Development of a new method for studying spinal reflex activities on single motoneurones in man.

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1.A new method for studying spinal reflex activities on single motoneurones was developed by recording single motor units. (1) The computer program was developed for this project. (2) A unitary H-reflex was evoked by stimulating the homonymous nerve. (3) The stimulus intensity was automatically adjusted with a computer so that the firing probability of a motor unit discharge converged to 50% : the intensity was intensified when the motor unit had been fired by a previous stimulus and it was weakened when not fired. The threshold intensity for the Ia fibers in the nerve was determined with the post-stimulus-time-histogram(PSTH)method, and it was subtracted from the intensity with the probability of 50%. The resulting intensity was termed 'critical firihg stimulus (CFS)', which indicated the excitability of the correspondent motoneurone. (4) Effects of peripheral inputs on a single motoneurone was assessed by change in CFS when conditioning stimuli were applied to a certain nerve in prio … More r to the test stimuli. (5) The thresholds of motor units was determined as the critical firing level(CFL), which was the size of compound H-reflexes with the firing probability of the motor unit to be 50%.2. The results obtained by the CFS method were compared with those by the conventional H-reflex method. (1) The pathways examined were reciprocal Ia inhibition and heteronymous Ia facilitation. (2) The data with the two methods were highly correlated. (3) The sensitivities of the two methods were almost the same. (4) The existence and extent of the reflex activities depended on the motor units.3. The heteronymous Ia facilitation from the quadriceps to soleus muscle was observed in 51.3% of the motor units examined, which was somewhat lower in frequency than in the cat and monkey.4. Change in reciprocal Ia inhibition from the common peroneal nerve to the soleus muscle during tonic pretibial contraction had been controversial owing to the difficulties in difflerentiating the effects on individual motoneurones from those on the motoneuronal pool. The CFS method, which revealed effects on individual motoneurones specifically, demonstrated increase in the inhibition.5. The CFS method promises more accurate evaluation of reflex activities on individual motoneu-rones, and is superior to the PSTH method in that examination is possible not only during voluntary contraction of the examined muscle but also in the resting state and even during antagonist contraction. Less