CHAPTER XCIII 



THE TENDON JERKS; SENSORY PATHWAYS IN 

 SPINAL CORD 



Certain responses are of importance largely because of their clinical 

 application. Of greatest interest in this connection are the tendon jerks. 

 The location of the sensory pathways in the spinal cord also demands at- 

 tention. 



The Tendon Jerks. One of the most important reflexes for diagnostic 

 purposes is that known as the knee-jerk, which is elicited in man by ap- 

 plying a smart tap to the patellar tendon of a person who is sitting on 

 a high stool or table so that the joint is passively flexed and the leg 

 hangs loosely from the knee joint. In this position the extensor muscles 

 are put slightly on the stretch, and when the patellar tendon is struck, 

 these muscles contract and cause the leg to become extended as in kick- 

 ing. This reflex, as we have seen, is also readily elicited in spinal 

 animals. Its importance from a clinical standpoint depends on the 

 fact that it may be altered not only in various general conditions of the 

 body, but also when any pathological condition disturbs the continuity of 

 the reflex arc concerned in maintaining the tonicity of the extensor mus- 

 cles of the thigh. The centers involved in this arc are situated about 

 the third or fourth lumbar segment, and the afferent impulses come 

 partly from the antagonistic flexor muscles and partly from the extensor 

 muscle itself. Abolition of the reflexes may therefore be produced either 

 by neuritis involving the afferent fibers or myelitis affecting the gray 

 matter of the cord. That certain of the afferent impulses come from the 

 hamstring muscles is shown by the fact that when the central end of the 

 cut motor nerve of the extensor muscles is stimulated electrically, the 

 knee-jerk becomes much less evident, a result which is also obtained by 

 massaging the muscles. 



Although such facts show clearly that the knee-jerk is of reflex na- 

 ture, yet there are difficulties in explaining the exact mechanism by 

 which the tap to the tendon produces the muscular contraction. The 

 chief difficulty is in accounting for the promptness with which the contrac- 

 tion occurs, the latent period being very much shorter than that of such 

 reflexes as the flexion or even the conjunctival. The total latent period 

 of the knee-jerk, as judged by the time elapsing between applying a tap 



