ZQ2 Comparative Animal Physiology 



The mechanical properties of muscle have been studied by stretching and 

 releasing under different conditions. Muscles have properties of viscous and 

 of elastic bodies. When vertebrate striated muscle is stretched it develops 

 elastic tension, and when the stretching force is removed the muscle returns 

 rapidlv to its initial length; its "viscosity" is low. When visceral short-hbered 

 smooth muscle is similarly stretched it lengthens rapidly at hrst, then more 

 slowly; it shows an initial elastic tension which falls off as the stretch con- 

 tinues; smooth muscle behaves as a plastic viscous body. According to Win- 

 ton--*" ' smooth muscle can be represented by a model of three phases: pure 

 elastic as a coiled spring, viscous-elastic as a spring damped in oil, and pure 

 viscous as \'anes in oil. Striated muscle can be represented by the first two 

 of these phases. Muscles differ in the proportionate effectiveness of their 

 elastic and viscous phases. 



Vertebrate striated muscle de\'elops elastic tension when stretched; in 

 addition it may develop tonus reflexly, i.e., initiated via the central nervous 

 system bv stimulation of stretch receptors within the muscle. Its tone results 



—I 1 1 1 1 1 — I 1 i 



Fis- 2H2. Tension curves of snail retractor pharynx. In each curve the muscle was 

 first stimulated, then after relaxation it was stretched by about 10 per cent of its length 

 and then released. A, in air; B, in air and 6 per cent COj. Time in seconds in each. Note 

 .similarity of rela.xation and release of tension. From Bozler.''' 



from continued discharge of motor impulses at low rates in a variable num- 

 ber of motor units. Relaxation is complete when such nervous discharge 

 ceases. In insects the contracted state in death feigning results from con- 

 tinued nervous acti\it\. That there is some "setting" or organization of me- 

 chanical elements in striated muscle is shown by the observation^ ''-^ that, if 

 the muscle is suddenly either released or stretched during continued isometric 

 contraction, tension is again developed over a time course corresponding 

 to its new length. The tension developed by an excited muscle depends on 

 the initial length of that muscle and is maximal at the resting length. ^•'*^ 



Short-Hbered smooth muscle is sometimes said to have no fixed resting 

 length. The tension in the mammalian bladder and in the gastral cavity of 

 a sea a.ncmone, in the coelom of Sipiincidus, or in the gill cavity of an as- 

 cidian, is rclati\ely constant, whether the cavity be empty or full. The mus- 

 cles are stimulated by stretching. Many hearts likewise are stimulated to 



