8 : 3/ Muscles 141 



cell is shown in Figure 2. The maximum cell thickness at the center of 

 the spindle is usually about 6 fx. 



Intact striated muscles rarely contract more than a small fraction of 

 their original length. Smooth muscles, in contrast, change their length 

 manyfold. This large change is believed to be due to a slipping of one 

 smooth muscle cell over another. In all cases of muscular contraction, 

 little if any change of volume occurs. 



Muscles are sometimes classified by criteria other than histological 



ones. In terms of function and innervation, 

 muscles are separated into voluntary and 

 involuntary. For an objective definition, 

 those muscles under direct control of the 

 frontal gyrus of the cerebral hemisphere 



i 1 might be called voluntary. By and large, 



^ striated muscles are voluntary and smooth 



Figure 2. A smooth muscle cell, muscles are involuntary, but this is not a 



hard and fa'st rule. Certain smooth muscles 

 are under conscious voluntary control in some individuals and not in 

 others. Likewise, very few individuals can voluntarily control all of 

 their striated muscles. 



Muscles may be classified by their kinetic properties. In terms of 

 speed of response, smooth muscles such as bladder and uterine 

 muscles often take several seconds to contract. Striated muscle, in 

 contrast, usually contracts rapidly, often reaching its maximum response 

 in a few milliseconds. Within the same animal, faster muscles are usually 

 paler and slower ones are usually darker. (The chicken is a particularly 

 good example of this. The wing muscles work rapidly and are pale, 

 whereas the slower leg muscles are dark.) This color is more closely 

 associated with an oxygen-storing protein called myoglobin than it is with 

 the histological structure. In the next section, the kinetics of the 

 contraction of striated skeletal muscles are described. 



3. Physical Changes during Muscular Contraction 



A. Changes of Tension and Length 



When a muscle is stimulated it twitches. If the muscle is held at con- 

 stant length, it develops a force, whereas if it is weighted down it contracts 

 and does work. The two simplest situations to study are constant 

 length (isometric) and constant force (isotonic). To eliminate the 

 nervous control, it is possible to remove the muscle from the animal body 

 or to cut the nerve fibers. 



