Spine - Upper vs Lower Motor Neuron

Upper motor neuron

• Originate in cerebral cortex and descent from white matter into spinal cord and ultimately synapse w anterior horn of lower motor neuron

• Together they form the two neuron system responsible for voluntary action of skeletal muscle

UMN function

• Initiation and direction of voluntary movement

  • Beginning at the premotor and supplementary motor complex a signal is sent to the LMN to cause muscle contraction

• Fine motor control

  • Lateral corticospinal tract controls distal extremities and associated fine motor movements

• Sensory gating

  • Corticospinal tract may act as a “gate” bc its fibers terminate in dorsal horn of spinal cord and synapse w interneurons that receive input from sensory receptors

  • Magnitude of gating changes according to specific motor tasks

UMN types

• Corticospinal tract: Fibers from this tract synapse w spinal nerves and are the major neuronal pathway providing voluntary motor function and connects the cortex to spinal cord which enables movement of the distal extremities

• Corticobulbar tract: UMN inn. to the cranial nerves which supply the face

• Rubrospinal tract: Midbrain structure, thought to play a role in fine control of hand movements

• Tectospinal tract: Coordinates movements of head with visual stimuli

• Vestibulospinal tract: Conveys balance information to spinal cord

UMN lesion

• Acute: immediate flaccidity of muscles of contralateral side of body and face and all reflex activity elevated and released

• Chronic: increased or uninhibited motor activity and loss of fine motor control

• Affects corticobulbar (voluntary movement of face, head, and neck) and corticospinal tract (voluntary movement of limbs and trunk). Efferent

LMN: Background & process

• Ventral horn of spinal cord to skeletal and smooth muscles and glands

• Acetylcholine transmits signals

• 1. Signal comes from primary motor cortex of brain (precentral gyrus)

• 2. Pass through UMNs

• 3. Once signal reaches medullary pyramids in medulla oblongata of brainstem, ~90% fibers go into spinal cord on contralateral side of body in the lateral corticospinal tract, the rest (~10%) make the anterior corticospinal tract and control proximal limb movement + posture

LMN types

• Somatic motor neurons (send sensory from peripheral to brain + controls voluntary movement)

  • Alpha

    • Inn. extrafusal fibers and controls skeletal muscle contraction

    • Found in either ventral horn of spinal cord or in brainstem

    • One axon inn. many muscle fibers w almost identical properties

  • Beta

    • Inn. both extrafusal and intrafusal fibers

      • Extra: generate force

      • Intra: detect stretch

  • Gamma

    • Inn. muscle spindles

      • Detect stretch

    • Don’t actually cause motor function - gets activated along w alpha neurons to control muscle contraction

    • Stretch signal sent down afferent nerve fibers to cause quick reflex

• Special visceral efferent motor neurons (branchial)

  • Inn. head and neck muscles

  • In brainstem

  • Branchial mn + sensory neurons make up CN 5, 7, 9, 10 & 11

    • CN 5: trigeminal (mastication, facial sensory)

    • CN 7: facial (facial expression, posterior digastrics)

    • CN 9: glossopharyngeal (swallowing)

    • CN 10: vagus (parasympathetic ns, muscles of throat/larynx)

    • CN 11: accessory (SCM & trapezius - only motor)

• General visceral mn

  • Both sympathetic & parasympathetic ns

    • Sympathetic (T1-L2)

      • Prevertebral or paravertebral ganglia —> inn. heart, colon, intestines, kidneys, lungs

      • Chromaffin cells of adrenal medulla

        • Produce catecholamines

    • Parasympathetic (S2-S4)

      • Inn. ganglia of heart, pancreas, lungs, and kidneys

      • Give rise to CN 3, 6, 9, & 10

        • CN 3: oculomotor (eye movement)

LMN injuries

• Lesion

  • Can be from ventral horn of spinal cord, peripheral nerve, nmj, or muscle. Affect cranial nerves and peripheral motor neurons. Efferent

  • Signs

    • Hyporeflexia

    • Muscle atrophy

    • Hypotonia

    • Muscle twitching

    • Negative Babinski reflex

    • Flaccid paralysis

• Spinal muscular atrophy

  • Congenital degeneration of ant horn

    • Type 1 - Werdnig-Hoffman disease —> respiratory failure in childhood —> death

    • Types 2 & 3 —> less severe, can’t ambulate

  • Weakness is symmetric

• Poliomyelitis

  • Poliovirus destroys ant horn - LMN paralysis

  • Weakness is asymmetric

  • Can also cause respiratory paralysis

Blood supply

• Primary motor cortex: ant and middle cerebral arteries (mostly middle)

  • ACA - lower limbs

  • MCA - upper limbs & face

• Upper motor cortex —> pyramidal tract —> lenticulostriate arteries

• Tract at brainstem —> basilar artery

• Caudal medulla level & lower motor cortex —> ant spinal artery

Radiculopathy implications

• Condition caused by compressed spinal root due to shrinkage of the IV foramen

• Causes:

  • Trauma (ex. spondylolisthesis)

  • Shortening of IV discs

  • Herniated/bulging discs

  • Cancer/tumors

• Symptoms:

  • Pain

  • loss of motor function

  • Loss of sensation

• LMN lesion but specifically at the nerve root