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 Fried reich'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 CS P at different stimulus intensities (2 X ST and 8 X ST) (c) pattern of resp onses 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 in creased abruptly between 3.5 X ST and 4 X ST (corresponding to the pain thr eshold). The SAP amplitude was saturated before CSP saturation. in the pati ents with Friedreich's ataxia, the CSP appeared only at higher stimulus int ensities (6 X ST-8 X ST). (b) The CSP evoked at 2 X ST showed a fast habitu ation 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 electromyogra phic suppression, significantly increasing from proximal to distal muscles. Conclusions: Our findings support the notion that low-threshold afferents p articipate in the production of the CSP in the upper limbs. The different a fferents may activate different central neural networks with separate funct ional significance. (C) 2001 Elsevier Science Ireland Ltd. All rights reser ved.