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Why can normal proximal sensory studies have decreased amplitude?

Phase cancelation

Why do mixed nerve studies have shorter latencies than sensory or motor studies?

Because the fastest Ia muscle spindle fibers are only recorded in mixed nerve studies.

What is the slowest speed that a normally myelinated nerve can have?

35 m/s

What is the longest latency when compared to the upper limit of normal that a normally myelinated nerve can have? Hint %.


How do axonal lesion affect the waveform?

They decrease the amplitude and can mildly slow the conduction velocity, but never below 35 m/s.

What types of lesion decrease waveform amplitude?

Axonal and demyelinating lesions with conduction block. Amplitude can also decrease due to increased phase cancellation with demyelination in sensory nerves.

What is the name for the pattern seen in NCS performed within 3 days of a nerve transection (i.e. hyperacute)?

Pseudo-conduction block, because the waveform is normal distally and has decreased amplitude when stimulated proximally.

When does Wallerian degeneration begin after a nerve injury in motor fibers and sensory fibers?

Earlier in motor fibers at 3-5 days and in sensory fibers 6-10 days.

What are the conduction velocity cutoff values for the arms and legs that signify almost undoubtly that a lesion is demyelinating?

35m/s in the arms and 30m/s in the legs

What is the % drop in CMAP that signifies conduction block?

20% decrease with proximal stimulation in either area or amplitude. But many use 50% as the cutoff because this is the upper limit to where temporal dispersion alone can drop the CMAP amplitude.

How do you use conduction block to differentiate between inherited and acquired neuroapathies?

Inherited neuropathies have uniform demyelination i.e. CMT and have slowing, but not conduction block where acquired such as GBS or CIDP have focal patchy demyelination causing conduction block.

What is normal F-wave persistence?


I what scenario may absent F-waves be normal?

In a sleeping or sedated patient.

Why do you turn the stimulator with anode away from the recording electrode for F-waves?

There is a theoretical possibility of anodal block, where the nerve is thought to hyperpolarize anode blocking antidromic transmission of the shock.

What cells create the F-wave?

Differing small populations of anterior horn cells (which is why F-waves vary from stimulation to stimulation).

What are 2 troubleshooting tests when F-waves are absent?

Assure supramaximal stimulation and the Jendrassk maneuver of clench teeth or making a fist with the contralateral hand to "prime" the anterior horn cells.

When tibial F-waves are absent, have decreased persistence, increased chronodispersion, or have prolonged latencies, what may this indicate?

A S1 radiculopathy or polyneuropathy, due to any cause even an acute neuropathy i.e. early AIDP.

Why is height of the patient an important consideration when interpreting F-wave latencies?

The latency is determined by factoring in the length of the nerve, so if a person is abnormally tall a prolonged F-wave latency may be normal.

How do you estimate the length from the stimulation site to the spinal cord when determining the F-estimate?

For tibial/peroneal studies you measure from the xiphoid to the ankle stimulation site and for the median/ulnar studies you measure from the C7 spinous process to the stimulation site.

What is the F-estimate equation?

F (latency) estimate= 2(D/CV)*10+1ms+DL, where D is the distance to hte spinal cord, CV is conduction velocity in m/s, 10 is the time conversion factor to ms, 1ms is "turnaround time" in the spinal cord, and DL is the distal motor latency in ms.

What can be diagnosed by abnormal F-waves?

A proximal neuropathy (AIDP), plexopathy, or radiculopathy.

What spinal levels are tested with the different F-waves?`

Median/ulnar: C8 and T1 (so if you have a C8 radiculopathy F-waves will be normal due to the T1 component).
Peroneal: L5
Tibial: S1

How are H-reflexes different from F-waves in their most basic sense?

H-reflexes involve a synapse and are therefore true reflexes whereas F-waves do not and H-reflexes are sensory responses where F-waves are motor responses.

Describe what creates an H-reflex?

Long duration, submaximal stimulation selectively activates the Ia muscles spindle nerve fibers of the afferent S1 reflex arc

What happens to the H-reflex as stimulation intensity increases?

The H-reflex's amplitude decreases as the M-wave increases as more muscle fibers become stimulated covering or blocking the descending H-reflex.

What is an axon reflex?

It is a late potential that is sometimes present between the M wave and F waves when recording F waves that look the same when ever they occur in time and shape.

What is the cause of the axon reflex?

It is caused by reinnervation and the firing of these reinnervated sprouts.

Why is it important to recognize A reflex waves?

