Spinal motor neuron excitability during the cutaneous silent period

The physiologic mechanisms generating the cutaneous silent period (CSP) remain uncertain. It is not known whether the CSP occurs because of inexcitability of the spinal motor neuron. We therefore, assessed excitability of the motor neuron during the CSP using F-wave responses. H-reflexes were also elicited during the CSP. Electrical stimulation to the fifth digit produced the CSP in the voluntarily contracting abductor pollicis brevis muscle (APB). Median nerve stimulation at the wrist elicited control F or H responses during isometric APB contraction (condition 1) and in resting muscle (condition 2). Control amplitudes were compared to those elicited in the midst of the CSP. In Condition 1, F-wave amplitudes and frequency during the CSP were unchanged compared with controls. However, F-waves were increased in amplitude and frequency during the CSP (P < 0.001) relative to responses elicited in resting muscle (condition 2). H-reflexes during the CSP were suppressed (P < 0.001) compared with controls elicited during contraction (condition 1), but facilitated relative to the resting state (condition 2) in which no H-reflexes were elicitable. We conclude that spinal motor neurons remain excitable to antidromic volleys at the same time that the corticospinal volley is inhibited to produce the CSP. Moreover, motor neuron excitability appears to be increased during the CSP compared to the relaxed state.