THE INFLUENCE OF EXPERIMENTAL LESIONS OF THE SPINAL CORD UPON THE KNEE-JERK. I.—ACUTE LESIONS

Abstract

One of the major physiological symptoms of spinal shock is an increased susceptibility to inhibition. The knee-jerk, for example, may be inhibited for a period of several seconds by applying a single break-induction shock to a sensory nerve of the hind-limb. In the intact animal, or in a decerebrate preparation, it is exceedingly difficult and often impossible to inhibit a knee-jerk by a corresponding stimulus. The present paper records a series of experiments in which the attempt was made to determine the pathways responsible for the differences between the decerebrate and spinal condition, as judged by the inhibition of the knee-jerk. Acute lesions were made at levels ranging from D.8 to L.1, after which knee-jerks were examined by means of a frictionless torsion-wire myograph of high natural vibration frequency, with simultaneous registration of the electrical response according to the technique described by Ballif, Fulton and Liddell. Complete lateral semisection of the spinal cord causes the knee-jerk on the side of the semisection to assume “spinal” characteristics, while the jerk on the opposite side remains “decerebrate.” The dorsal two-thirds of the spinal cord may be cut through without causing the knee-jerk to become “spinal” in type on either side. In acute preparations isolated transections of the ventral quadrant of the spinal cord on one side cause the knee-jerk on the same side to become “spinal.” We conclude from these observations that the descending pathways responsible for the differences between the “decerebrate” and “spinal” conditions pass in the ventral quadrant of the spinal cord, probably in the medial half. The vestibulospinal tracts therefore suggest themselves since they are the most important descending fibres in this area. Transection of the spinal cord caudal to the knee-jerk centres causes great enhancement in their activity, diminution in susceptibility to inhibition, and, in a “spinal” preparation, the prompt reappearance of the stretch reflex.