Because they typically only occur at submaximal stimulation (so if you're recording F-waves you should be supramaximal), they are an indicator of reinnervation, and for unknown reasons they are often seen during the 1st several days in GBS.

What is tested with the blink reflex?

CN V, CN VII, and their connections in the pons and medulla

What clinical exam finding is the correlate to the blink reflex?

The corneal blink reflex

What is the afferent and efferent limb of the blink reflex?

Afferent= V1 and CN V nucleus
Efferent= Nucleus and tract of VII

What do R1 and R2 represent in the blink reflex?

R1=Disynaptic reflex between the CN V nucleus ipsilateral to stimulation and the ipsilateral CN VII nucleus. R1 is always present.

R2=Multisynaptic pathway btw CN V and ipsilateral/contralateral CN VII nucleus. These are variable and extinguish with repeated stimulation.

What is measure with blink reflex?

First whether or not there is a signal, the latencies of ipsilateral R1 and bilateral R2's, and the latencies differences btw R1's and the difference btw R2's.

What is the use of the blink reflex?

It can help localize lesions to the trigeminal nerve, facial nerve, pons, medulla, or a general demyelinating lesion based on R1 and R2 latencies and patterns.

What is the physiology of 2-3Hz slow repetitive stimulation?

ACH is incrementally depleted from the primary store, so fewer quanta (ACH vesicles) are released into the cleft with each stimulation, however there are still sufficient amounts of ACH to bind with ACHR to stay above the end plate potentials "safety factor" and cause the all of nothing depolarization of all muscle fibers controlled at that NMJ.

What is the physiology of 10-50Hz rapid repetitive stimulation?

In this the ACH is depleted and the primary quanta are repleted with the secondary stores like with slow rep. stim, but also Ca in the presynaptic cleft builds up because the rapidity of the stim. cause Ca influx faster than it can be pumped out, thus increase the ACH that is ultimately released increasing the end plate potential way above the safety threshold at first, so the subsequent drop of ACH due to using up the quanta does not affect the muscle depolarization.

How does decrement with slow rep stim and increment in rapid rep stim when done after slow rep stim. occur in NMJ diseases?

In NMJ diseases there is less safety factor and not much reserve, so with slow rep. stim. as less ACH is released you get the decrement and then with rapid rep. stim. you greatly increase the amount of ACH that is released increasing the safety factor causing more muscle depolarization and the increment.

4 key things to describe in EMG report

- location
- duration
- severity
- prognosis

What is the basic functional element of the neuromuscular system?

the motor unit

What are the 7 components of the motor unit (from proximal to distal)?

- anterior horn cell
- nerve root
- spinal nerve
- plexus
- peripheral nerve
- neuromuscular junction
- muscle fiber

Where is the alpha-motor neuron located?

cell body of the motor nerve; located in the anterior horn of the spinal cord

What regulates the characteristics of the motor unit?

The alpha motor neuron

What is the innervation ratio?

The amount of muscle fibers belonging to an axon

Do muscles with stronger/grosser movements have a higher or lower innervation ration?

Higher (more muscle fibers per axon)

What is the relationship between innervation ratio and force generated by a muscle?

Higher innervation ratio = greater force

What is a typical innervation ratio for a muscle in the leg?

600 muscle fibers : 1 neuron

What is a typical innervation ratio for a muscle around the eye?

1 muscle fiber : 1 neuron

Define the neuromuscular junction

The location in the motor unit where electrical AP is converted to chemical energy to initiate a a muscle action potential.

What type/category of motor neurons can can EMG study?

Alpha motor neurons (Ia fibers)

What are the 2 general ways that alpha motor neurons are further described?

- size
- physiology

What order are alpha motor neurons recruited?

In order of the size of the motor unit (smaller muscle fibers first)

The sequential activation of motor units allowing for smooth increase in contractile force is described by what principle?

Henneman Size Principle

Alpha motor neurons innervate ______

extrafusal fibers (skeletal muscle)

Gamma motor neurons innervate ______

intrafusal fibers (muscle spindle)

Beta motor neurons innervate _____

intrafusal and extrafusal fibers (skeletal muscle and the muscle spindle)

What are extrafusal fibers?

skeletal muscle

What are intrafusal fibers?

muscle spindle fibers

4 basic characteristics of type I muscle fibers

- smaller cell body
- thinner diameter axon
- lower innervation ratio
- slower twitch muscle

4 basic characteristics of type II muscle fibers

- larger cell body
- thicker diameter axon
- higher innervation ratio
- faster twitch muscle

What is the Henneman size principle?

