TENDON JERKS; SENSORY PATHWAYS IN SPINAL OORD 



to the tendon and the electrical response observed in tin- vastus internus 

 muscle by the string galvanometer, was found by Jolly in the spinal cat 

 to be 0.0055 of a second, whereas measured in the Bame way the latent 

 period of the flexion reflex was tumid to lie just twice a> long; i. <•.. 0.0101 

 a second. These differences were explained by Jolly as indicating thai 

 the knee-jerk is a simple reflex, involving bul two neurons, whereas the 

 flexion reflex involves three and therefore 1ms twice as long a hv 

 period. r>y subtracting from the total latent period the time occupied 

 in the transmission of the impulse along the nerves and the time lost at 

 the afferenl and efferenl nerve endings, we secure a figure -_ r i \- i 1 1 «_r the time 



lost in the synapses between the neurons. Tins synapse time, ris it is 

 called, was found by Jolly to be 0.002] of a second for the knee-jerk 

 and <).()()4:! of a second for the flexion reflex.' Snyder obtained somewhal 

 similar results in man by the same method. 



Some authors, particularly (lowers, do not, however, believe that the 

 knee-jerk is of the nature of a simple reflex, bul explain it as being due 

 to a contraction of the extensor muscles brought about by direct 

 mechanical stimulation while the muscle is in a hyperexcitable condition 

 as a result of a reflex increase in its tonicity. Cowers believes that by 



putting tl stensor muscles on the stretch and the hamstring mua 



in the relaxed condition, afferent impulses are transmitted to the cord 

 which excite the efferenl neurons of the extensor muscles, so as to throw 

 them into a hypertonic condition, during which the tapping of the ten- 

 don directly excites a contraction. Of course this hypothesis would ac- 

 count once and for all (■)!■ the remarkably short latency of the knee- 

 jerk, hut on the other hand it leaves US many difficulties to explain: 

 such, for example, as the fact that, although tapping the tendon produces 

 the jerk, similar tapping of the muscle itself has no effect. 



The effective stimulus of the jerk is a slight passive increase of the 

 tension to which the extensor muscle itself is subjected, and not a stimu- 

 lation of receptors in the tendon, for it still occurs after the tendon has 

 i denervated. The importance of die relationship of the hamstring 

 nerve to the knee-jerk becomes evidenl in connection with recipr< 

 action; thus, when the flexor is contracted, as in the flexion reflex, the 



knee-jerk disappears (page H l I , whereas when the hamstring nerves an- 



cut, it is augmented. 



Whatever its nature may he. the knee-jerk is of value because of the 

 ease with which it can he altered not only by conditions affecting the 

 reflex arc concerned, hut also by changes occurring here in the 



central nervous mi. The hest known of these conditions is that 



known as reinforctment. This is brought about by having the patient 

 make some voluntary muscular effort at the moment that the tap is ap- 



