[Effect of motor cortex stimulation on recruitment pattern during reflex activity of fast and slow motor units in the cat (author's transl)].
In order to learn whether change of the fixed recruitment order is induced by the activity of motor cortex or not, the effects of motor cortex stimulation on stretch or vibration reflex of single motor units were examined in 43 motor units of the triceps surae muscle in cats. On the single alpha axon identified functionally there was observed two types of firing pattern in response to the muscle stretch; a phasic change and a tonic change in firing pattern. Then, according to the firing pattern, the fast or the slow motor units defined by the twitch contraction time was classified into the fast-phasic unit, the fast-tonic unit, the slow-phasic unit, and the slow-tonic unit. The axonal conduction velocity was significantly correlated with the reciprocal of twitch contraction time (r = 0.812, p < 0.001). Since the conduction velocity reflects the motoneuron size, larger motoneurons would correspond to the fast motor units and smaller ones to the slow units. By repetitive stimulation of the contralateral pericruciate cortex, the activities of the fast-phasic units were facilitated (11 units), those of the fast-tonic units were either facilitated (3) or inhibited (4), and those of the slow-tonic units were inhibited (24 units) except that only one slow-phasic unit was facilitated. Namely, larger motoneurons with higher conduction velocity had a shorter contraction time and a phasic discharge pattern, and were facilitated by the cortical stimulation, while the smaller ones with lower conduction velocity had the opposite properties. Further, the cortical control described above, were confirmed by the experiment which examined the simultaneous responses to the MG vibration of two or three axons in a natural filament of the MG nerve. These findings suggest that a stereotyped recruitment order according to the motoneuron size could be modified by the input system from the motor cortex.
D000200:Action Potentials; D000818:Animals; D002415:Cats; D004558:Electric Stimulation; D004569:Electroencephalography; D005260:Female; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009132:Muscles / Q000294:innervation; D009431:Neural Conduction; D011999:Recruitment, Neurophysiological; D012018:Reflex