Silent period in upper limb muscles after noxious cutaneous stimulation in man

We studied the effect of electrical stimulation of the C5-C8 dermatomes on voluntary electromyographic activity (EMG) recorded from the ipsilateral first dorsal interosseus (FDI), abductor digiti minimi, flexor and extensor carpi, triceps brachii, biceps brachii, and orbicularis oculi muscles of healthy humans. Finger stimulation (C6-C8) produced an EMG inhibition (silent period, SP), which progressively decreased in duration from distal to proximal muscles; in the biceps it induced a slight facilitation and in the orbicularis oculi muscle, it had no effect. Stimulation of the C5 dermatome induced no response in either distal or proximal muscles. Only high-intensity stimuli evoked clear silent periods. The threshold for evoking an SP was almost double that required for sensory action potentials, 3.25 times the sensory threshold, and decidedly above the pain threshold. An indirect estimation of the conduction velocity of SP afferent fibres placed them in the A-delta group of myelinated fibres. In double-shock experiments, used to study the recovery cycle of the SP in the FDI muscle after finger stimulation, neither low- nor high-intensity conditioning stimuli delivered 100-500 ms before the test stimulus changed test SPs. Experiments designed to evaluate motoneuronal excitability showed that in relaxed FDI muscle, finger stimulation markedly reduced the F wave at the 50 ms time interval, the time when the SP normally occurs. Our findings demonstrate that the activation of A-delta afferents from the fingers inhibits the C7-T1 motoneurons postsynaptically, through an oligosynaptic spinal circuit. We propose that the strong inhibitory effect exerted by noxious cutaneous stimuli on all distal muscles may contribute to a defence action which is specific for the human upper limb.