Cutaneous afferents mediating the cutaneous silent period in the upper limbs: evidences for a role of low-threshold sensory fibres

Objectives: To evaluate the contribution of the low-threshold afferents to the production of the cutaneous silent period (CSP) in the upper limbs. Methods: The CSP was studied in 10 healthy adults and 4 patients with Friedreich's ataxia. The following neurophysiological aspects were studied: (a) relationship between sensory threshold (ST), sensory action potential (SAP) amplitude and CSP parameters; (b) habituation and recovery cycle of the CSP at different stimulus intensities (2 X ST and 8 X ST) (c) pattern of responses in distal and proximal muscles at different stimulus intensities (2 X ST and 8 X ST). Results: (a) The CSP occurred at low intensities (I X ST and 2 X ST) and increased abruptly between 3.5 X ST and 4 X ST (corresponding to the pain threshold). The SAP amplitude was saturated before CSP saturation. in the patients with Friedreich's ataxia, the CSP appeared only at higher stimulus intensities (6 X ST-8 X ST). (b) The CSP evoked at 2 X ST showed a fast habituation and slow recovery cycle whereas the opposite behaviour was found at 8 X ST. (c) Low-threshold stimuli induced an inhibitory response restricted to the distal muscles. High-intensity stimulation produced an electromyographic suppression, significantly increasing from proximal to distal muscles. Conclusions: Our findings support the notion that low-threshold afferents participate in the production of the CSP in the upper limbs. The different afferents may activate different central neural networks with separate functional significance. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.