Week 4 HUBS - Somatic Motor Systems

Upper Motoneurons (UMNs)

• Located entirely within the CNS; axon synapses on a LMN
• Some UMNs are interneurons; the pathway to a LMN may involve >1 UMN
• UMN excitation can produce either excitation or inhibition of the LMN
• Pyramidal cells: largest cortical neurons; soma about 100\,\mu\mathrm{m}; project to brainstem/spinal cord via corticobulbar and corticospinal tracts

Lower Motoneurons (LMNs)

• Cell body in a somatic motor nucleus (brainstem or spinal cord) in the CNS
• Axon projects into the PNS via ventral roots/spinal nerves or cranial nerves
• Innervates skeletal muscle; LMN activation directly causes muscle contraction

Primary Motor Cortex

• Located in the precentral gyrus; drives voluntary skeletal movements
• Topographical map: medial = lower limbs; lateral = face
• Disproportionate representation: finer movements require larger cortical area

Corticospinal & Corticobulbar Pathways

• 1) Pyramidal cells of primary motor cortex initiate voluntary movements
• 2) Corona radiata
• 3) Internal capsule
• 4) Cerebral peduncles
• 5) Tracts through pontine nuclei
• 6) Pyramidal tracts in the medulla
• 7) Lower motor neurons (cranial nerve nuclei in brainstem; ventral horn of spinal cord)

Corticospinal Pathways

• Lateral corticospinal tract: movements of limbs; contains \approx 90\% of descending UMNs; decussates in the medulla
• Anterior (ventral) corticospinal tract: movements of trunk/axial muscles; \approx 10\%; decussates at termination
• Topography is maintained along the tract

Corticospinal Tract Organization

• Voluntary motor control travels from Primary Motor Cortex to ventral horn of the spinal cord
• Projections also reach basal ganglia and brainstem nuclei
• Anterior tract controls trunk; lateral tract controls limbs

Corticobulbar Tracts

• Example: CN XII (Hypoglossal nerve)
• UMN = lateral primary motor cortex (head/face)
• LMN = hypoglossal nucleus in the medulla

Descent Systems: Extrapyramidal vs Pyramidal

• Descending motor pathways include direct (pyramidal) and indirect (extrapyramidal) systems
• Major extrapyramidal tracts: Reticulospinal, Vestibulospinal, Tectospinal, Rubrospinal
• Medial pathways support posture and tone; lateral pathways support distal limb movement

Lateral Pathway

• UMNs in brainstem/midbrain
• Rubrospinal tract: cortex -> red nucleus -> spinal ventral horn
• Movements primarily involve distal upper limbs

Medial Pathways

• Unconscious, but not involuntary, movements (maintaining posture, regulating tone)
• Head, neck, trunk, proximal limbs; reflexive movements
• Vestibulospinal tract: vestibular nuclei to spinal cord; uses head position and movement info
• Tectospinal tract: superior/inferior colliculi; uses visual/auditory information for head/neck movements
• Reticulospinal tract: reticular formation to spinal cord; integrates wide range of info to maintain tone and coordinate movement

Spinal Cord Injury: Pathway Deficits

• Dorsal horn lesions → loss of sensory input
• Ventral horn lesions → loss of LMNs & motor output
• Deficits depend on lesion level: higher lesions produce greater deficits
• Consider effects on visceral function in higher lesions

LMN Dysfunction (Polio)

• LMN degeneration leads to widespread paralysis
• Deficits: ipsilateral (same side as lesion)
• Features: weakness/paralysis, flaccid muscles, atrophy, reduced/absent reflexes

UMN Dysfunction (Stroke)

• UMN deficits due to brain tissue damage (ischemic or hemorrhagic)
• Deficits are mainly contralateral if lesion is above the medulla; ipsilateral if below
• Features: weakness/paralysis; muscle atrophy limited (LMNs still innervated)
• Increased muscle tone (spasticity) due to loss of inhibitory non-corticospinal UMNs
• Reflexes often exaggerated due to loss of inhibitory inputs on LMNs

Motor Neuron Disease (ALS)

• Also known as Amyotrophic Lateral Sclerosis or Lou Gehrig’s Disease
• Progressive neurodegenerative disease affecting both UMNs and LMNs
• Loss of motor neurons → skeletal muscle atrophy
• Disease spreads to other body parts regardless of starting location; respiratory muscles may be involved
• Cognitive function largely unaffected

Quick Recap

• UMNs: CNS-only, synapse to LMNs; may involve interneurons
• LMNs: motor nuclei to muscles; direct cause of contraction
• Major pathways: Corticospinal (lateral/trunk), Corticobulbar; Extrapyramidal (reticulo-, vestibulo-, tectospinal, rubrospinal)
• Medial vs Lateral: posture/tone vs distal movement
• Injury patterns distinguish UMN vs LMN signs and lesion level
• ALS links UMN and LMN degeneration with progressive weakness