Pyramidal Effect on Alpha and Gamma Motoneurons

Reports on the movements evoked by pyramidal stimuli are conflicting. To avoid the effects of spread of current we have studied the response to impulses confined to the pyramidal tract in two ways: 1) By transection of all other structures in the medulla oblongata. We maintained the arterial blood pressure above 100 mm Hg by vagotomy and intravenous infusion of noradrenalin and dextran. The tone in flexor muscles predominated. 2) By dissection of the pyramids away from the medulla oblongata. Reticulospinal and vestibulospinal tracts being intact, the tone in extensor muscles predominated. Pure pyramidal stimuli activated flexors and inhibited extensors after both procedures; complex movements were not observed. We recorded action potentials of alpha and gamma motoneurons in the nerve to the tenuissimus muscle. Pyramidal stimuli discharged both types of motoneuron, alpha motoneurons having the higher threshold, longer latency and shorter duration of the burst of potentials.