Upper motor neuron anatomy

UMN Origin & Path - The Command Center

• Origin: Primary motor cortex (precentral gyrus, Brodmann area 4).
• Organization (Motor Homunculus): Somatotopic map controlling contralateral muscles.
  • Medial to Lateral: Leg → Trunk → Arm → Face.

• Supraspinal Pathway: Axons descend from the cortex to the brainstem.

⭐ High-Yield: The posterior limb of the internal capsule contains both motor and sensory fibers and is a common site for lacunar strokes, leading to pure motor hemiparesis.

Pyramidal Decussation - Crossing the Midline

• Location: Occurs at the caudal medulla (cervicomedullary junction).
• Action: Most corticospinal tract fibers cross the midline.
  • ~85-90% of fibers decussate to form the Lateral Corticospinal Tract, controlling contralateral limbs.
  • ~10-15% of fibers do not cross, continuing as the Anterior Corticospinal Tract, managing bilateral axial/proximal muscles.

⭐ High-Yield: Lesions above the pyramidal decussation (e.g., in the cerebral cortex or internal capsule) result in motor deficits on the opposite side of the body.

Spinal Cord Tracts - The Final Highways

• Lateral Corticospinal Tract (LCST):

  • Located in the lateral funiculus.
  • Controls distal limb muscles (e.g., hands, feet) for fine motor skills.
  • Somatotopy: Cervical fibers are most medial, sacral fibers are most lateral.
  • 📌 Mnemonic: CSL - Cervical is Central, Sacral is to the Side.

• Anterior Corticospinal Tract (ACST):

  • Located in the anterior funiculus.
  • Controls proximal/axial muscles for posture and balance.
  • Most fibers cross at their target spinal cord level.

⭐ Clinical Pearl: The LCST's somatotopy is key. Central cord lesions (e.g., syringomyelia) often impact medial cervical fibers first, causing upper extremity weakness while sparing the lateral sacral fibers for the legs.

UMN Blood Supply - The Power Grid

• Cortex (Motor Homunculus):
  • Face & Arm UMNs: Supplied by the Middle Cerebral Artery (MCA).
  • Leg UMNs: Supplied by the Anterior Cerebral Artery (ACA).
• Subcortical Pathway:
  • Internal Capsule: Blood from lenticulostriate arteries, deep branches of the MCA.
• Spinal Cord:
  • Corticospinal Tracts: Perfused by the Anterior Spinal Artery (ASA), covering the anterior two-thirds.

⭐ The lenticulostriate arteries are end-arteries highly susceptible to hypertensive damage, making them a common cause of lacunar strokes affecting the internal capsule.

• Cell bodies are located in the primary motor cortex (precentral gyrus).
• Axons descend through the posterior limb of the internal capsule.
• Most fibers decussate in the caudal medulla at the pyramidal decussation.
• Descends as the lateral corticospinal tract to synapse on LMNs in the spinal cord's anterior horn.
• MCA supplies the motor cortex for the face and arms; ACA supplies the legs.
• UMN lesions cause spastic paralysis, hyperreflexia, and a positive Babinski sign.