Upper Motor Neuron Systems & Descending Pathways

Overview of the Upper-Motor Neuron (UMN) System

  • After covering sensory systems, focus shifts to motor systems that enable coordinated, voluntary movement.
  • UMN system = cortical & brain-stem neurons whose axons descend to influence lower motor neurons (LMNs).
    • Includes motor cortex (corticospinal & corticobulbar tracts).
    • Includes 4 additional brain-stem tracts: vestibulospinal, reticulospinal, rubrospinal, tectospinal.
  • Cerebellum & basal ganglia (previously introduced) modulate these pathways; cerebellum will be detailed later.

Primary Motor Cortex (M1, Brodmann Area 4)

  • Location: Precentral gyrus (most caudal part of frontal lobe), anterior to central sulcus; mirror-image somatotopy to S1.
  • Histology
    • Lacks granular Layer 4 → called agranular cortex.
    • Layer 5: giant pyramidal Betz cells (output to spinal & cranial motor nuclei).
  • Neurochemistry: Dense dopaminergic innervation → explains motor side-effects of dopamine-related drugs.
  • Outputs
    • Corticospinal tract (CST) → spinal ventral horn (limbs & trunk).
    • Corticobulbar tract (CBT) → brain-stem nuclei (head & face).
  • Interconnections
    • Reciprocal with premotor/supplementary motor areas (PM/SMA; Brodmann 6, 44, 45).
    • Projects to basal ganglia, motor & somatosensory thalamus, S1, and pontine nuclei (→ cerebellum).

Supplementary & Premotor Areas (SMA/PM)

  • Plan, sequence, and prepare complex movements; activate with tasks demanding fine coordination.
  • PET imaging examples
    • Arm/Hand movement → high glucose uptake in contralateral hand/arm region of M1 + PM cortex.
    • Speech → uptake shifts to face/oropharyngeal M1 region, PM areas, and temporal-lobe speech perception zones.

Corticospinal Tract (CST)

  • Massive, myelinated fiber bundle forming internal capsule between caudate & putamen.
  • Descends ipsilaterally through midbrain, pons, medulla → pyramidal decussation where ≈90%\approx90\% fibers cross.
    • Crossed fibers → lateral CST (contralateral control below medulla).
    • Uncrossed 5!–!10%5!–!10\% → anterior/ventral CST (ipsilateral descent, some cross near termination).
  • Termination: Mostly on spinal interneurons; occasional direct synapse on alpha-motor neurons (especially fine finger control).
  • Function: Voluntary, precise, fractionated movement; overrides simple spinal reflexes.

Corticobulbar Tract (CBT)

  • Origin: Face/head region of M1 & PM.
  • Path: Internal capsule → midbrain/pons/medulla cranial-nerve nuclei.
  • Projection rule: Bilateral innervation except
    • Lower facial muscles (CN VII, caudal division) → contralateral only.
    • Tongue muscles (CN XII) → contralateral only.
  • Governs speech, mastication, swallowing, facial expression, tongue protrusion.

Rubrospinal Tract (RST)

  • Origin: Red nucleus (midbrain tegmentum).
  • Immediate ventral decussation → descends contralaterally, mainly to cervical enlargement.
  • Targets proximal upper-limb LMNs; relatively small in humans but
    • Demonstrates plasticity with strength training.
    • Provides alternate motor control after CST lesions (stroke, spinal injury).

Reticulospinal Tracts (ReST)

  • Dual origins in brain-stem reticular formation.
    1. Medial (pontine) ReST
    • Origin: Pontine reticular formation.
    • Descends ipsilaterally.
    • Facilitates extensor (anti-gravity) muscles.
    1. Lateral (medullary) ReST
    • Origin: Medullary reticular formation.
    • Descends bilaterally.
    • Facilitates flexor muscles.
  • Contribute to posture, locomotion, and (with RST) fine upper-limb control.

Tectospinal Tract (TST)

  • Origin: Superior colliculus (part of midbrain tectum; integrates visual & auditory cues).
  • Crosses below inferior colliculus → descends contralaterally to upper cervical levels.
  • Bilateral termination in ventral horn LMNs controlling neck muscles.
  • Function: Reflexive orienting of head/neck & eye alignment toward salient visual or auditory stimuli (foveation).
  • Experimental evidence: Anterograde tracers (e.g.
    WGA) injected into superior colliculus label contralateral cervical projections.

Vestibulospinal Tracts (VST)

  • Input: Purkinje cells from cerebellar flocculonodular lobe → vestibular nuclei.
  • Two divisions
    1. Lateral VST
    • Origin: Lateral/pontine vestibular nucleus.
    • Descends ipsilaterally entire cord.
    • Excites axial & proximal extensor muscles for posture/balance.
    1. Medial VST
    • Origin: Medial (medullary) vestibular nucleus.
    • Descends bilaterally to cervical/upper thoracic levels.
    • Influences neck & upper-back flexor muscles; stabilizes gaze during head motion.
  • Works with TST to maintain equilibrium during rapid head movements.

Integration & Functional Themes

  • Multiple descending systems share overlapping targets; redundancy allows recovery if one pathway fails.
  • CST provides voluntary, fractionated control; brain-stem tracts supply posture, orientation, basic gait patterns.
  • Dopaminergic modulation (from midbrain) affects cortical excitability & explains drug-induced dyskinesias.
  • Cerebellum (via vestibular & reticular nuclei) refines timing & balance; basal ganglia (via thalamus) set movement initiation/selection.

Clinical Correlates

  • Upper Motor Neuron Syndrome (after CST lesion)
    • Immediate: Contralateral flaccid weakness/hemiparesis.
    • Days-weeks: Spasticity, hyperreflexia, clasp-knife rigidity.
    • Positive Babinski sign (dorsiflexion & fanning of toes) re-emerges.
  • Lesion localization
    • Above pyramidal decussation ⇒ deficits contralateral to lesion.
    • Below decussation (spinal hemisection) ⇒ ipsilateral motor loss; accompanies dorsal-column sensory changes (Brown-Séquard pattern).
  • Amyotrophic Lateral Sclerosis (ALS)
    • Degenerates both UMNs & LMNs.
    • Early: UMN signs (limb weakness, spasticity) or CBT signs (bulbar onset in ≈25%\approx25\%).
    • Late: LMN degeneration → muscle wasting (atrophy), fasciculations; ventral nerve-root loss visible on pathology.
  • Recovery via alternate tracts
    • RST & ReST may mediate residual or rehabilitated movement post-CST stroke.

Experimental & Anatomical Techniques Mentioned

  • Positron Emission Tomography (PET)
    • Measures regional glucose uptake; reveals somatotopic activation during specific tasks (hand vs speech).
  • Anterograde Tracing (e.g. Wheat-Germ Agglutinin, WGA)
    • Injected into neuron soma; travels along axon → maps projection fields (e.g.
      TST from superior colliculus).
  • Nissl & dopamine-marker staining
    • Identify agranular cortex (absent Layer 4) and high dopaminergic density in M1.

Practical & Ethical Considerations

  • Understanding UMN pathways guides neurosurgical planning (e.g.
    internal-capsule stroke risk) and rehabilitation strategies.
  • Dopaminergic drug effects on M1 necessitate caution in psychiatric/neurological pharmacotherapy.
  • Motor recovery research (plasticity of RST/ReST) informs physical-therapy protocols and underscores neuroplastic potential.