A text-book of physiology : for medical students and physicians / by William H. Howell.
- William Henry Howell
- Date:
- 1907
Licence: Public Domain Mark
Credit: A text-book of physiology : for medical students and physicians / by William H. Howell. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![give contractions If stimulated directly by electricity. This fact has been interpreted to mean that, in the neuromuscular complex involved in a mus- cular contraction—namely, motor nerve cell, motor nerve fiber, and muscle fiber—the first named fatigues most easily, and that the ordinary fatigue curve obtained from the ergograph does not represent pure muscle fatigue, but fatigue of the neuromuscular apparatus as a whole, the point of complete fatigue being reached in the neural component of the mechanism before the muscle itself loses its power of contraction. This interpretation, however, is not entirely certain. Wedenski has called attention to the fact that in the neuromuscular apparatus the motor end-plate is a sensitive link in the chain, and that, when the nerve is stimulated strongly with artificial stimuli at least, this structure falls into a condition in which it fails to conduct the nerve impulse to the muscle. It may be, therefore, that in sustained volun- tary contractions the end-plate fails first, and thus is directly responsible for the failure of the apparatus to perform further work. That the fatigue in ordinary voluntary contractions affects the muscles before the motor nerve centers is indicated by the experiments of Storey.* Making use of a weight ergograph and experimenting tipon the abductor indicis he found that after fatiguing this muscle to voluntary contractions with a certain weight, re- moval of the weight enabled the individual to make contractions as high and as rapid as before the fatigue. On the other hand, if after removing the weight the muscle was stimulated electrically the contractions were lower and slower than before the fatigue. So far as our knowledge goes, therefore, fatigue as it appears in sustained voluntary contractions is due probably primarily to a loss of irritability in the muscle and in the motor end-plates. The motor nerve fibers do not fatigue, and as regards the motor nerve centers it is not possible as yet to say what may be their relative susceptibility to fatigue. Sense of Fatigue.—It should be noted in passing that in con- tinued voluntary contractions we are conscious of a sense of fatigue, which eventually leads us, if possible, to discontinue our efforts. This sensation must arise from a stimulation of sensory nerve fibers within the muscle or its tendons, and it may be regarded as an important regulation whereby we are prevented from pushing our muscular exertions to the point of straining. Muscle Tonus,—In addition to the conditions of contraction and of relaxation the living muscle exhibits the phenomenon of tone. By muscle tone we mean a state of continuous shortening or contraction which under normal conditions is slight in extent and varies from time to time. This condition is dependent upon the connection of the muscle with the nerve centers, and we may assume that under normal circumstances the motor centers are continually discharging subminimal nerve impulses into the muscles which cause chemical changes similar in kind to those set up by an ordinary voluntary effort, but less in degree; the result being thai the muscles enter into a, slate of contraction which, while slight in exlent , is more or less continuous. According lo this view, the whole neuromuscular apparatus is in a condition of tonic ac- tivity, and this slide may be referred in Hie long run to the con- t inual inflow of sensory impulses into the centra] nervous system. That is, I lie tonus <>f the skeletal muscles is not. only dependent * Story, American Journal of Physiology, L903, 8, '■'<-^>.](https://iiif.wellcomecollection.org/image/b21218183_0058.jp2/full/800%2C/0/default.jpg)
