REFLEX STIMULATION OF CONTINUOUSLY OSCILLATORY FIRING ALPHA-MOTONEURONS AND GAMMA-MOTONEURONS IN PATIENTS WITH SPINAL-CORD LESION

Single nerve-fibre action potentials (APs) were recorded extracellular ly from lower human sacral nerve roots, and simultaneous single-fibre impulse patterns of alpha and gamma-motoneurons and secondary muscle s pindle afferents were analysed. Identified alpha and gamma-motoneurons fired oscillatory due to the sustained stretch reflex of the external sphincters induced by an anal catheter (and, possibly, the bladder ca theter). The motoneurons and the secondary muscle spindle afferents tr ansiently synchronized their firing upon repetitive touch, pin-prick a nd dimpling stimulation of the perianal skin inside the anal reflex ar ea, by reducing the duration of their oscillation period untill resett ing of the oscillation cycle. In one case, the anal reflex area extend ed approximately 6 cm laterally from the anus. The responses to pin-pr ick stimulation were different from those to touch stimulation in thre e aspects. Firstly, the response time till the shortening of the oscil lation period was longer than the oscillation period (similar to 100 m s) for pin-prick, and it was shorter for touch. Second, the response t o pain stimulus was longer (shortening of several oscillation periods) and stronger than for touch stimulation. Pin-prick stimulation reduce d the oscillation period to between 5 and 40 ms (mean = 18 ms), and to uch stimulation to between 8 and 28 ms (mean = 15 ms). Third, transien t synchronization of afferents and efferents was most pronounced for p in-prick stimulation. The shortest latency following touch was approx. 10 ms when measuring from the afferent volley running in the directio n of the spinal cord, and 30 ms when measuring from the beginning of t he skin touch. It is discussed that repetitive touch stimulation reinf orced the sustained stretch reflex of the anal sphincter which is poss ible with no network reorganization (variation of the same network sta te) and therefore fast, whereas repetitive pin-prick stimulation repla ced the sustained stretch reflex by the protection reaction of the ana l sphincter (change from one network state to a different one) which m ade time consuming network reorganization necessary. Different sacral reflexes were analysed by studying time-related activation changes of group conduction velocities in velocity distributions. During the refl ex response to stretch of the external anal sphincter, the alpha(2)-mo toneurons (FR) (and the secondary muscle spindle afferents) were stron gly activated whereas upon eliciting the bulbocavernosus reflex (squee zing of the glans penis) the alpha(3)-motoneurons (S) were mainly acti vated. Sacral reflexes are discussed with respect to the organization and reorganization of preformated neuronal networks, and the synchroni zation of oscillatory firing networks is discussed with respect to the overlapping of synfire chains.