CHAPTER 16 



Muscle and Electric Organs 



T 



I HE SPEED OF LOCOMOTION of an animal, and hence its ability to get 



I food and to escape from predators, is partly limited by the reaction 



B time of its muscles. In addition to locomotion, muscles perform 



functions associated with digestion, excretion, and reproduction, and most 



animals have muscles of several characteristic reaction times. 



Contractile fibers occur in Protozoa, as, for example, the myonemes of Vor- 

 ticella and Stentor. In sponges, contractile epidermal cells can close the 

 oscula, while animals of all higher phyla have cells speciahzed for contraction. 

 The cellular pattern of muscle evolved very early, and even coelenterates 

 have some striated fibers. 



The cellular mechanism of contraction and relaxation resides in certain 

 fibrous proteins which can undergo reversible folding.^- ^°*^' ^^^ How these 

 proteins differ in fast and slow muscles is totally unknown, and a comparative 

 study of the contractile protein, actomyosin, might well give evidence re- 

 garding the fundamentals of contraction. 



A comparative study of muscles suggests that their properties can be con- 

 veniently grouped into five general categories: (1) time relations of contrac- 

 tion—contraction rate, time constants of excitation, conduction rate, refractory 

 period, and frequency of stimulation for fused response; (2) dependence on 

 facilitation (summation of nerve impulses); (3) maintenance of tension, re- 

 laxation rate, and tonus; (4) spontaneous rhythmicity; and (5) chemical 

 compounds in activation and liberation of energy. 



GROSS FUNCTIONS OF MUSCLE 



Muscles have been classified in many ways and no single classification is 

 entirely satisfactory. We can first put muscles into two groups in terms of 

 their function in the animal: 



1. Muscles with origins and insertions on skeletal structures, either endo- 

 skeletal or exoskeletal, or on skin. These muscles are generally phasic, or 

 muscles of movement. Examples are the muscles moving body appendages 

 such as legs, wings, and mouth parts, muscles extruding or retracting a pro- 

 boscis or tentacles, and muscles closing the valves of a pelecypod. Many 

 muscles occur in antagonistic pairs— contraction of one reflexly inhibiting con- 

 traction of the other, one causing movement in one direction, the other caus- 

 ing movement in the opposite direction; other muscles pull against an elastic 

 ligament, for example, clam adductors. Usually phasic muscles form part of 

 a lever system and function by shortening (isotonic contraction) or by de- 

 veloping tension while at a constant length (isometric contraction). No 



576 



