CORTICAL STIMULATION AND REFLEX EXCITABILITY OF SPINAL-CORD NEURONS IN MAN

The H reflex technique was used to evaluate the influence exerted by c ortical conditioning on the excitability of the alpha-motoneurone pool and on IA interneuronal activity (reciprocal inhibition). In ten subj ects at absolute rest electrical and magnetic stimulation of the motor cortex was transcranially applied during flexor carpi radialis H refl ex eliciting and in conditions of reciprocal inhibition induced by rad ial nerve stimulation. The time courses showed that at intensities bel ow motor threshold, electrical brain conditioning induced an increase in the amplitude of the test reflex when the cortical shock was given 4 ms after the test H reflex. On the contrary, reciprocal inhibition w as reduced by electrical cortical conditioning when the scalp stimulat ion was applied 2-3 ms after the test stimulus. Magnetic transcranial stimulation induced an increase of H reflex amplitude when the test sh ock was administered 5 and 2 ms prior to the scalp shock; it did not m odify the degree of reciprocal inhibition. The experimental findings c ould be considered the electrophysiological manifestation of a differe ntial cortico-spinal control on the pathway alpha-motoneurone/IA inter neurone. Considerations on the delay allow the hypothesis of a further synapse between the cortico-spinal ending and the IA interneurone. Di screpancies with magnetic conditioning might be ascribed to a preferen tial transynaptic action of magnetic mode of neural activation.