8/24 Lower Motor Neurons - Glendinning Flashcards
alpha motor neurons
aka
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)
- orderly motor unit recruitment by size (“size principle”)
- 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
paresis
plegia
palsy
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
electromyography
measure action potentials to identify the source of muscle weakness
- measures of denervation include: fibrillations, positive sharp waves, fasciculations
fibrillation
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
fasciculations
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
SMA I
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
symptoms:
- 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
