Waves h-delay m
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It is generally accepted that the 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. Although elicitable in a variety of muscles, it is best obtained in the small foot and hand muscles. The F-wave is a long latency muscle action potential seen after supramaximal stimulation to a nerve. Much controversy remains, however, on whether its absence bilaterally in otherwise asymptomatic individuals is of any clinical significance. Its absence or abnormal latency on one side strongly indicates disease if a local process is suspected. The H-reflex is useful in the diagnosis of S1 and C7 root lesions as well as the study of proximal nerve segments in either peripheral or proximal neuropathies. If no facilitation maneuvers are performed, the difference in latency between both sides should not exceed l ms. Whatever these values however, the best normal value in localized processes is the patient’s asymptomatic limb. H-reflex latency can be determined easily from charts, according to height and sex or from published normal values. When the motor response becomes maximal, the H-reflex disappears and is replaced by a small late motor response, the F-wave. With further increases in stimulation strengths, the M response becomes larger and the H-reflex decreases in amplitude. As the stimulation strength is increased, the direct motor response appears. Typically, it is first seen at low stimulation strength without any motor response preceding it. The H-reflex can normally be seen in many muscles but is easily obtained in the soleus muscle (with posterior tibial nerve stimulation at the popliteal fossa), the flexor carpi radialis muscle (with median nerve stimulation at the elbow), and the quadriceps (with femoral nerve stimulation). Understandably, the latency of this reflex is much longer than that of the M response, and a sweep of 5-10 ms/division is necessary to see it. The result is a motor response, usually between 0.5 and 5 mv in amplitude, occurring at low stimulation strength, either before any direct motor response (M) is seen or with a small M preceding it.
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The stimulus travels along the Ia fibers, through the dorsal root ganglion, and is transmitted across the central synapse to the anterior horn cell which fires it down along the alpha motor axon to the muscle. 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.
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It is elicited by selectively stimulating the Ia fibers of the posterior tibial or median nerve. The H-reflex is the electrical equivalent of the monosynaptic stretch reflex and is normally obtained in only a few muscles.