Physiology and biochemistry in modern medicine . ected hamstring- muscles simultaneously with a visible relaxationof the extensors (Fig. 212). If the leg is held properly, this relaxation maybe marked enough to cause a slight flexion at the joint; and in any case,if the knee-jerk is regularly elicited by equal taps applied to the patellartendon, it will be found that, while the flexion is being produced, theknee-jerk is very much less than normal, if not entirely absent, thus in-dicating that the tone of the extensor muscles is diminished. This ex-periment is very striking when performed on a
Physiology and biochemistry in modern medicine . ected hamstring- muscles simultaneously with a visible relaxationof the extensors (Fig. 212). If the leg is held properly, this relaxation maybe marked enough to cause a slight flexion at the joint; and in any case,if the knee-jerk is regularly elicited by equal taps applied to the patellartendon, it will be found that, while the flexion is being produced, theknee-jerk is very much less than normal, if not entirely absent, thus in-dicating that the tone of the extensor muscles is diminished. This ex-periment is very striking when performed on a decerebrate animal, inwhich, as we shall see, the extensor muscles of the limb are in a per-manent state of hypertonicity (Fig. 213). Before it is permissible to conclude that this reciprocal inhibition is anecessary event in the movement of a joint, Ave must however show thatit occurs at exactly the same time as the flexion of the antagonist. Sher-rington has succeeded in doing this in a considerable variety of experi- /Ant Crural ). Sciatic N.(Ischiddicus) Fig. 213.—Diagram showing the muscles and nerves concerned in reciprocal innervation. (After Sherrington.) ments, one of which we may cite here. If, in a spinal dog, the tendonsof the flexor muscles of the knee joint of one hind limb and the ex-tensor tendons of the opposite limb are cut, then the former limb willbe unable to flex properly, but will nevertheless exhibit reciprocalinhibition of the intact extensor muscle, while the latter limb will flex,but require passive extension to bring it back to its old position. Ifsuitable stimuli are simultaneously applied to the skin of both legs andthe movements of th,e isolated muscles recorded, the onset of inhibitionof the intact extensor of the one leg and the contraction of the flexorsof the opposite leg will be found to agree with regard to latent periods,strength of required stimulus, summation and indeed all the other phys-iologic properties of reflex action.
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