EMG Basics

Question 1

Where is the cell body for efferent?

Answer 1

alpha motor horn in spinal cord

Question 2

What is a motor unit?

Answer 2

alpha motor neuron, axon, NMJ, all muscle fibers innervated

Question 3

What type of injury is a radiculopathy?

Answer 3

pre-ganglionic

Question 4

Where is the cell body for afferent nerves?

Answer 4

dorsal root ganglion

Question 5

How does an EMG cathode propagate an action potential?

Answer 5

cathode terminal generates a negative impulse that attracts positive charges from the axon.

Question 6

What is orthodromic?

Answer 6

Stimulating in direction of normal flow of action potential (motor testing)

Question 7

What is antidromic?

Answer 7

Stimulating in opposite direction of normal flow of action potential (sensory testing)

Question 8

What is an F wave?

Answer 8

long latency muscle action potential seen after supramaximal stimulation to a nerve

Question 9

What is a CMAP?

Answer 9

summated action potentials of all the fibers of a muscle

Question 10

What nerve are you testing when electrodes are placed over the APB?

Answer 10

median nerve

Question 11

How do you measure onset latency of CMAP?

Answer 11

Measure from start of stimulus to start of deflection off baseline

Question 12

How do you measure amplitude of CMAP?

Answer 12

Measure from initial deflection off baseline to peak

Question 13

How do you measure duration of a CMAP?

Answer 13

Measure from initial deflection from baseline to return to baseline

Question 14

What is a SNAP?

Answer 14

Sum of sensory nerve fiber AP elicited by stimulation of nerve or dorsal root

Question 15

What is the standard for testing nerve conduction?

Answer 15

supramaximal stimulation

Question 16

How is SNAP and CMAP generated?

Answer 16

Subtracting reference electrode from active electrode recording

Question 17

What is the gain?

Answer 17

Amplification of the signal on the screen

Question 18

What is the sweep speed?

Answer 18

Recording duration of the signal on the screen represented on the X-axis

Question 19

Why would you change the sweep speed?

Answer 19

When looking for temporal dispersion in a demyelinating process

Question 20

How is an F-wave elicited?

Answer 20

F-wave is elicited when the stimulus travels antidromically along the motor fibers and reaches the anterior horn cell at a critical time to depolarize it. The response is then fired down along the axon and causes a minimal contraction of the muscle

Question 21

What is an H-reflex?

Answer 21

H-reflex is the electrical equivalent of the monosynaptic stretch reflex and is normally obtained in only a few muscles.

Question 22

How is an H-reflex elicited?

Answer 22

selectively stimulating the Ia fibers of the posterior tibial or median nerve. Such stimulation can be accomplished by using slow (less than 1 pulse/second), long-duration (0.5-1 ms) stimuli with gradually increasing stimulation strength.