A smaller alpha motor neuron has a lower threshold of excitation causing it to be recruited first. Larger alpha motor neurons have larger thresholds causing them to be recruited when more force is needed.

What order do the "neurium" layers go in from outside in?

- epineurium
- perineurium
- endoneurium

Define endoneurium

Connective tissue surrounding each individual axon and its myelin sheath

Define perineurium

Connective tissue surrounding bundles or fascicles of myelinated and unmyelinated nerve fibers

What is the purpose of the perineurium?

- strengthens the nerve
- acts as a diffusion barrier

May individual neurons cross from one bundle to another throughout the course of the nerve?


Define epineurium

loose connective tissue surrounding the entire nerve that holds the fascicles together and protects it from compression

Define resting membrane potential

the voltage of the axon's cell membrance at rest

What are "leak channels"?

Channels that allow K and Na to move passively in and out of the cell membrane

What is the normal resting membrance potential of an axon?

-70 to -90mV

How many K and Na are involved in the K/Na pump?

3Na out for every K in

The resting membrane potential is maintained by the

Na/K pump

Most important event in generating an action potential is...

sodium conductance

How does the stimulator in NCS cause the nerve to depolarize?

Positive ions accumulate under the negative pole of the stimulator (cathode) and lower the membrane potential. The membrane becomes increasingly permeable to Na ions which eventually rush in through the voltage gated channel and depolarize the membrane (sodium conductance)

What are the 3 conformations of the voltage gated sodium channel?

- resting
- activated
- inactivated

About how long to sodium channels stay open during an action potential?

about 25 microseconds

What are the general conceptual effects of cold on the sodium channel?

channel open and closes later

Is there a difference in the waveform effects in NCS for focal vs. generalized cooling?

Yes, generalized cooling has more significant effects in all domains

Classically, cooling causes an increase in the amplitude of NCS - but sometimes you see a decrease...why?

- temporal dispersion
- negative phase cancellation

What are the general effects of cooling that can be expected with NCS waveform morphology?

- latency prolonged
- amplitude increased
- duration increased
- conduction velocity decreased

Why does the movement of Na into a channel end up causing a propagating action potential?

Because the path of least resistance is along the length of the axon (both directions)

During an action potential when sodium is rushing in - what prevents it from going right back out


The process of propagating a current from one node to another is called

saltatory conduction

Define orthodromic

action potential is monitored traveling in the direction of its typical physiology conduction (usually described as away or toward the spinal cord)

Define antidromic

action potential is monitored traveling in the opposite direction of its typical physiology conduction (usually described as away or toward the spinal cord)

The repolarization phase of an action potential is dependent on

Na channel inactivation and K channel activation

What are the 2 conformational phases of the voltage gated K channels?

- resting
- slow activation

What is the "overshoot phenomenon"?

The hyperpolarization that occurs because of the slow activation of K channels

What is the motor endplate?

The distal portions of the motor axon and the muscle fibers that they innervate

Define the presynaptic region of the neuromuscular junction

the bulbous area at the axon's terminal zone

How many storage compartments are there for acetylcholine in the presynaptic region of the axon and how many quanta does each compartment store?

- main store = 300,000

How long does the migration of acetylcholine from the axon's main and mobilization compartments to immediate release take?

4-5 seconds

Define the synaptic cleft of the region of the neuromuscular junction in a motor neuron

the regions where acetylcholine crosses from the presynaptic region towards receptors on the postsynaptic region

How wide is the synaptic cleft of the neuromuscular junction?

about 200-500 angstroms

Acetylcholinesterase degrade acetylcholine into...

Acetate and choline

The convolutions in the postsynaptic region increase the surface area by how many times


What are "presynaptic active zones" in the motor neuron?

Areas on the presynaptic membrane where acetylcholine is released

The postsynaptic Ach receptor requires __ molecules of Ach to be activated


During the periods of inactivation in a motor unit, a spontaneous release of Ach quanta occurs every __ seconds


Spontaneous release of Ach quanta in motor neurons results in ____

MEPP (miniature endplate potential)

The calcium associated with depolarization and release of Ach in motor neurons stays in the terminal axon for how long?


Normally, the end-plate potential amplitude is ___ times amount needed to initiate an action potential


The "safety factor" in an end-plate potential depends on what 2 factors

- quantal count (numbner of quata released)
- quantal response (ability of receptors to respond)

What are the defining edges of the sarcomere?

