Lesson 11 – Motor Pathways (Corticospinal & Corticobulbar)

Basic Lesson Concepts

  • Goal of Lesson 11 (Banks, 2025):
    • Understand the corticospinal tract (multiple origins ➜ multiple terminations).
    • Learn the clinical value of Brodmann numbers for cortical functional areas.
    • Review descending motor pathways of the cranial nerves (collectively = corticobulbar pathways).
    • Survey common spinal-cord injuries and their characteristic motor/sensory deficits.
    • Reference text: Nolte, 7th ed., Chap. 18, pp. 459{-}473.

Corticospinal (Pyramidal) System

  • Primary descending pathway for voluntary movement below the head.
  • Constituents
    • Upper-motor neurons (UMNs)
    • Cell bodies = pyramidal cells of cortical layer V (of VI).
    • Somatotopic origin distribution:
      • Pre-central gyrus (primary motor cortex) \approx 45\% ➜ Brodmann 4.
      • Premotor cortex \approx 30\% ➜ Brodmann 6.
      • Post-central gyrus \approx 25\% ➜ Brodmann 3,1,2 (sensorimotor modulation).
  • Functional nickname: “pyramidal tract.”

Brodmann’s Cytoarchitectonic Map & Motor Cortices

  • Korbinian Brodmann, 1909: 52 cytoarchitectonic areas.
    • Horizontal lamination + vertical cell columns → distinct functional parcels.
    • Modern research blurs purely cytoarchitectonic boundaries, but numbering still used clinically.
  • Key frontal-lobe motor territories
    • Area 4 (Primary Motor Cortex): ~40\% of giant Betz cells originate here.
    • Area 6 (Premotor / SMA): sequencing of goal-directed actions.
    • Areas 9–12 (Prefrontal): goal setting, problem solving, decision making.

Somatotopic Mapping (Motor & Somatosensory Homunculus)

  • Penfield & Boldrey 1934 via electrical stimulation.
  • Principle: Cortical “real estate” ∝ agility + movement repertoire of body part.
    • Larger representations: hands, face, tongue.

Upper Motor Neuron Trajectory

  1. Corona radiata ➜ internal capsule (posterior 1/3 of posterior limb).
  2. Corticobulbar fibers travel medial to corticospinal within capsule.
  3. Descent through brain-stem:
    • Midbrain: central 3/5 of crus cerebri (basis pedunculi).
    • Pons: disperse as irregular bundles in basis pontis.
    • Medulla: reunite as prominent pyramids.
  4. Pyramidal decussation (caudal medulla)
    • 85{-}90\% fibers cross → lateral corticospinal tract (LCST).
    • 8{-}10\% remain uncrossed in anterior funiculus → anterior corticospinal tract (ACST).
      • Many ACST fibers cross in anterior white commissure (AWC) just before synapse; special role = neck & shoulder control.
    • 2{-}5\% stay ipsilateral within LCST.
  5. Spinal-cord descent & exit pattern
    • Cervical cord: \approx 60\% LCST fibers exit → upper limbs.
    • Thoracic cord: \approx 10\% ACST fibers exit → trunk.
    • Lumbar/Sacral: \approx 30\% LCST fibers exit → lower limbs.
  • Remember: All ascending & descending tracts are bilateral (crossed/uncrossed contingents exist).

Lower Motor Neurons (LMNs)

  • α-motor neurons = “final common pathway.”
  • Ventral horn somatotopy:
    • Mediolateral axis: proximal ↔ distal muscles.
    • Dorsoventral axis: flexor ↔ extensor muscles.
  • Axons exit via ventral roots ➜ 31 peripheral nerves.
    • Ventral rami: limb muscles & trunk flexors.
    • Dorsal rami: trunk extensors.

Spinal Cord Segmentation (Structural Review)

  • Continuous dorsal rootlets enter at posterolateral sulcus; ventral rootlets leave at anterolateral sulcus.
  • Rootlets → dorsal & ventral roots → spinal nerve → divide into dorsal ramus (smaller) & ventral ramus (larger) (both mixed).
  • Each dorsal root bears a dorsal-root ganglion just proximal to union with ventral root.

