CONDITIONING EFFECT IN SINGLE HUMAN MOTONEURONS - A NEW METHOD USING THE UNITARY H-REFLEX

1. A new method for the study of spinal reflexes using single motor un its is described. 2. The excitability of a motoneurone is assessed as the 'critical firing stimulus' (CFS), which is the difference between the test stimulus intensity needed to reach the threshold for the lowe st threshold Ia fibres and the intensity which evokes firing of a moto r unit with the probability of 50% (FP50%). The intensity with FP50% i s obtained by modulating stimulator output. When the motor unit is fir ed by a stimulus, the next intensity is decreased, and vice versa. The Ia threshold is defined as the threshold for homonymous monosynaptic peaks in PSTHs during contraction of the muscle examined. 3. A conditi oning effect is represented as a change in CFS, the extent being expre ssed as a percentage of the unconditioned CFS. 4. Effects obtained by conditioning stimulation with the new and conventional H reflex method s are compared. The sensitivities are almost the same and the extents of the effects have highly correlated linear relations for the two met hods. 5. The advantages of the new method are (1) that it shows reflex activities on a single motoneurone, (2) that it is applicable both to muscles at rest and during contraction, and (3) that it quantifies co nditioning effects as percentages of the size of test Ia EPSPs.