Ch 5 - EDX Medicine and Clinical Neuromuscular Physiology Flashcards
Where is the cell body of a motor neuron located?
Anterior horn of the SC
Where is the cell body of a sensory neuron?
Located in dorsal root ganglion outside the SC in intervertebral foramen
What is an axon?
Projection from sensory or motor neurons that propagate current flow and transport nutrition
Where do axons of motor neurons project?
projects axons distal
Where do axons of a sensory neuron project?
Bipolar cells with 2 axons
1 projects proximal and one distal
What is the innervation ratio?
Amount of muscle fibers innervated by one axon
What is the innervation ratio of a muscles of gross movement?
High
The higher the ratio the higher the force
What is a myotome?
Group of muscles innervated by one spinal segement
What is a dermatome?
Sensory innervation of one spinal segment
What is a NMJ?
Sites where electric impulse along an axon converts to a chemical rxn and translated to an electrical impulse at postsynaptic mem to initiate a muscle fiber action potential (AP)
What is a motor unit?
Anterior horn cell Motor nerve axon Peripheral nerve NMJ Muscle fibers
What is the Henneman size principle?
Smaller alpha motor neuron has low threshold for excitation and is recruited first in voluntary contraction
Describe Type I muscle fibers
Small cell body
Thin diameter axon
Low innervation ratio
Slow twitch muscle fibers
Describe Type II muscle fibers
Large cell body
Thick diameter axon
High innervation ratio
Fast twitch muscle fibers
What type of fibers does EDX evaluate?
Ia (large, myelinated) fibers
What does the cathode of EDX do?
Neg cathode attracts positive charges and lowers membrane potential allowing Na to rush through voltage-gated channels and generate an AP
How do cooler temperatures impact EDX?
Prolongs latency
Inc: amp, duration
Dec: CV
What is saltatory conduction?
Propogation of current between myelin in Na channels in nodes of Ranvier
What is orthodromic recording?
AP recorded in direction of physiologic conduction
What is antidromic recording?
AP recorded in opposite direction of physiologic conduction
How is a miniature endplate potential (MEPP) created?
Spontaneous release of Ach ~ 5 sec during resting state at NMJ
How is an endplate potential (EPP) produced?
nerve depolarization opens voltage-gated Ca channels and release Ach into synaptic cleft which inc # of MEPP
Describe a Type I muscle fiber
Small Dark Early recruitment Highly resistant to fatigue Mild effort (4-8 hz) Slow, prolonged firing freq Fine, precise movements Small innervation ratio Small amp/duration Aerobic
Describe a Type IIA muscle fiber
Large Dark Late recruitment Fatigue resistant Intermed effort (20-30 Hz) Fast, unsustained firing freq Gross movements Large innervation ratio Large amp/duration Anaerobic
What is demyelination?
Injury to myelin sheath but axon remains intact resulting in slower signal conduction
What is conduction block?
Failure of an AP to proprogate past an area of demyelination along axon that is structurally intact
How does conduction block present on NCS?
> 50% drop in CMAP b/w proximal and distal stimulus sites across area of injury
What are the EDX findings of Demyelination on NSC?
Prolonged latency
Dec CV
Inc temporal dispersion
Dec amp across site of injury
What are the EDX findings of Demyelination on EMG?
Normal insertional activity Normal resting activity \+/- myokymia \+/- dec recruitment Normal MUAP
What is axonal degeneration?
Degeneration of axon starts distally and ascends proximally
What is Wallerian degeneration?
At site of nerve lesion, axon degenerates distally
How long does Wallerian degeneration take?
Motor axons: 7 days
Sensory axons: 11 days
What are the EDX findings of axonal injury on NSC?
Normal latency
Dec amp
Normal temporal dispersion
Mild dec CV
What are the EDX findings of axonal injury on EMG?
ABN insertional activity
ABN resting activity
Dec recruitment
ABN MUAP
What is collateral sprouting?
Neurite sprouts off axon of an intact motor unit to innervate denervated muscle fibers of an injured motor unit
How long does axonal regrowth take?
1 mm/ day
1 inch/month
Describe regrown axons
Dec diameter
Thinner myelin
Shorter internodal distance
Describe reinnervation on EMG
Low amp
Long duration
Polyphasic potentials (nascent potentials)
What are the types of nerve injury based on Seddon Classification?
