64 ESSENTIALS OF PHYSIOLOGY. 



The reflex character of the knee jerk has been denied on the ground 

 that it is a single twitch, whereas in genuine reflexes there is a rhythmic 

 discharge ol" impulses to the muscle. This argument is discounted by 

 the fact that the sudden extension of the leg which is produced by 

 applying pressure to the foot of a spinal animal is undoubtedly a reflex 

 action and is also a single twitch. 



The knee jerk takes place very rapidly, the reduced reflex time 

 being only 0'002 second ; and though it must be regarded as a true 

 reflex, the impulse passes, in all probability, through only one synapse 

 in the spinal cord. 



In man the part of the spinal cord concerned with the knee jerk is 

 the third and fourth lumbar segments ; and the knee jerk is abolished 

 eitheF^rdestructi^n^f this part of the spinal cord or by division of 

 the afferent or efferent nerves of the extensor muscles of the thigh. 

 The knee jerk is absent in locomotor ataxia, in which the posterior 

 lumbar nerve roots are diseased and the afferent path of the reflex arc 

 is interrupted. Exaggeration of the knee jerk may be brought about 

 by section of the hamstring muscles or of the afferent nerves from these 

 muscles. 



The knee jerk can also be increased by impulses from the higher 

 parts of the central nervous system. If, for example, the fists are 

 firmly clenched at the moment at which the knee jerk is elicited, the 

 jerk is more marked, this being called reinforcement of the knee jerk. 

 Further, it is often exaggerated in disease of the cerebral cortex or of 

 the pyramidal tracts, probably owing to the cutting off of restraining 

 impulses which normally pass to the spinal cord from the brain. 



The knee jerk and other similar reflexes serve to protect joints and 

 ligaments from injury when a sudden strain occurs, which tends to 

 separate the joint surfaces or to stretch ligaments ; and the extreme 

 rapidity of the reflex is no doubt associated with this protective function. 



The production of the knee jerk and other tendon reflexes is 

 dependent on the existence of muscle tone. The skeletal muscles are 

 in a constant condition of slight tonic contraction, which is due to the 

 continuous discharge of impulses from the spinal cord to the muscles, 

 as is showrPBy the f ol lo wi^^oBseTVSttDEI When the brain of a frog is 

 pithed and the animal is suspended by its head, the muscles do not 

 become flaccid and the limbs remain very slightly flexed. When the 

 spinal cord is destroyed, or the posterior roots of the spinal nerves are 

 divided, the limbs become fully extended and the muscles lose their 

 tone. 



In mammals, muscular tone is lost when the skeletal muscles are 

 cut off from the central nervous system, or during deep anaesthesia. 



