Suppression of an ongoing muscle contraction following noxious digital stimulation is called cutaneous silent period (CSP) which is under the influence of several physiological factors. In this study, we aimed to evaluate the influence of group Ia afferents on the cutaneous silent period (CSP) by applying 2-min vibration. CSP was obtained from abductor pollicis brevis muscle after stimulating index finger. The recordings were repeated three times-before, during and after vibration-which was applied over the tendon of flexor carpi radialis muscle. Onset latency, duration and magnitude of total CSP, inhibitory phases I1 and I2, and of the long-loop reflex were measured and compared. Suppression indices of CSP, I1 and I2 increased significantly during and after vibration, indicating significantly less exteroceptive EMG suppression outlasting the time of vibration. Vibration also caused mild shortening of I2 end latency (p = 0.048) and I2 duration (p = 0.019). Our findings indicate that vibration exerts a powerful influence on CSPs and causes reduction in the magnitude of exteroceptive EMG suppression during and after vibration. Although vibration is known to activate Ia afferents, we cannot exclude contribution of other afferents, e.g. mechanoreceptors, as well as pre- or postsynaptic inhibitory effects on ensuing interneurons, or enhanced vibration-related excitatory influence.
Keywords: A delta fiber; Alpha motoneuron; Cutaneous silent period; Exteroceptive suppression; Group Ia afferent fibers; Vibration.