906 CENTRAL NERVOUS SYSTEM 



would have been produced by a tetanic contraction of the same height. 

 Since the metabolic changes underlying tonic contraction are not great, 

 it is not surprising that tension can be maintained by this form of con- 

 traction for long periods without fatigue. 



It is a peculiar fact that although tonus is maintained in skeletal 

 muscle only by means of a reflex arc, it is impossible to produce it by 

 any known form of artificial stimulation applied to either the efferent 

 or afferent nerve trunks. Under the normal reflex control the tonic re- 

 action may involve either a shortening or a lengthening to a new state 

 of tone, in which the tension remains the same. 



In the tonic contraction of skeletal muscle only a comparatively low 

 degree of tension can be maintained. Consequently it does not require 

 much force to move a joint out of the posture in which it is held by 

 the tone of its muscles. In certain muscles the tension exerted by the 

 tonic contraction is not inconsiderable, however. In the tonic rigidity 

 which certain muscles assume after the cerebrum is removed the ex- 

 tensors of the limbs may exert a tension well in excess of that required 

 to support the weight of the animal. In order to exert the maximum 

 tension, however, muscles must resort to the tetanic mode of contraction. 



Tetanic Contraction of Skeletal Muscle 



When a skeletal muscle is stimulated directly, or through its nerve 

 with a single shock from an induction coil, it gives a momentary con- 

 traction or twitch. On page 178 it was explained that if a series of 

 stimuli are applied to a muscle the resulting twitches may follow one 

 another so rapidly that the muscle does not relax between them, and 

 a maintained or continuous contraction results to which the name teta- 

 nus is applied. That tetanus is really a series of discrete contractions 

 is shown by the nature of the electrical changes, or action currents set 

 up by a muscle contracting in this way, for it is found that each part 

 of the muscle becomes alternately positive and negative as each twitch 

 is set up in it (see page 188). It is extremely unlikely that the volun- 

 tary and reflex contractions of our muscles ever consist of single twitches, 

 since a nervous discharge must usually consist of a series of impulses, 

 in order that summation may occur and the resistance of the synapses 

 be overcome. Whenever our movements are not due merely to changes 

 in muscular tone, they are tetanic in nature. 



The action currents produced by the voluntary contraction of skel- 

 etal muscle enable us to discover the rate at which component twitches 

 of the tetanic contraction occur, which is about 50 per second (Fig. 223). 

 This rate is determined by the fact that immediately after one contractile 

 process has occurred a second cannot take place until a brief time, known as 

 the refractory period, has elapsed. In the case of skeletal muscle one fif- 



