The spatial organization of central sensitization of hind limb flexor reflexes in the decerebrated, spinalized rabbit

This study was designed to investigate the relationship between the locatio n of a noxious stimulus and the magnitude and duration of the plastic effec ts induced by that stimulus in withdrawal reflexes acting about the knee an d the ankle, in rabbits. Reflexes were evoked in the nerves to the anterior tibial and semitendinosus flexor muscles by electrical stimulation at the toes. Repetitive, high intensity electrical stimulation of nerve trunks (su ral, medial gastrocnemius, superficial peroneal, tibial, 100 pulses, 20 V, I ms at 0.5 Hz) was generally found to be a poor method for inducing centra l sensitization in these flexor reflexes. 'Natural' noxious stimulation ind uced more reliable enhancement of both reflexes. Mechanical (clamp) or chem ical (mustard oil) stimulation of the heel induced prolonged (median durati on > 30 min) increases in reflexes to both muscles. Mechanical (clamp and s uperficial pinch) or chemical (mustard oil) stimulation of the toes, and in jection of bradykinin into the gastrocnemius muscles or into the soft tissu es of the sole of the foot, also led to enhancement of both reflexes, with the median duration of potentiation between 7 and 30 min. The effects obtai ned from deep tissue stimulation were generally weaker than those obtained after stimulation of superficial structures. These data show that there wer e no major differences in the effects obtained from the heel iv the toes, o r between the two reflexes. It appears that the spatial organization of the spinal mechanisms underlying central sensitization of flexor withdrawal re flexes is rather more crudely drawn than that pertaining to the reflexes th emselves. Furthermore, the data indicate that in the present preparation, a fferents from deep tissues are no more effective in generating central sens itization than those from superficial structures. (C) 2001 European federat ion of chapters of the International Association for the study of pain.