Internal Capsule and Pyramidal Decussation

Internal Capsule

• Anterior limb: contains fibers controlling upper limbs
• Genu: contains fibers controlling lower extremities
• Posterior limb: contains fibers of the corticospinal tract (CST)
• CST fibers follow a somatotopic arrangement

Corticospinal Tract (CST)

• Divided into anterior (ventral) and lateral CST
• Anterior CST:
  • Uncrossed fibers (10-15%)
  • Eventually cross before terminating on anterior horn cells in cervical and upper thoracic spinal cord segments
  • Shorter tract
• Lateral CST:
  • Crossing of fibers (75-90%)
  • Forms pyramidal decussations
  • Terminates at the anterior gray column of all spinal cord segments
  • Longer tract
• Pyramidal tract is a crossed tract:
  • Right motor side controls left half of the body
  • Left motor strip controls right half of the body

Lesions

• Upper motor neuron lesion (supranuclear lesion):
  • Affects tract before facial nucleus
  • Ventral portion innervates lower half, which will affect the facial nerve
  • Results in paralysis on the contralateral lower half of the face
  • Also called central facial paralysis
• Lower motor neuron lesion:
  • Affects facial nucleus or facial nerve
  • Lesion is on or after the dorsal and ventral portions of the facial motor nucleus
  • Results in paralysis on the ipsilateral half of the face
  • Also called peripheral facial paralysis (Bell's palsy)

Projections

• Fibers would pass through corona radiata and genu of the internal capsule and terminate in the brainstem
• Fibers project to:
  • Motor nuclei of CN III, IV, V, VI, VII, IX, X, XI, XII (except CN I, II, and VIII)
  • Parts of reticular formation in pons and medulla
  • Sensory relay nuclei (nucleus gracilis, nucleus cuneatus, sensory trigeminal nuclei, and nucleus of the solitary fasciculus)
• Projections are bilateral, except for facial motor nucleus (CN VII) and hypoglossal nucleus (CN XII)

Pyramidal and Extrapyramidal Tracts

• Rubrospinal tract:
  • Origin: red nucleus in the tegmentum of the midbrain
  • Fibers cross immediately in the ventral tegmental decussation
  • Descend down the brainstem to enter the lateral funiculus of the spinal cord
  • Terminate on internuncial neurons in all spinal cord levels
• Interstitiospinal tract:
  • Origin: interstitial nucleus of Cajal
  • Uncrossed fibers
  • Forms part of MLF
  • Terminate in the anterior horn of upper cervical levels of the spinal cord
  • Function: modulates reflex postural movements in response to visual and auditory stimuli

Vestibulospinal Tracts

• Medial vestibulospinal tract:
  • Origin: medial vestibular nucleus
  • Fibers are both crossed and uncrossed
  • Descend in MLF and anterior funiculus of spinal cord
  • Terminate on Rexed laminae VII and VIII
• Lateral vestibulospinal tract:
  • Origin: lateral vestibular nucleus
  • Fibers are both crossed and uncrossed
  • Descend in MLF and anterior funiculus of spinal cord
  • Terminate on Rexed laminae VII and VIII
• Function: for maintenance of upright posture, excites neck and back motor neurons

Reticulospinal Tracts

• Pontine (medial) reticulospinal tract:
  • Origin: pons
  • Fibers descend in anterior funiculus of spinal cord
  • Terminate on Rexed laminae VII and VIII
  • Function: facilitatory to extensor motor neurons
• Medullary (lateral) reticulospinal tract:
  • Origin: medulla
  • Fibers descend in lateral funiculus of spinal cord
  • Terminate on Rexed laminae VII and VIII
  • Function: inhibitory to extensor motor neurons