UMN vs LMN Lecture Notes Flashcards

UMN vs LMN

Overview

• The presentation discusses the differences between Upper Motor Neurons (UMNs) and Lower Motor Neurons (LMNs).

Motor Pathways

• Pyramidal Tract:

  • Primary tract for voluntary movement.

  • Provides a direct pathway between the cerebral cortex and the spinal cord.

  • Named from the medullary pyramids of the medulla oblongata.

  • Divides into the corticospinal and corticobulbar tracts.

    • Corticospinal tract fibers synapse with spinal nerves.

    • Corticobulbar fibers synapse with cranial nerves.

  • Majority of corticospinal tract axons decussate at the pyramidal decussation; axons continue their descent contralateral from their cell bodies of origin and enter the spinal cord at the lateral funiculus; tract is now the lateral corticospinal tract.

  • About 10% of the corticospinal tract axons do not decussate and continue their descent down the brain ipsilateral to their cell bodies of origin; enter the ventral aspect of the spinal cord and are known as the anterior corticospinal tract

  • UMN lesions above the pyramidal decussation- symptoms contralateral to the site of the lesion

  • UMN lesions below the decussation present symptoms ipsilateral to the lesion

  • Causes of UMN lesion: cerebrovascular accidents, traumatic brain injury, anoxic brain injury, malignancy, infections, inflammatory disorders, neurodegenerative disorders, and metabolic disorders

• Extrapyramidal Tracts:

  • Provide indirect pathways for the coordination of movement.

  • Important for maintaining posture and regulating involuntary motor functions.

  • Examples:

    • Reticulospinal tract from reticuluar formation

    • Vestibulospinal tract from vestibular nuclei

    • Rubrospinal tract from the red nucleus

    • Tectospinal tract from the superior colliculus

  • Functions:

    • Postural tone adjustment.

    • Preparation of predisposing tonic attitudes for involuntary movements.

    • Performing movements that make voluntary movements more natural and correct.

    • Control of automatic modifications of tone and movements.

    • Control of the reflexes that accompany the responses to affective and attentive situations.

    • Control of movements that are originally voluntary but become automatic through exercise and learning.

    • Inhibition of involuntary movements (hyperkinesias), particularly evident in extrapyramidal disease.

Upper Motor Neurons (UMN)

• First-order neurons responsible for carrying electrical impulses that initiate and modulate movement.

• Pyramidal neurons in the precentral gyri (primary motor area) form the corticospinal tract.

• UMNs synapse in the anterior horn with LMNs.

Corticospinal Tract

• Majority of axons decussate (cross) at the pyramidal decussation.

• Axons descend contralaterally in the lateral funiculus, forming the lateral corticospinal tract.

• Lateral corticospinal tract axons control distal extremities and synapse directly on lower motor neurons.

  • Responsible for precise, agile, and highly skilled movements (e.g., buttoning a shirt, playing the piano).

• About 10% of corticospinal tract axons do not decussate and descend ipsilaterally as the anterior corticospinal tract.

  • Terminate in skeletal muscles that control movements of the trunk and proximal parts of the limbs.

UMN Lesions

• Damage to motor neurons above the nuclei of cranial nerves or anterior horn cells of the spinal cord.

• Symptoms (Upper Motor Neuron Syndrome):

  • Weakness (affects extensors of the arm and flexors of the leg).

  • Spasticity.

  • Clonus.

  • Hyperreflexia.

  • Babinski sign (extension of the large toe and fanning of other toes upon stroking the sole of the foot).

  • Hoffman sign (flexion and adduction of the thumb when flicking the middle finger).

• UMN Lesion Location and Symptoms:

  • Above the pyramidal decussation: symptoms contralateral to the lesion (e.g., right lesion causes left-sided weakness).

  • Below the decussation: symptoms ipsilateral to the lesion (e.g., left-sided spinal cord lesion causes left-sided weakness).

• Causes:

  • Cerebrovascular accidents.

  • Traumatic brain injury.

  • Anoxic brain injury.

  • Malignancy.

  • Infections.

  • Inflammatory disorders.

  • Neurodegenerative disorders.

  • Metabolic disorders.

Lower Motor Neurons (LMN)

• Directly innervate skeletal muscle.

• Cell bodies are in the anterior horn of the spinal cord and at cranial nerve nuclei.

• Cholinergic.

Types of Motor Neurons

• Branchial Motor Neurons:

  • Located in the brainstem; form LMNs of the cranial nerve nuclei.

  • Innervate muscles formed by the branchial (pharyngeal) arch; includes muscles innervated by cranial nerves V, VII, IX, and X.

• Visceral Motor Neurons:

  • Component of the autonomic nervous system (ANS).

  • Regulate smooth muscles and glands.

  • Divided into sympathetic and parasympathetic nervous systems.

• Somatic Motor Neurons:

  • Located in the ventral horn of the spinal cord.

  • Directly innervate skeletal muscle.

  • Receive stimuli from UMNs.

  • Subtypes:

    • Alpha motor neurons: Innervate extrafusal muscle fibers (primary means of skeletal muscle contraction).

    • Beta motor neurons: Innervate both extrafusal and intrafusal fibers (poorly characterized).

    • Gamma motor neurons: Innervate muscle spindles and dictate their sensitivity; fine-tune muscle contraction (alpha-gamma coactivation).

LMN Lesions

• Presentation:

  • Muscle atrophy.

  • Fasciculations (muscle twitching).

  • Decreased reflexes (hyporeflexia).

  • Decreased tone.

  • Negative Babinski sign.

  • Flaccid paralysis.

• Examples:

  • Poliomyelitis.

  • Spinal muscular atrophy.

Extrapyramidal Tracts and Related Diseases

• Diseases/Conditions:

  • Parkinson's disease (PD).

  • Huntington's chorea (HC).

  • Sydenham chorea.

  • Multiple systemic atrophy (MSA).

  • Progressive supranuclear palsy.

• Clinical Features:

  • Hypokinesias (scarcity of movement).

  • Akinesia (absence of movement).

  • Bradykinesia (slowness in the execution of voluntary movements).

• Hyperkinesias:

  • Abnormal involuntary movements.

    • Choreic movements: Sudden, irregular, incomplete, aimless, variable movements.

    • Athetotic movements: Arrhythmic, slow, exaggerated, tentacular movements.

    • Hemiballism: Intense choreic-like movements that persist during sleep.

    • Tics: Rapid muscle contractions reproducing stereotyped movements, can be voluntarily inhibited.

    • Tremors: Rhythmic, slow pace (4-5 oscillations per second), more pronounced at rest, attenuates during voluntary movements.

    • Spasms: Involuntary movements (tonic or clonic).

    • Myoclonus: Rapid, sudden contractions involving isolated muscles or bundles of muscle fibers.

Bilateral Injuries at the Brainstem Level

• Can cause quadriplegia.

• Hypertonia (decerebrate or decorticate rigidity) can be observed, depending on the level of injury.

Descending (Efferent) Pathways:

Pyramidal Tracts

• Lateral corticospinal tract

• Anterior corticospinal tract

Extrapyramidal Tracts

• Rubrospinal tract

• Reticulospinal tracts

• Olivospinal tract

• Vestibulospinal tract

Sensory and Ascending (Afferent) Pathways

Dorsal Column Medial Lemniscus System

• Gracile fasciculus

• Cuneate fasciculus

Spinocerebellar Tracts

• Posterior spinocerebellar tract

• Anterior spinocerebellar tract

Anterolateral System

• Lateral spinothalamic tract

• Anterior spinothalamic tract

• Spino-olivary fibers