Late responses

1. Which type of nerve fibers are activated during the H-reflex?
2. What potentiates and abolishes the H-reflex?
3. What are the normal H-reflex values?
4. What are limitations of the H-reflex?
5. What is the typical height of an F-wave?
6. What are F waves commonly used for?
7. Why are multiple stimulations needed to obtain an F wave?
8. What is an A-wave? (Axon wave)

Answers:
1. IA afferent nerve fibers activated, creating an analogue to a monosynaptic reflex. It is usually used to test for S1 radiculopathy.
2. The waveform is potentiated by agonist muscle contraction and abolished by antagonist contraction or increased stimulation.
3. Normal is 28-30 ms or a side to side difference of less than 2 ms.
4. Focal lesions are diluted so it is difficult to determine injury location, it can be normal with incomplete lesions, it does not distinguish between acute and chronic, and once it is abnormal it stays abnormal.
5. 5% of the CMAP height.
6. F waves are used in polyneuropathies and plexopathies. A side to side difference of more than 2 ms in UE and 4 ms in LE is significant.
7. Configuration and latency change with each stimulation due to the polysynaptic response in the s.c., where Renshaw cell are activated by the stimulus and inhibit firing of alpha neurons, which keeps impulses from traveling the same path each time.
8. Occurs when stimulus travels antidromically along motor nerve and becomes diverted by a neural branch formed by collateral sprouting, occuring between the CMAP and F wave response. This represents nerve damage.