Lecture on Corticobulbar Tracts

Key Learning Objectives

• Understand the origin and destination of corticobulbar tracts.
• Comprehend the function of corticobulbar tracts.
• Visualize the corticobulbar tract pathway in sagittal and coronal views.

Origin of the Corticobulbar Tract

• Motor Cortex
  • Primary Motor Cortex
    • Located on the pre-central gyrus.
    • Contributes 30% to the corticobulbar tract.
  • Motor Association Cortex
    • Composed of pre-motor and supplementary motor cortex.
    • Each contributes 15% (total 30%) to the tract.
  • Primary Somatosensory Cortex
    • Located on the postcentral gyrus in the parietal lobe.
    • Accounts for 40% of the tract.
• Motor Homunculus
  • Corticobulbar tract originates from the most lateral and inferior portion (head, neck, tongue).

Destination of the Corticobulbar Tract

• Cranial Nerve Nuclei Synapses
  • Trigeminal Nerve Nucleus (Cranial Nerve V)
    • Muscles of mastication.
  • Facial Nerve Nucleus (Cranial Nerve VII)
    • Muscles of facial expression.
  • Nucleus Ambiguus
    • Associated with Cranial Nerves IX (Glossopharyngeal), X (Vagus), and XI (Accessory).
    • Supplies soft palate, uvula, pharynx, larynx.
  • Hypoglossal Nucleus (Cranial Nerve XII)
    • Muscles of the tongue.
  • Indirect Supply to Cranial Nerves III, IV, and VI
    • Via paramedian pontine reticular formation and medial longitudinal fasciculus.

Function of Corticobulbar Tracts

• Involved in:
  • Chewing
  • Facial expression
  • Deglutition (swallowing)
  • Speech
  • Tongue movements

Pathway of the Corticobulbar Tract (Sagittal View)

1. Cerebral Cortex: Originates from motor cortex.
2. Corona Radiata: Fan-like structure fibers pass through.
3. Internal Capsule: Fibers move through this area.
4. Midbrain (Crus Cerebri): Fibers descend through.
5. Pons and Medulla: Extends down, giving off collaterals to cranial nerve nuclei.
6. Upper and Lower Motor Neurons: Cortex to nuclei is upper motor neuron; nuclei are lower motor neurons.

Pathway of the Corticobulbar Tract (Coronal View)

• Corona Radiata and Internal Capsule
  • Fibers run through corona radiata, then through the internal capsule (specifically the genu).
• Midbrain Crus Cerebri to Pons and Medulla
  • Descends down and gives off collaterals to bilateral nuclei.
• Ipsilateral and Contralateral Fibers
  • Trigeminal and nucleus ambiguus receive bilateral supply.
  • Facial nucleus receives bilateral to upper face, contralateral to lower face.
  • Hypoglossal nucleus receives bilateral to upper part, contralateral to lower part.

Clinical Significance

• Facial Nucleus Lesions:
  • Determine stroke vs Bell’s Palsy using upper vs lower facial muscle involvement.
  • Stroke: Upper face intact due to bilateral supply; lower face droops (contralateral issue).
  • Bell’s Palsy: Affects entire half of the face.
• Hypoglossal Nucleus Lesions:
  • Contralateral tongue deviation indicates cortical bulbar tract lesion.
  • Lesions affect contralateral genioglossus function, causing deviation.

Summary

• The corticobulbar tract is crucial for head, neck, and face motor control, with significant clinical implications for understanding and diagnosing conditions like strokes and cranial nerve palsies.

Remember: A thorough understanding of the dual supply and contralateral specifics is key in clinical practice for diagnosing nerve issues.