and VI via the paramedian pontine reticular formation and medial longitudinal fasciculus

Functions

• Chewing: Muscles of mastication (CN V)
• Facial Expression: Muscles of facial expression (CN VII)
• Deglutition and Speech: Soft palate, uvula, pharynx, and larynx (Nucleus Ambiguus: CN IX, X, XI)
• Tongue Movements: Muscles of the tongue (CN XII)

Pathway (Sagittal View)

1. Cell bodies in motor cortex
2. Descend through the corona radiata
3. Pass through the internal capsule
4. Down through the midbrain (crus cerebri)
5. Continue through the pons and medulla
6. Collaterals synapse on cranial nerve nuclei (CN V, VII, IX, X, XI, XII)

• Upper Motor Neuron: Cortex to nuclei
• Lower Motor Neuron: From nuclei to target muscles

Pathway (Coronal View)

• Right corticobulbar tract example:
  • Passes through corona radiata, internal capsule (genu), and midbrain (crus cerebri)
  • Continues through pons and medulla
  • Bilateral and contralateral collaterals to nuclei
    • Trigemininal Nucleus (CN V): Bilateral supply
    • Facial Nerve Nucleus (CN VII):
      • Upper portion: Bilateral supply
      • Lower portion: Contralateral supply
    • Nucleus Ambiguus: Bilateral supply (CN IX, X, XI)
    • Hypoglossal Nucleus (CN XII):
      • Upper portion: Bilateral supply
      • Lower portion: Contralateral supply

Clinical Significance

• Facial Nerve Lesions:
  • Upper face: Bilateral cortical supply
  • Lower face: Only contralateral cortical supply
  • Lesion in corticobulbar tract: Drooping in lower contralateral face; upper face function preserved
  • Bell's Palsy: Whole side (upper and lower) affected
• Hypoglossal Nerve Lesions:
  • Upper tongue muscles: Bilateral cortical supply
  • Genioglossus: Only contralateral cortical supply
  • Lesion in corticobulbar tract: Tongue deviates to contralateral side of lesion

Summary

• Corticobulbar tract originates from motor cortices, synapses at multiple cranial nerve nuclei, and controls various muscle groups for mastication, facial expression, and tongue movement
• Important for diagnosing lesions based on motor deficits observed in facial and tongue muscles