Abstract des Autors

The present study was undertaken to examine the effects of acoustic stimulation on human spinal motoneuron excitability. For this purpose, we used the soleus (Sol) H-reflex as a test reflex, and three different types of acoustic stimuli as conditioning stimuli. The features of the acoustic stimuli were as follows, 1) click sound (CS), 2) tone burst composed of 11 click sounds (TBL, interstimulus interval 10 ms), 3) tone burst composed of 21 click sounds (TBH, interstimulus interval 5 ms). The intensity and frequency of each sound was 110 dB and 0.5 kHz, respectively. Significant facilitation of the Sol H-reflex occurred at conditioning-testing (C-T) intervals of 50 ms in all subjects when the TBL stimulus was used (50.3+/-8.2 ms). This facilitatory effect appeared early and later when TBH (43.7+/-3.7 ms) and CS (59.2+/-4.5 ms) stimuli were used, respectively. The maximum facilitatory effect appeared at a C-T interval of 100 ms (98.0+/-0.8 ms) and the amount of peak facilitation at that time was 156.1+/-1.4% (relative to the control value). Thereafter, the amount of facilitation decreased sharply up to a C-T interval of 500 ms. No significant inhibition of the Sol H-reflex was observed between a C-T interval of 0 and 500 ms in all subjects. Irrespective of whether the magnetic cortical stimulation was given before or after the Sol H-reflex at a short interval (- 2 to 2ms), the acoustic facilitation of the Sol H-reflex was not changed with a C-T interval of 50 ms. Additional facilitation due to magnetic cortical stimulation was, however, obtained in one of the four subjects when a C-T interval of 100 ms was used. These results suggest that the acoustic facilitation of the Sol H-reflex is composed of three different facilitatory mechanisms; 1) a pathway with a high threshold and shorter latency (about 40 ms), 2) a pathway with a lower threshold and medium latency (about 60 ms), and 3) a pathway with a relatively high threshold and longer latency (longer than 200 ms). In addition, we discussed the mechanisms that underlie the facilitatory effects of the Sol H-reflex. Verf.-Referat