CHAPTER XCVII 

 MUSCULAR CONTRACTION 



Voluntary and reflex nervous activity expresses itself in the reactions 

 of the muscles of the body. In order to interpret the normal working 

 of the motor mechanism, and the abnormal manifestations which dis- 

 ease of the nervous system imposes on the muscles of the body, the na- 

 ture of the contractile processes in skeletal and smooth muscle must be 

 understood. 



A muscle is an elastic body. That is to say, for every length to which 

 it is stretched it will exert a definite tension on its origin and insertion, 

 tending to pull them together until it has returned to its unstretched 

 length. When a muscle "contracts," it does not assume a smaller vol- 

 ume; rather it tends to change its shape so as to become shorter and 

 thicker. The contracted muscle has become a body with new elastic 

 properties, i. e., for each length to which it is stretched, it now exerts 

 a greater tension on its origin and insertion than it did in the "uncon- 

 tracted" condition. Consequently if opposition is presented to the short- 

 ening of the stimulated muscle it will exert a tension on the opposing 

 object equivalent to the tension necessary to stretch the contracted mus- 

 cle to its resting length. The muscle's length does not change, and hence 

 it is said to contract isometrically. If, however, the opposition is not 

 strong, i. e., is due to a light weight, the tension developed on contraction 

 will overcome the opposition, and the muscle will shorten, thus lifting 

 the weight. Since the tension exerted by the weight is the same at all 

 times during the contraction of the muscle, such a contraction is called 

 isotonic. When it shortens against a weight the muscle assumes a new 

 length at which it exerts a tension equal to the weight which is lifted. 

 A muscle is consequently a machine for developing tension, and this 

 tension may or may not do work in lifting a weight, or moving a joint, 

 depending on whether or not the tension developed is great enough to 

 overcome the opposition. 



The elastic properties of skeletal muscle may be modified in two dis- 

 tinct ways which differ in respect to the energy required for the processes 

 which cause the muscle to tend to shorten. A tonic contraction is one in 

 which the processes which cause the muscle to change its elastic con- 

 dition so as to take on a shorter length when at a given tension (or 

 to exert a greater tension at a given length) are maintained with great 



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