Z line to Z line

During normal muscle contraction the I band and the H zone ____ in size


During maximal muscle contraction the H zone...


Muscle contraction is initiated by

muscle fiber depolarization

How quickly does muscle fiber depolarization spread?

3-5 meters per second

How can the muscle fiber depolarization penetrate deeper into the muscle?

T-tubule system (calcium is released from the sarcoplasmic reticulum)

What is Ohm's law?

E = IR

Electromotor source (volts)
Current (I) amperes
Resistance (Ohms)


compound muscle action potential


sensory nerve action potential

What are the limitations of using a needle recording electrode in NCS?

Because you are only recording a few fibers you can't validly analyze the amplitude or the conduction velocity of the waveform you record with a needle.

Describe a monopolar electrode. Where is the reference?

22-30 gauge Teflon coated needle with exposed tip of 0.15-0.2mm
- requires external reference

What are the advantages of the monopolar electrode?

- inexpensive
- conical tip allows for omnidirectional recording
- less painful
- larger recording area
- records more positive sharp waves

What is the relative recording area size of monopolar vs. concentric needles?

monopolars record twice as much field

What are the disadvantages of monoplar electrodes?

- requires a separate reference
- unstandardized tip area
- Teflon can fray
- more interference

Describe a concentric electrode. Where is the reference?

The 24-26 gauze needle serves as the reference, the active is a bare inner wire

What are the advantages of the concentric electrode?

- standardized active area
- fixed location from the reference
- less interference
- no separate reference
- can be used for quantitative EMG

What are the disadvantages of the concentric electrode?

- beveled tip = unidirectional recording
- smaller recording are
- MUAPs have smaller amplitudes
- more painful


motor unit action potentials (what you see on EMG)

Describe a bipolar concentric electrode

Has active and reference electrode wires within the needle lumen

Define ground electrode

A zero-voltage, neutral, surface reference point placed between the recording electrode and the stimulating electrode

Define anodal block

A theoretical local block that occurs when reversing the stimulator's cathode and anode; this hyperpolarizes the nerve, thus inhibiting the production of the action potential.

Define threshold stimulus

electrical stimulus occuring at an intensity level just sufficient enough to produce a detectable evoked potential from the nerve

Define maximal stimulus

Electrical stimulus at an intensity level where no further increase in evoked potential occurs as a higher stimulus

Define supramaximal stimulus

20% above maximal stimulus

What effect does supramaximal stimulus have on latency?


What technical NCS error can occur with stimulus duration greater than 0.3ms?

falsely prolonged distal latency since nerve is stimulated for a longer period of time

Recommended stimulus duration in NCS


6 sources of environmental electrical noise interference in EMG/NCS

- EMG audio feedback
- needle artifact
- 60Hz interference
- preamplifier
- fluorescent lights
- the patient

Signal to noise ratio =

(signal amplitude) x (square root # averages performed) / noise amplitude

Stimulus artifact represents

the current spread across the skin to the electrode

List 3 ways you can reduce the stimulus artifact

- ground between the stimulator and recording electrode
- appropriate anode and cathode placement
- improving electrode contact by cleaning the skin

What does the differential amplifier do?

- responds to alternating currents
- cancels waveforms recorded at active and reference pickups
- amplifies remaining potentials

Optimal parameters for a differential amplifier

- high impedance
- common mode rejection
- low noise from within the system

What's an acceptable common mode rejection ratio (CMRR)?

Greater than 90dB

What does the common mode rejection ratio tell you?

The larger the CMRR the more efficient the amplifier

NCS: Differential signal =

active - reference

Filters are made of

resistor and capacitors

What is the frequency band width in electrodiagnostics?

The frequencies between the low and high frequency filters that the machine is allowed to see

What are the typical filter settings for sensory NCS

20Hz - 2kHz

What are the typical filter settings for motor NCS


What are the typical filter settings for EMG?


What effects on waveform morphology occur with elevating the low frequency filter

- shortens peak latency
- reduces the amplitude
- potentials go from bi- to triphasic
- does not change the onset latency

What effects on waveform morphology occur with reducing the high frequency filter

- prolongs the peak latency
- reduces amplitudes
- creates a longer negative spike
- prolongs the onset latency

What are the x and y axes on the screen display for NCS

x = sweep speed
y = sensitivity

NCS sweep speed is measured in


NCS sensitivity is measured in

mV or uV

In NCS what are the units of gain?

no units; it's a measurement of output to input

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