Nerve Conduction Velocity

Motor nerves

• electically stimulated, generating action potentials in alpha motor neurons

• propagate along mtoro fibres activate NMJ and trigger muscle action potentials (CMAP/M-wave) leading to muscle contraction

recording and stimulation set up

• electrodes over the muscle records biphasic CMAP (M-wave)

• CMAP amplitude reflects the number for depolarized MU

• Cathods (negative electrode) is placed over the nerve and anode (postive) is placed proximally

• nerve segment is distanced between two stimulation sites

Latency and nerve conduction velocity

• latency is time from stimulus onset ot beginning of CMAP

• latency includes

  • nerve conduction time

  • neuromuscular transmission time

  • muscle action potential propagation

• nerve conduction velocity (NCV) = (nerve segment length)/(difference in latencies from proximal nad distal sites)

Factors affecting NCV

• biological: age, height, gender

• environmental/technical: temperature, nerve pathology, segment length, measurement errors

• regression models predict NCV using age, height and/or gender

clinical relevance

• NCV reflects the functional state of myelinated motor nerves, neuromuscular transmission and muscle fibres

• useful for diagnosing nerve damage or disorders (eg. neuropathies, demyleination)

tips to minimizing measurement error

1. use alcohol swab to clean the skin at surface EMG nad nerve stimulation sites

   1. allow to dry before applying electrodes

2. begin with low voltage, fid the best position for stimulation of nerve

   • gradually increase the voltage ot supra-maximal level

3. clip the SS2L cable to relieve cable strain and avoid pulling on the electrodes

4. keep the stimulus and recording leads and cable speared to minimize electrical interference

5. if the arm is cold should be warmed to ambient temp prior to recording

preparation and safety precautions

• ensure safety: avoid circuit cross the heart

  • remain seated and report any dizziness

• skin preparation: use alcohol swab to clean EMG and stimulation sites then allow drying

• temperature check: if the arm is cold warm to ambient temperature to minimize variability

Electrode placement

• recording electrodes (stickers) on the hand

  • ground: ulnar head or dorsal surface of hand

  • active (white): on the belly of th abduction pollicis brevis muscle

  • reference (red): distal to active electrode

    • in line with muscle fibres

• stimulation electrodes

  • probe used

  • S1: black cathode placed on midline of anterior wrist joint, yellow anode is proximal

  • S2: black cathode placed proximal to medial epicondyle and cubital fossa and yellow anode placed proximal

Running the experiment

1. participant sits comfortable with arm resting on desk at 15-35º of elbow flexion

2. apply get ot anode and cathode before stimulation

3. stimulator setup

   1. use HSTM01 probe: white - anode, black - cathode

   2. hold the probe at S1 (wrist) with cathode distal

   3. press and hold red button to deliver stimulation

4. BioPac system setup

   • open Biopac BSL navigate to PRO experiments → NCV experiment

   • set BSLSTMB stimulator unit to 0V and turn it off

   • run the calibration when prompted

5. start the recording

   • press “start” in BioPac student lab software

   • turn on the stimulator and set it to 5V

   • turn down the red button to deliver the stimulus

6. finding the best stimulation postion

   • gradually increase stimulus in 5V increments until a response is observed

   • expected response: thumb twitching

     • thenar muscles contracting

   • typical threshold response: between 25-40V

   • if not response, reposition the probe

7. confirm and record the M-wave

   • if an M-wave appears clipped reduce gain setting and re-record

   • mark and label data as “S1 Median nerve”

   • record three M-wave responses at S1 for accuracy

repeat for S2

1. mark and label S1 cathode postion

2. reposition the stimulating probe to S2 (elbow position)

3. decrease the stimulus level to 0V before starting

4. repeat steps 3-8 for S2

5. record threee M-waves at S2 for accuracy

calculation for nerve conduction velocity

• NCV (m/s) = nerve segment (m) / conduction time (s)

• NCV = (distance between S1 and S2)/(delta T for S2 - Delta T for S1)

• regression equations for upper/lower extremity NCV

  • upper extremity: NCV = 66.22 + age(-0.09) + height (-0.03)

  • lower extremity: NCV = 90.15 + age(-0.11) + height(-0.22)

Create a graph that shows the stimulation pulse and resultant M-waves at each stimulation intensity (make sure to plot the stimulation and EMG channels on separate Y axes)

• Create time as =A2+(1/50) and insert down due to sampling rate and keeping it in ms

• Create smooth scatter plot of EMG (mV) and Stim pulse (Hz) on either Y and time in ms on the X axis

Using the highest stimulation level for each site, calculate the M-wave latency from the onset of stimulus to the onset of M-wave. (Signals have already been time-aligned for you).

