Corticospinal Tracts

Descending Tracts Study Notes

1. Overview of Descending Tracts

• Definition: Pathways carrying motor commands from the brain down to the spinal cord.

• Two major categories:

  • Corticospinal tracts (main focus here).

  • Subcortical tracts (rubrospinal, vestibulospinal, pontine/medullary reticulospinal – covered separately).

2. Corticospinal Tracts

• Originate primarily in the cortex.

• Function: convey voluntary motor commands to skeletal muscle.

• Two divisions:

  1. Lateral corticospinal tract – controls distal limb muscles (fine, precise movement).

  2. Anterior/ventral corticospinal tract – controls axial muscles (trunk, posture, gross movements).

3. Neuron Organization

• Upper Motor Neurons (UMN): cell bodies in cortex → project down to spinal cord.

• Lower Motor Neurons (LMN): cell bodies in anterior horn of spinal cord → project to skeletal muscle.

4. Corticonuclear Fibers (contrast)

• Fibers from cortex to cranial nerve nuclei (e.g., CN III, IV, XI, XII).

• Control muscles of head/neck (extraocular, trapezius, tongue).

• Not corticospinal, but related descending pathways.

5. Cortical Origins of Motor Commands

• Precentral gyrus → Primary Motor Cortex.

• Premotor cortex → learned/repetitive/planned movements.

• Supplementary motor area → complex movement planning.

• Primary somatosensory cortex (postcentral gyrus) → ~30–40% of descending fibers.

• Prefrontal cortex → initiates voluntary movement idea, sends command to motor areas.

6. Motor Modulation (Movement Planning + Correction)

• Basal nuclei: modify motor programs (prevent overshoot/undershoot).

• Cerebellum: integrates planned movement + proprioceptive feedback → refines execution.

• Together ensure smooth, coordinated movements.

7. Pathway of Corticospinal Fibers

1. Origin: Pyramidal cells (esp. large Betz cells) in layer V of cortex.

2. Corona radiata → condense into internal capsule (posterior limb).

   • Clinical note: lenticulostriate artery infarct here → contralateral paralysis.

3. Midbrain: descend via cerebral peduncles (crus cerebri).

4. Pons: fibers disperse around pontine nuclei (which send info to contralateral cerebellum via middle cerebellar peduncle).

5. Medulla: fibers enter pyramids.

   • At caudal pyramids: pyramidal decussation.

     • ~80% cross → form lateral corticospinal tract (contralateral).

     • ~15–20% remain uncrossed → form anterior corticospinal tract (ipsilateral, later cross at spinal level).

8. Spinal Cord Targets

• Lateral corticospinal tract (in lateral funiculus):

  • Synapses on LMNs in anterior horn.

  • Controls distal limb muscles → fine voluntary movement.

  • Involves alpha motor neurons (extrafusal fibers) and gamma motor neurons (intrafusal fibers, tone).

  • Alpha–gamma coactivation ensures both contraction + spindle tension.

• Anterior corticospinal tract (in anterior funiculus):

  • Eventually crosses in spinal cord at level of termination.

  • Controls axial muscles → posture, gross movements.

9. Functional Differences

• Lateral corticospinal tract → distal limb, precision, dexterity.

• Anterior corticospinal tract → axial muscles, posture, balance, larger movements.

10. Key Clinical Correlations

• Lesion above decussation (cortex/internal capsule/brainstem) → contralateral weakness.

• Lesion below decussation (spinal cord) → ipsilateral weakness.

• Internal capsule stroke (lenticulostriate artery) → dense contralateral hemiplegia.

Summary:
Descending tracts, particularly the corticospinal tracts, are the main voluntary motor pathways. They originate from multiple cortical motor areas, are refined by the basal nuclei and cerebellum, descend via internal capsule → brainstem → pyramids of medulla, and either cross (lateral corticospinal) or stay ipsilateral then cross later (anterior corticospinal). Together, they control both fine distal limb movements and gross axial motor control.