Late Responses

Late Responses

The H reflex
With recording electrodes over the soleus, and stimulation of the posterior tibial nerve at the popliteal fossa, a short latency CMAP is recorded as a direct response via orthodromic alpha motor axon stimulation.
In addition, an indirect, longer latency response may be recorded via Ia orthodromic stimulation through the reflex arc. This is referred to as the H-reflex.With a small intensity stimulus, the longer latency, indirect H reflex response may be evoked prior to the direct, shorter latency CMAP response, given the fact that the sensory axons are larger and more myelinated than the motor axons, and have a lower threshold of activation. In practice, it is difficult to evoke only an indirect H response, without the direct response, but the H reflex can usually be seen as a larger amplitude response than the direct response. After evoking the H reflex with a small intensity stimulus, increasing the stimulus intensity will inhibit the H reflex, but the CMAP direct response will continue to be seen. H reflex inhibition occurs through renshaw cell inhibition, collision between action potential mediated thru the Ia and antidromic alpha motor axons, or because the axon hillock may be in refractory period from the preceding antidromic alpha motor neuron mediated action potential.Clinically, the H reflex is readily obtained from the soleus, and serves as an evaluation of the S1 reflex arc. It is less reliably evoked from the flexor carpi radialis as a function of the C7 reflex arc.
In non-clinical setting, H reflex may be evoked from other muscles as well, with less reliability.

300-Hreflex
H-reflex (soleus recording) electrode set up

450-hreflx
H reflex recording with small M wave

The F response
With recording from any muscles, and electrical stimulation of the nerve supplying that muscle, a direct CMAP response is evoked and recorded on the oscilloscope. This is the result of orthodromic activation of the alpha motor axons. Activation of the motor axons antidromically, with an action potential rebounding orthodromically from the motor cell body produces an indirect response, longer in latency than the direct response. This is known as the F response.The F response is mediated strictly up and down the alpha motor system. The fact that the F response continues to be seen in de-afferented limbs suggests that the response is not mediated through a reflex arc. Supramaximal stimulus facilitates the F response, whereas it will inhibit the H response.The F wave amplitude is much smaller than the direct CMAP amplitude, depending on which motor cell bodies fire back. The latency will also vary. Several F responses will be recorded, and the one with the shortest latency used for data.

The F wave latency allows for indirect evaluation of a proximal nerve segment. The proximal nerve segments can be technically difficult to assess nerve conduction velocity through standard techniques.

F response
F response with supramaximal evoked M wave

F response
 Antidromic stimulation of motor neurons (alpha motor neuron)
Conduction to and from spinal cord
No reflex involved; found in de-afferented limbs
Found in all skeletal muscles
Enhanced with supra-maximal stimulation
Low amplitude response ( less than 10% M response)
Variable latency, depending on which percent of motor neurons fire back
Indirect assessment of proximal conduction
H reflex
Electrical equivalent of the phasic muscle stretch reflex
Limited to ant-gravity muscles
Most commonly used to assess S1 nerve root integrity
H reflex amplitude, latency consistent with successive stimuli
Inhibited by supra-maximal stimulation
Collision from anti-dromic impulse of alpha motor neuron
Refractory period of axon hillock due to passage of preceding antidromic response
Renshaw inhibition

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Ray Jurewicz
412-731-0173
E-mail: rj@NerveStudy.com Web Site design by Larry Berman and Chris Maher

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