Ch 5 - EDX Medicine and Clinical Neuromuscular Physiology Flashcards by Chris Garrison

Where is the cell body of a motor neuron located?

Anterior horn of the SC

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Where is the cell body of a sensory neuron?

Located in dorsal root ganglion outside the SC in intervertebral foramen

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What is an axon?

Projection from sensory or motor neurons that propagate current flow and transport nutrition

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Where do axons of motor neurons project?

projects axons distal

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Where do axons of a sensory neuron project?

Bipolar cells with 2 axons

1 projects proximal and one distal

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What is the innervation ratio?

Amount of muscle fibers innervated by one axon

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What is the innervation ratio of a muscles of gross movement?

High

The higher the ratio the higher the force

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What is a myotome?

Group of muscles innervated by one spinal segement

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What is a dermatome?

Sensory innervation of one spinal segment

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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)

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What is a motor unit?

Anterior horn cell
Motor nerve axon
Peripheral nerve
NMJ
Muscle fibers

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What is the Henneman size principle?

Smaller alpha motor neuron has low threshold for excitation and is recruited first in voluntary contraction

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Describe Type I muscle fibers

Small cell body
Thin diameter axon
Low innervation ratio
Slow twitch muscle fibers

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Describe Type II muscle fibers

Large cell body
Thick diameter axon
High innervation ratio
Fast twitch muscle fibers

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What type of fibers does EDX evaluate?

Ia (large, myelinated) fibers

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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

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How do cooler temperatures impact EDX?

Prolongs latency
Inc: amp, duration
Dec: CV

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What is saltatory conduction?

Propogation of current between myelin in Na channels in nodes of Ranvier

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What is orthodromic recording?

AP recorded in direction of physiologic conduction

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What is antidromic recording?

AP recorded in opposite direction of physiologic conduction

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How is a miniature endplate potential (MEPP) created?

Spontaneous release of Ach ~ 5 sec during resting state at NMJ

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How is an endplate potential (EPP) produced?

nerve depolarization opens voltage-gated Ca channels and release Ach into synaptic cleft which inc # of MEPP

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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

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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

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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