MNCV

Question 1

Myelinated fibers

Term to describe the flow of electrical current in a myelinated fiber

Answer 1

Saltatory condution: AP jumps between nodes of Ranvier

Question 2

Myelinated fibers

Conduction velocity is ____ to fiber diameter

Answer 2

Proportional

Question 3

Myelinated fibers

Which cells produce myelin in the PNS

Answer 3

Schwann cells

Question 4

Action potentials

2 types of refractory periods in an AP

Answer 4

• Absolute: NO AP can be generated. ~1 ms
• Relative: Stronger stimulus needed to generate AP

Slide 11

Question 5

Name the 3 large classification of nerve fibers

Answer 5

• A-fibers: Myelinated somatic
• B-fibers: Preganglionic autonomic
• C-fibers: Post-ganglionic autonomic and non-myelinated somatic

Slide 13

Question 6

Which types of fibers are
a) the fastest
b) the slowest

Answer 6

a) A-fibers (largest)
b) C-fibers (smallest)

Slide 13

Question 7

Nerve conduction studies

What are the 3 waveforms measured in nerve conduction studies

Answer 7

1. Compound sensory nerve AP (CSNAP)
2. Compount muscle AP (CMAP)
3. Mixed nerve AP

Question 8

Nerve conduction studies

Describe electrode placement.
Understand anodal block and how to avoid it.

Answer 8

*We use sets of stimulating electrodes (cathode + anode) placed proximally to stimulate a specific nerve

*Then place recording electrodes over the target muscle, and ground electrode between recording and stimulating electrodes

Stimulating electrodes:
- Cathode (-, black) is closest to the nerve
- Anode (+, red) is SQ or farthest from the recording electrodes (to avoid anodal block). See slide 6 of SNCV.
- 2 sets of stimulating electrodes (+-) must be separated by at least 10 cm

Ground:
- Between set of recording and stimulating electrodes

Recording electrodes
- Active (black): at muscle motor point
- Reference (red): remotely

Note on anodal block:
- cathode depolarizes vs anode hyperpolarizes
- Anodal block occurs if the anode is placed too close to the recording electrode, hyperpolarizing and blocking the electrical current. Same principle for SNCV.

anode stinks, so far from recording.

Question 9

Motor nerve action potentials

What is the recorded wave called ?

Answer 9

M wave (Compound Muscle Action Potential, CMAP)

Question 10

MNCV

How is MNCV determined (formula)

Answer 10

NCV (m/s) = Distance (mm)/(L1-L2 ms)

D=between the two stim electrodes.
L1=proximal latency, L2=distal late

Question 11

Define..

CMAP latency

Answer 11

Time from onset of the stimulus artifact –> onset of the first deflection (proximal vs distal)

slide 30

Question 12

Define..

Onset latency

Answer 12

Conduction time of the fastest conducting axons

Question 13

Define..

Residual latency

Answer 13

Collective delay throught the fine intramuscular branches and at the NMJ.

Better than ‘‘distal latency’’ as a measure of distal nerve disease

Unaffected by limb lenght

FORMULA SLIDE 33

Question 14

define

CMAP amplitude

Answer 14

Distance (mm) from peak to peak

slide 30

Question 15

Define..

CMAP duration

Answer 15

Time (ms) from initial deviation from baseline to final return to baseline

slide 30

Question 16

***

Adult MNCV values are reached at what age in
a) dogs
b) cats

Answer 16

a) 6m-1y
b) 3m

Question 17

At what age does MNCV start decreasing in
a) dogs
b) cats

Answer 17

a) 7 yo
b) 10 yo

Question 18

As animal ages, CMAP ____ in duration and ____ in amplitude

Answer 18

increase
increase
(become wider and higher)

Question 19

in dogs..

Effect of T° on MNCV?

Answer 19

Every 1°C↓ leads to 1.7-1.8 m/s MNCV↓

Question 20

Effect of limb lenght?

Answer 20

• slower MNCV
• Smaller CMAP amplitude
• Longer CMAP duration

Due to longer ‘‘tapering segment’’ in longer peripheral nerves

Question 21

Normal CMAP waveform

Answer 21

• Biphasic, with initial negative (upward) deflection
• 15-20 mV amplitude (peak-peak)
• CMAP morphology should be the same at the proximal and distal stimulations (only difference = latency)
• Physiologic temporal dispersion: proximal stimulus can have slightly longer duration and lower amplitude than distal stimulus

Question 22

3 types of CMAP alterations?

Answer 22

1) decreased amplitude
2) increased latency
3) absent response

Question 23

Name the 3 types of nerve injuries in increasing order of severity

Answer 23

• Neuropraxia: Conduction alteration without neuronal structural change. Recovery: days-weeks
• Axonotmetis: Physical interruption of axons –> Wallerian degeneration. Recovery: months-years, nerve fiber regeneration is 1-4 mm/day.
• Neurotmesis: complete severance of a nerve including connective tissue. Recovery: slow, incomplete or absent.

Question 24

Significance of..

↑CMAP latency

Answer 24

Demyelination

Note: Peripheral neuropathies will lead to increased latencies, but not always lower MNCV, as the surviving axons may conduct normally.

Question 25

↓CMAP amplitude

Answer 25

• axonopathy (neuropraxia or axonotmesis)
• ↓ NM transmission (e.g. botulism)
• Severe myopathy

Question 26

Temporal dispersion

Answer 26

When there is a difference between the conduction times between different axons of the same nerves

Creates a polymorphic waveform with ↑CMAP duration

Slide 59

Question 27

Phase cancellation

Answer 27

Cancellation between peaks of oposite polarity due to temporal dispersion. Creates a lower CMAP amplitude.

Slide 59

Question 28

Absent response (flat line)

Answer 28

fails to distinguish between neuropraxia, axonotmesis, neurotmesis

Question 29

Conduction block, definition and significance

Answer 29

• ↓ in proximal vs distal CMAP amplitude by > 50% without significant dispersion
• Occurs with demyelination, neuropraxia, axonotmesis

Slide 58