Unilateral Overflexion of the Limbs Following Experimental Lesions in the Pons and Upper Medulla. 20

Abstract

A characteristic and constant unilateral overflexion of the limbs, during progression or isolated movement of the limbs, has been observed in 12 to 15 cats following hemisection of the brain-stem at the level of the pons or upper medulla—the vestibular nuclei being uninvolved. The abnormal action occurs in the limbs on the opposite side from the lesion. Maximal rather than graded flexion occurs at the ankle, knee, shoulder, or hip joints. In some instances in progression the flexion was maintained to the extent that the animal fell to the affected side because of the failure of the fore limb to extend in time to catch the weight of the body. This overflexion has been seen to involve a fore limb or a hind limb individually. In cases where the lesion extended across the mid-line so as to involve a small medial segment of the opposite half of the stem the ability of the animal to right and to maintain the righted position was impaired or eliminated. In these cases the overflexion occurred when rhythmic running movements were executed. If the animal was passively held in the righted position, typical walking movements embodying overflexion occurred.

Overflexion did not occur following: (1) hemisection of the stem through the thalamus or mid-brain, or even after complete unilateral removal of the mid-brain. (2) Unilateral or bilateral section of the lateral quarter segments of the pons or upper medulla. It was therefore not due to the absence of a rubro-spinal tract. (3) Hemisection of the spinal cord at upper cervical levels. (4) Injury or removal of the lateral lobes or vermis of the cerebellum.

The observations suggest that this disturbance in muscular movement results from injury to structures located in the medial quarter segments of the pons and upper medulla.