Electromyography (EMG) Nerve conduction velocity (NCV)

Question 1

…. are electrical tests to help diagnose problems that can occur in the peripheral nervous system, not the CNS.

Answer 1

EMGs and NCVs

Question 2

EMGs can be recorded irrespective of how a muscle is activated, and can be found as the result of :

Answer 2

1. In response to electrical stimulation of the nerve to the muscle
2. Initiating a reflex contraction of the muscle
3. Voluntary actions

Question 3

Name the waves

Answer 3

Question 4

Describe the effect of myelin sheath’s damage on EMG and NCV

Answer 4

Damage may injure the sheath that surrounds the nerve (the myelin sheath) creating a block to conduction but leaving the nerve itself intact, so the EMG amplitude will be normal, while the conduction velocity is reduced (longer latency).

Question 5

Describe the effect of axonal affection on EMG

Answer 5

amplitude of EMG will be decreased.

Question 6

An estimate of the conduction velocity can be obtained from the….. response.

Answer 6

ankle jerk reflex

Question 7

When the Achilles tendon is tapped, the…. muscle contracts after a brief delay (i.e. latency).

Answer 7

soleus

Question 8

Explain the detection of EMG to a reflex contraction of a muscle

Answer 8

• An estimate of the conduction velocity can be obtained from the ankle jerk reflex response.

• When the Achilles tendon is tapped, the soleus muscle contracts after a brief delay (i.e. latency).

• This contraction is detected by the EMG.

• This detects competent peripheral nerves and synapses in the spinal cord.

• If the velocity at which action potentials travel along a nerve is reduced, reflex responses occur at a longer latency.

Question 9

Compare the conduction velocity of : Ia sensory fibers and alpha motor fibers

Answer 9

• The sizes, and hence conduction velocities, of the muscle spindle afferent (Ia sensory nerve fibre) and the alpha motor nerve fibers are about the same.

Question 10

How can we measure the total distance travelled by the action potentials in the reflex arc ?

Answer 10

• If the distance from the muscle spindle to central synapse (in the spinal cord) and back can be found, the total distance travelled by the action potentials in the reflex arc will be known

• Conduction velocity= distance/latency