In man, electrical stimulation of the fingers produces an inhibition of the voluntary EMG activity due to the activation of AS fibers. When comparing distal and proximal muscles the EMG suppression is of longer duration in distal than in proximal muscles. The mechanisms whereby the inhibition is produced have not been sufficiently clarified. Recently, Priori et al. (1998), stimulating the tendon of extensor digitorum muscle, showed that the EMG suppression may originate from a dysfacilitation of the alpha motoneurons due to presynaptic inhibition of la afferents produced by A delta afferents. In contrast, Inghilleri et al. (1997) showed that finger stimulation directly changes the level of motoneuron excitability postsynaptically in hand muscles. These effects are probably related to activity of the same afferent input to motoneurons innervating proximal or distal muscles due to a different inhibitory spinal pathway. In order to address the question whether the afferent input could produce an inhibition of alpha motoneurons at pre- or postsynaptic levels, we studied the changes in excitability of cervical motoneurons at different levels. The experiments were performed in 10 normal subjects. High intensity electrical shocks (40-50 mA, 0.5 ms) were given to the second finger. A Novametrix D200 stimulator was used with a 10-cm diameter coil centered at the vertex and the electric current in the coil flowing clockwise as viewed from above. H reflex was recorded in the forearm flexor muscles after median nerve stimulation. An EMG recording was taken from surface electrodes over the right FDI and extensor and flexor forearm and biceps brachii muscles. After finger stimulation the inhibition of ongoing EMG was maximal in the FDI, less pronounced in the forearm and absent in the biceps muscles. With the subject at rest we investigated the excitability of spinal motoneurons by giving a magnetic transcranial shock (test stimulus) 60 ms after finger stimulation (conditioning stimulus) at a time when tonic voluntary activity is usually completely suppressed. When we used as test a muscle response evoked by transcranial stimulation, the inhibition was present in the FDI and less pronounced in the forearm muscles, and a facilitation was observed in the biceps muscle. When we used as test the H reflex, we observed an inhibition. We conclude that activation of Aδ fibers produces a different gradient of postsynaptic inhibition of FDI and forearm motoneurons and facilitates biceps motoneurons probably through a complex spinal pathway that contributes to a withdrawal response specific to the arm.
|Issue number||4 SUPPL.|
|Publication status||Published - 2000|
ASJC Scopus subject areas
- Clinical Neurology