Neuropraxia
Axonotmesis
Neurotmesis
Describe Neuropraxia
Nerve compression
Axon intact
Local myelin injury
Conduction block
What is seen on NSC in Neuropraxia?
Signal normal distal to lesion
ABN proximal/across lesion
What is seen in EMG in Neuropraxia?
normal/dec recruitment
Describe Axontmesis
Nerve crush injury
Axonal disruption
CT/Scwann cell intact
Conduction failure
What is seen on NSC in Axontmesis?
No conduction proxmial/distal to lesion wk-months after injury
What is seen on EMG in Axonotmesis?
ABN activity
Describe Neurotmesis
Nerve transection injury
Axonal interruption
CT disruption
Conduction failure
What is seen on NSC in Neurotmesis?
No conduction proximal/distal to lesion after 2 weeks
What is seen on EMG in Neurotmesis?
ABN activity
What is the Suderland Classification of nerve injury
Type 1: conduction block Type 2: Axonal injury Type 3: type 2+ endoneurium injury Type 4: type 3+ perineurium injury Type 5: type 4+ epineurium injury
What does the cathode of the stimulating electrode do?
Generates a - impulse and attracts + charges from the axon
What does the anode of the stimulating electrode do?
Generates a + impulse and attracts - charges from the axon
What is anodal block?
Reversing stimulator’s cathode and anode hyperpolarizes the nerve and inhibits generation of AP
What is threshold stimulus?
Electrical stimulus sufficient to produce potential from a nerve
What is maximal stimulus?
Electric stimulus in which no further increased evoked potential will occur with added stimulus intensity
What errors can occur with submaximal stimulus?
Falsely lower amp & prolonged latency
can give impression of axonopathy or conduction block
What is the supramaximal stimulus?
Electrical stimulus intensity at leaset 20% above maximal stimulus
What can too high of a stimulus intensity lead to?
Decreased conduction times
Shortened latency
Altered waveforms
Amp unchaged
How can stimulus artifact be minimized on NCS?
Placing ground electrode b/w recording electrode and simulator
Appropriate anode and cathode placement
Cleansing skin of dirt, perspiration and lotion
What does the High Frequency (Low Pass) Filter (HFF) do?
Removes signals with frequencies higher than its cutoff setting and signals below not affected.
Affects faster portions of the summated waveforms
What does the Low Frequency (High Pass) Filter (LFF) do?
Removes signals with frequencies lower than cut off and frequencies above not affected.
Affects slower portions of summated waveforms
What are the filter settings for NCS and EMG?
Motor NCS: 2Hz-10kHz
Sensory NCS/EMG: 20Hz-10kHz
What does increasing the LFF do to waveforms on NCS?
Reduces peak latency
Reduces amp
Changes potentials from bi- to triphasic
Does not change onset latency
What does reducing the HFF do to waveforms on NCS?
Prolongs peak latency
Reduces amp
Creates longer negative spike
Prolongs onset latency
What is the sweep speed?
Time allocated for each x-axis division and measured in miliseconds
What is the sensitivity?
Height allocated for each y-axis and measured in milivolts (mV) or microvolts (uV).
What is a normal sweep speed setting?
Sensory: 5 ms
Motor: 2 ms
EMG: 10 ms
What is a normal sensitivity setting?
Sensory: 10 uV
Motor: 5 uV
EMG: 100 uV (insertional acitivity)
1 mV (recruitment pattern)
What is the onset latency?
Time required for electric stimulus to initiate an evoked potential. Recorded from initial deflection from baseline
What does onset latency reflect in sensory fibers?
Fastest fibers
What does the peak latency represent?
Initiation of conduction along the majority of axons. Recorded at peak waveform
What is conduction velocity?
Speed an impulse travels along a nerve dependent on myelin sheath
What are the age variations in conduction velocity?
Newborns 50% of an adult
1 yo 80% of adults
3-5 yo equal to adults
Dec 1-2 m/s per decade >50yo
What is considered a normal limb temperature for NCS?
Upper: 32 deg C
Lower: 30 deg C
How does temperature affect CV?
Dec 2.4 m/s per 1 deg C drop
5% dec for each 1 deg C below 29 deg C
What is amplitude?
Max voltage diff b/w two points reflecting # of nerve fibers activated and synchronicity of firing
How is amplitude measured?
Peak-to-peak or baseline-to-peak
How is duration measured?
Initial deflection from baseline to return
What is the duration?
Summation and firing rate of numerous axons