• Find time of M-wave onset and subtract stim onset from it 

  • Ex. M-wave-stim = latency

calculate maximum amplitude of the M-wave

• find max of each stimulation column =max(B:B)

Using the tables below as a guideline (you may format them differently), calculate all relevant information required to determine the experimental nerve conduction velocity

	Nerve Segment	Converted Nerve Segment
	cm	m
Proximal nerve segment (S2 - active electrode)	Given in the Q	=S2/100
Distal nerve segment (S1-active electrode)	Given in the Q	=S1/100
S2-S1 Nerve Segment	=S2-S1	=(S2/100)-(S1/100)

	M-wave latencies	Converted M-wave latencies
	msec	s
Upper extremity (median nerve)
Elbow site (proximal)	Calculation completed above for both sites M-wave-stim = latency	latency/1000 to convert to s
Wrist site (distal)	Calculation completed above for both sites M-wave-stim = latency	latency/1000 to convert to s

	Conduction Time
	s
Upper extremity (median nerve)	Proximal latency - distal latency

calculate the predicted nerve conduction velocity using the appropriate regression equation

Participant Comparison	Nerve Conduction Velocity	Regression equation
Upper extremity (median nerve)	Experimental (m/s)
Participant A	=(S2-S1 nerve segment in m)/(conduction time)	=66.22+age*(-0.09)+height(cm)*(-0.03)
Participant B	Same as above with their values

what is a CMAP

• CMAP is an acronym for compound motor unit action potential, aka an M-wave.

What is an M wave

M-wave is the label that represents the activity generated by individual muscle fibers innervated by the stimulated motor axons. M-wave response is measured as the EMG activity (M-wave) or force associated with the response; shape and size depends on number and size of activated muscle fibres, and the temporal dispersion of their action potentials

two confounding variables to the M-wave

•  Cross-talk EMG - pick up signals or other disruptors surrounding EMG

  • alters shape of M-wave

• Stimulus artifact

  • especially seen at the wrist and ankle sites

Why is it important to accurately measure the nerve segment and conduction time? (consider the equation.)  What would be the effect of inaccurate measures?  For example, what might be the compound effect of inaccurate measures for both nerve segment and conduction time?  Describe how measurement errors would affect data.

1. if mis-measured then the velocity calculation will be off (v=d/t)

2. Single subject- if have small errors in opposite directions (e.g. distance under and time over or visa versa) get large inaccuracy (2 small errors become multiplied to be a big error)

3. Group data if one person under and one over then believe that there is larger variability between people even if average values correct. If errors consistently in same direction then average value is off.

4. scaling effects - small errors in measurement lead to increased variability when comparing between people (additive effect of error)

were the experimental results comparable to linear regression equation? Why or why not?

Participant A: experimental nerve conduction was faster (78.59 m/s) than linear regression equation (57.55 m/s).

Participant B: experimental nerve conduction was slower (48.28 m/s) than linear regression equation (59.17 m/s).

• Participant A has better nerve health than predicted for their age indicating they may have higher body temperature, increased myelination and is well hydrated.

• Participant B has slower nerve conduction than predicted for their age indicating they may have a lower body temperature, fatigue or some characteristics of individual nerves leading to reduced conduction velocity. 

describe 2 such factors that have not previously been identified within your lab report. How and why do these factors affect nerve conduction velocity? 

• Age - greater age causes decreased NCV (demyelination, degeneration of axons, decreased blood supply)

• Height - greater height causes decreased NCV (longer nerve length, signal attenuation, etc)

• Myelination increases speed of transmission and efficiency due to jumping between nodes of Ranvier

  • Voltage gets a speed boost through the myelinated sections of the nerve

• An increase in temperature may lead to increase nerve conduction velocity while the opposite occurs at cooler temperatures.

  • affects channel gating increasing the acceleration of sodium channel activation which in turn increases conduction velocity

  • opposite occurs in cooler temperatures slowing the acceleration of sodium channel activation reducing the conduction velocity
    

Why is it important to stimulate at a supramaximal level (above the minimum voltage for maximal M-wave amplitude)?

• Consistent m-wave amplitude


• Low-stimulus intensity = delayed response (increased latency)

Were each of the Participants within the expected NCV range for their age group?

• Participant A should have a NCV in the range of 48-65 m/s

  • linear regression equation is within this range at 57.55 m/s

  • the experimental value is above this range at 75.59 m/s.

• Participant B should have NCV in range of 50-66 m/s

  • the linear regression equation is within this range with a value of 59.17

  • the experimental value is slightly below this range at 48.28 m/s