[The appearance manner of C-response in healthy individuals]

The existence of long-latency responses following M and H waves in the complex muscular action potential elicited by stimulation of peripheral nerves was reported by Upton et al. This electrical potential, called the C-response, is applied to examinations in central nervous system diseases. However, the pathway and details on fundamental types of waveforms have not yet been clarified. The main purpose of this study is to classify the waveforms of the C-response, based on the analysis of waveforms. To investigate the type of C-responses, we developed a modified superimposing method. We also investigated the types in different age groups. Fifty-seven healthy individuals (30 males and 27 females) aged between 20 and 73 (average age: 43.1 years old) were enrolled as subjects. All subjects were instructed to oppose the thumb against the little finger of their dominant hands so that the abductor pollicis brevis was in voluntary isometric contraction; subsequently, electrical stimuli were repeatedly applied to the median nerve at the wrist. The stimuli had a strength of 110% of the threshold value of the M wave. The electrical potential was recorded with surface electrodes placed on the muscle belly of the abductor pollicis brevis. In each measurement, 200 waveforms were averaged. A Neuropack 8 (Nihon Kohden, Co., Ltd.) was used for recording and analysis of electromyograms. Measured negative peak latencies (ms) were divided by the subjects' heights (m) to obtaining the corrected latencies per unit height. The time axis of a waveform was also corrected with each height, and shifted so that the latency of the negative peak of the H wave was observed at the same position (modified superimposing method). Then, the position where the negative peak of the C-response appeared most frequently was examined. We clarified that C-responses have three types. C-responses have two major negative peaks basically, C1 and C2 (the latency of C1 is shorter than that of C2). Type 1 has only C2; Type 2 has C1 and C2; Type 3 has both C1 and C2 but the latency of C2 is shorter than that of Type 2. Type 1 was observed in 37 cases (64.9%), Type 2 in 15 cases (26.3%), and Type 3 in 5 cases (8.8%). The incidence of each C-response type depended on the age of the subjects, Type 1 was observed frequently in young subjects, and Types 2 and 3 were observed more frequently as the age of the subjects increased.