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)