Pathway Schematics

  • Classic diagram (Carpenter):
    • Red = UMNs (decussate in medulla; descend contralaterally through brain-stem, ipsilaterally in spinal cord).
    • Black = LMNs.

Extrapyramidal (Indirect) Motor Pathways

  • “Extrapyramidal tracts” = all descending tracts other than corticospinal.
    1. Rubrospinal
    2. Reticulospinal
    3. Vestibulospinal
    4. Tectospinal
  • Mediate automatic & learned motor programs (walking, running, cycling, etc.).

Corticobulbar (Corticonuclear) Pathways

  • Influence motor cranial-nerve nuclei (all CNs except I, II, VIII).
  • Term “bulb” = historical synonym for medulla/brain-stem.
  • Fine motor control (lips, tongue) via direct synapse; most others via reticular-formation interneurons.

Cranial-Nerve Nuclei With NO Direct Corticobulbar Input

  • CN III (Oculomotor), IV (Trochlear), VI (Abducens).
    • Rapid horizontal gaze: PPRF.
    • Vertical gaze: MLF.

Five Key Nuclei Commonly Discussed

  1. Trigeminal Motor (CN V) – muscles of mastication.
  2. Facial (CN VII) – muscles of facial expression.
  3. Hypoglossal (CN XII) – tongue muscles.
  4. Nucleus Ambiguus (CN IX/X) – pharyngeal & laryngeal muscles.
  5. Spinal Accessory (CN XI) – SCM & trapezius.

General Rules

  • Bilateral corticobulbar innervation (both hemispheres) to:
    • Trigeminal Motor (CN V).
    • Part of Facial nucleus controlling forehead.
    • Nucleus Ambiguus (CN X).
  • Contralateral-predominant innervation to:
    • Facial neurons controlling lower face.
    • Hypoglossal nucleus (CN XII).
  • Projection fibers:
    • Originate in face/mouth region of motor cortex + other frontal/parietal areas.
    • Descend with corticospinal tract, peel off near target nucleus on both sides.
    • Hence no single “corticobulbar decussation.”

Clinical Correlate

  • LMN lesion of CN VII ➜ Bell’s palsy (ipsilateral upper & lower face weakness).
  • UMN lesion (e.g., cortical stroke) ➜ contralateral lower-face weakness (forehead spared).

Spinal Cord Syndromes (Motor & Sensory Deficit Patterns)

  1. Transverse Cord Lesion

    • All ascending & descending pathways partially/fully interrupted below lesion level.
    • Sensory level present (dermatomal line).
    • Causes: trauma, tumors, multiple sclerosis (MS), transverse myelitis.

  2. Hemicord Lesion (Brown-Séquard Syndrome)

    • Ipsilateral UMN-type weakness (LCST damage).
    • Ipsilateral loss of vibration/proprioception (posterior columns).
    • Contralateral loss of pain & temp (anterolateral system) 1–2 segments below lesion.
    • Causes: penetrating injury, MS, unilateral tumors / herniated discs.
    • High-yield exam topic!

  3. Posterior Cord Syndrome

    • Loss of vibration & position sense below lesion.
    • Large lesions may affect LCST → UMN weakness.
    • Causes: trauma, posteriorly placed tumors, (rare) MS.

  4. Anterior Cord Syndrome

    • Loss of pain & temperature below lesion (anterolateral pathway).
    • LMN weakness at lesion level (anterior horn cell loss).
    • Causes: trauma, MS, anterior spinal-artery infarct.

Practical / Ethical / Clinical Connections

  • Brodmann mapping remains standard in neurosurgical planning despite evolving functional imaging.
  • Understanding fiber decussation crucial for localizing strokes & predicting deficits.
  • Extrapyramidal tracts highlight importance of subcortical plasticity for rehab after corticospinal damage.
  • Awareness of bilateral vs contralateral corticobulbar control guides prognosis for facial, tongue, swallowing deficits.
  • Early recognition of spinal-cord syndromes enables timely interventions (e.g., decompression, steroids, vascular therapy).