458 MUSCLE-NERVE PHYSIOLOGY 



complete at 40 to 41 C. A muscle cannot recover its irritability after heat 

 rigor has set in strongly. 



If the time of the contraction is measured at different temperatures it will 

 be found to be greatly delayed at 2 to 4 C., and very much quicker than nor- 

 mal at 33 to 35 C. As in fatigue, the effect falls chiefly on the contraction and 

 relaxation phases and only slightly on the latent period. The latent period 

 is more sharply influenced by temperature than by fatigue. 



Influence of Blood Supply. In the normal human muscle there 

 is a delicately balanced vaso-motor mechanism by which the amount of blood 

 flowing through a muscle is immediately increased when the muscle is in con- 

 traction. This blood stream is of course carrying nutritive materials to the 

 muscle and taking away wastes. If the blood supply to a muscle is cut off, 

 then the muscle can only draw on its stored supply of potential energy, which in 

 active contraction is sooner or later exhausted. Under such conditions the 

 muscle increases in irritability for a few minutes and then gradually loses 

 both its irritability and its power to contract. Even mammalian muscles 

 have been kept alive and normal in their activity for several hours by irrigat- 

 ing them with defibrinated and aerated blood (von Frey). Mammalian 

 muscles will remain irritable for 30 minutes, or longer if cooled, after being 

 shut off from their blood supply and isolated from the body, but both irrita- 

 bility and contractility soon disappear entirely. 



Effect of Nerve Supply. The voluntary or skeletal muscle normally 

 contracts in the body only when stimulated through its motor nerve. If the 

 motor nerve is severed, the muscle is cut off from its normal source of activity, 

 hence will undergo the changes resulting from disuse, which will be presently 

 discussed. Aside from this, it is held by most observers that there are dis- 

 tinct nutritive or trophic nerves which exercise a controlling influence over the 

 growth, development, and general nutritive processes going on in muscle. 



When a motor nerve is cut, the muscle at first exhibits heightened irrita- 

 bility to all forms of stimuli. In a couple of weeks it decreases in its power 

 to respond to rapidly changing stimuli like induced currents. It responds 

 more readily to mechanical shocks and to galvanic currents for six or seven 

 weeks, then gradually loses the power of contracting through as many months. 

 The changes are due to protoplasmic degeneration. It is not clear in what 

 degree these changes are due to loss of trophic nerve influence, to inac- 

 tivity, and to changes in nutritive conditions. Since degeneration occurs 

 when the vascular supply is maintained, it would seem that the nutritive con- 

 dition must be chargeable to one or the other of the first two factors, probably 

 to both. 



Use of muscle increases its power and also its irritability. A properly 

 regulated exercise is well known to contribute to the health and development 

 of muscles. In cases of paralysis, mechanical or electrical stimulation is 

 applied directly to the muscle in an effort to supply artificial exercise until the 



