Surround inhibition phenomenon tested by the method of cutaneous silent period

Purpose: Surround inhibition (SI), where excitability is suppressed in the area around the activated neural network for the motor system, is a physiological mechanism by which motor commands are focused to select of specific movements. Previously, the SI mechanism has been tested with various paradigms in which sensory and motor systems are examined. The cutaneous silent period (CSP), an A-delta (A-δ) fiber-mediated spinal inhibitory reflex, is considered an advanced cutaneous motor reflex after high-intensity afferent stimulation. In this study, we aimed to investigate the reflex properties of CSP as a neurophysiological method due to dual stimulation, in terms of SI phenomenon. Methods: In this study, right handed nineteen healthy individuals (Female: 10, Male: 9, mean age: 36.00 ± 6.31, age range: 23-47) were participated in the study. Changes were examined the that may occur during the acquisition of CSP by using the paradigm previously applied for somatosensory evoked potentials (SEP). Electromyographic (EMG) CSP recording was performed with superficial electrodes placed on the abductor pollicis breves (APB) and abductor digiti minimi (ADM) muscles of right and left hand during voluntary muscle activity, following painful and repetitive nociceptive stimulation of digit II/III (Median nerve-M) and V (Ulnar neve-U) of the adjacent two nerves one by one and together using ring electrodes. CSP onset and end latency, CSP duration, and EMG suppression of the averaged and rectified CSP responses were evaluated. In the same individuals, cortical SEP N20 responses were obtained by stimulating the M and U nerves individually and together from the wrist at three times the intensity of the sensory threshold. SEP-N20 latency and amplitude values were determined. Results: In all cases, the amplitude of SEP-N20 obtained by dual stimulation of the M and U nerve was found to be smaller than the arithmetic sum of the SEP-N20 amplitude values obtained by individual stimuli of M and U nerves (R-p: 0.048, L-p: 0.0041). Similarly, There was no significant difference CSP duration between dual stimulation and individual stimulation of the M and U nerves, and the CSP duration obtained by dual stimulation of the M and U nerve were smaller than the arithmetic sum of the data obtained by individual stimulation of the M and U nerve in the CSP responses that recorded in APB and ADM muscle (R-p: 0.001; L-p: 0.001). Discussion: In healthy individuals, When we try to look at CSP from the environmental inhibition window, the fact that the duration of the CSP responses generated by dual stimuli is not different from the duration of the CSP responses obtained by stimulating each nerve separately and also it does not reach their arithmetic sum. These results show that we are far from proving this prediction. However, we believe that our data may open a window for the recognition of both reflex mechanism and spinal circuits and inhibitory systems. Keywords: Cutaneous silent period (CSP), Surround inhibition (SI), somatosensory evoked potentials (SEP)