15: Basic EMG and Analysis of MUAP's

Question 1

What are larger motor neurons associated with regards to axons, myelin sheath, CV, threshold to depolarization, and type of muscle fiber innervated?

Answer 1

Larger axons, thicker myelin, faster, higher threshold to depol, type II fast twitch muscle fibers

Question 2

What factors affect duration and how?

Answer 2

1. Increases with more fibers and increased motor unit territory
2. Increases with age
3. Decreases with increased temp
4. Muscle being studied (proximal and facial muscles with shorter duration)

Question 3

What does MUAP amplitude reflect?

Answer 3

The few muscle fibers closest to the needle

Question 4

What factors are associated with increased amplitude on EMG?

Answer 4

1. Proximity of needle to the motor unit
2. Increased number of muscle fibers in the motor unit
3. Increased diameter of muscle fibers
4. More synchronized firing of the muscle fibers

Question 5

What defines an unstable MUAP, and when can it be seen?

Answer 5

Change in amplitude or number of phases/serrations, or both;
Myasthenia, LEMS, and also other neuropathic or myopathic disorders where there is denervation

Question 6

Firing faster than what frequency will not result in increased force production?

Answer 6

30-50 Hz (tetanic fusion frequency)

Question 7

When can poor activation be seen?

Answer 7

CNS disease; pain manifestation; poor cooperation; fxnal disorders

Question 8

What is the notable finding for EMG with acute axonal loss lesions?

Answer 8

Reduced recruitment

Question 9

What could be seen on EMG with true conduction block?

Answer 9

Reduced recruitment with normal MUAP morphology

Question 10

Pattern for acute myopathy?

Answer 10

Early recruitment, SDSA MUAP’s with polyphasia due to less synchronous firing

Question 11

What can be seen with endstage muscle from myopathy?

Answer 11

Reduced recruitment due to loss of motor units

Question 12

What is necessary for reinnervation via axonal regrowth?

Answer 12

Intact AHC

Question 13

What set of disorders can appear like myopathic disorders with regards to MUAP morphology?

Answer 13

NMJ disorders with greater and more persistent block

Question 14

What can happen with dystonia?

Answer 14

MUAP firing increases in the antagonist muscle