8/24 Lower Motor Neurons - Glendinning Flashcards Preview

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Flashcards in 8/24 Lower Motor Neurons - Glendinning Deck (20)

alpha motor neurons


lower motor neurons

"lower" bc it's the last part of the motor nervous system - part that goes out to muscle


  • neurons that innervate muscles
  • heavily myelinated, fast-conducting neurons
  • locations
    • cell bodies located in CNS (ventral horn lamina IX or brainstem motor nuclei)
    • axons in PNS in ventral root, spinal nerve, peripheral nerve, or cranial nerve


upper motor neurons

begin in cortex or brainstem and project in long-pathways to brainstem or spinal cord motor neurons


somatotropic organization of motor neurons in ventral horn

motor neurons innervating...

  • proximal muscles have medially located nuclei
  • distal muscles have laterally located nuclei
  • flexors are more dorsal
  • extensors are more ventral


application: ventral horns in thoracic region dont have any distal muscles to innervate, SO ventral horn is thin!




spinal cord LMN with important functions

C3-C5 : motor neurons to phrenic nerve controlling diaphragm

S3-S4 Onuf's nucleus : motor neurons innervating urethral and external anal sphincter → voluntary control of urination/defecation

S2-S4 : motor neurons to pelvic floor muscles


LMN in cranial nerves

originate in brainstem

aka "bulbar" motor neurons


CN III, IV, VI : eyes

CN V : jaw opening

CN IX, X, XI : laryngeal and pharangeal muscles (speaking/swallowing), trapezius and SCM

CN VII : facial nerve

CN XII : tongue muscles


motor unit

  • one motor neuron and all of the muscles it innervates 
  • number varies from (1:10 extraocular → 1:1000 leg muscles)


motor neurons and muscle type

motor neurons determine muscle fiber types within the motor unit

units classified by force and fatiguability

  • all muscle fibers innervated by a motor neuron will be the same type
    • ATPase weak rxn - S (slow, fatigue resistant; smallest)
    • ATPase mild rxn - FR (fast, fatigue resistant; intermediate)
    • ATPase strong reaction - FF (fast, fatiguable; larger)

most muscles will have a combo of all three types of motor units present


how do you increase the force of a muscle contraction?

recruit motor units!

small (S) → large (FF)


1. orderly motor unit recruitment by size ("size principle")

2. after a motor unit is recruited, its finir rate can also increase


lower motor neuron syndrome

  • weakness or paralysis
  • atrophy
  • hyporeflexia or areflexia
  • decreased tone (resistance to passive movement)
    • flaccid paresis/paralysis
  • fibrillations, positive sharp waves or fasciculations, measured by EMG


sx will be in muscles that are innervated by motor neurons!


weakness terms






paralysis : weakness so severe muscle can't be contracted

paresis : weakness or partial paralysis

plegia : severe weakness or paralysis

  • diplegia : bilateral lower limb weakness
  • quadriplegia : all 4 limbs
  • hemiplegia : one sided weakness

palsy : imprecise term for either weakness or no movement


muscle atrophy

without motor neuron innervation...

  • muscles don't contract → lose mass
  • muscles lose trophic support from motor neurons



measure action potentials to identify the source of muscle weakness

  • measures of denervation include: fibrillations, positive sharp waves, fasciculations



only detected with EMG

  • short-duration, spontaneous biphasic or triphasic potentials produced by single muscle fibers
    • indicative of denervated muscle
  • believed to represent an unstable muscle fiber cell membrane



larger potentials caused by spontaneous activity in a motor UNIT or several motor UNITS

  • caused by LMN lesions, esp in anterior horn cell disease like ALS
  • large potentials suggestive of denervation and reinnervation


conditions that cause LMN syndrome

  • peripheral nerve, spinal nerve, or cranial nerve lesions
  • cauda equina lesions
  • strokes/tumors affecting alpha motor neurons in bentral horn or brainstem
  • polio (viral infection of alpha motor neurons)
  • amyotrophic lateral sclerosis (ALS - also affects upper motor neurons)
  • Guillan Barré (demyelinating disease)
  • Werdnig-Hoffman disease (degen of anterior horn)


spinal muscle atrophy (SMA I-IV)

caused by abnormalities in chromosome 5

group of diseases caused by progressive degeneration of anterior horns

  • begin in infancy

most common/severe : SMA I Werdnig-Hoffman disease 



Werdnig-Hoffman disease

"acute infantile spinal muscle atrophy" aka "floppy baby" syndrome

  • autosomal recessive
  • median age of death : 6-7 months
  • 1/10k live births


  • weakness and muscle wasting in limbs, resp system, bulbar muscles (issues sucking, swallowing, breathing)


type grouping

when denervation occurs, nerves from another motor unit can "sprout" and innervate the denervated muscle

this changes the type of the denervated muscle to the type confered by the sprouting neuron


→→→ chronic denervation leads to less diversity in muscle type


nerve conduction tests in LMN syndrome

measure size of muscle response (CMAP)

  • decreased in problems with muscle, motor neuron, or neuromuscular jx


measure conduction velocity

measute time to CMAP/distance between electrodes

  • decreased with demyelinating conditions


motor and sensory deficits depend on location and nature of lesions

think about the symptoms that you see and work backwards to figure out where the lesion might be

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