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 Friedreichʹs 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×ST and 8×ST); (c) pattern of responses in distal and proximal muscles at different stimulus intensities (2×ST and 8×ST). Results: (a) The CSP occurred at low intensities (1×ST and 2×ST) and increased abruptly between 3.5×ST and 4×ST (corresponding to the pain threshold). The SAP amplitude was saturated before CSP saturation. In the patients with Friedreichʹs ataxia, the CSP appeared only at higher stimulus intensities (6×ST–8×ST). (b) The CSP evoked at 2×ST showed a fast habituation and slow recovery cycle whereas the opposite behaviour was found at 8×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.