STUDIES ON THE KNEE JERK

Abstract

In an animal with intact cord but deeply anesthetized, morphine in any effective dose (1/8 to 1/4 gr.) depressed the spinal cord as judged by the amplitude of the knee jerk. In the unanesthetized animal with cord transected in the lumbo-thoracic region (acute experiment), morphine sulphate in the same dose also depressed the spinal cord as judged by the amplitude of the knee jerk. In unanesthetized dogs recovering from spinal shock, morphine sulphate in moderate doses (1/4 to 18 gr.) likewise effected a depression of the spinal cord as judged by the disappearance of spontaneous reflex movements posterior to the tran-section and the difficulty of eliciting reflexes posterior to the section by various afferent stimuli. The increase in amplitude of the knee jerk under such conditions is interpreted as due to a release of the knee jerk centers from the inhibitory control of impulses from a variety of organs and tissues and absence of mechanical antagonistic restraint due to diminution in tone of muscles antagonistic to the quadriceps extensor. It is concluded that the primary effect of morphine sulphate in the doses used is a depression of the reflex excitability of the cord and the marked briskness of the knee jerk observed temporarily following injection of morphine sulphate in the intact animal is due partly to an isolation of the knee jerk centers from the influence of impulses which normally act on it and partly to the diminution in tone of the antagonistic muscles which tend to reduce the amplitude of the kick.