384 THE BIOLOGY OF MARINE ANIMALS 



Consequently, the maximal rate of effective excitation via the nerve net 

 is about 86 per min, at which frequency partially fused contractions 

 (clonus) are produced (Fig. 10.6, p. 427). An absolute refractory period 

 reigns in vertebrate cardiac muscle throughout most of systole, to be suc- 

 ceeded by a relative refractory period during late systole and diastole; 

 recovery is complete by the end of diastole. The heart remains inexcitable 

 to direct electrical stimulation throughout the absolute refractory period, 

 and gives a submaximal contraction during the relative refractory period 

 (Fig. 3.7, p. 103) (25). 



It is likely that muscular contractions in most animals are produced by 

 trains of nervous impulses, but this is not invariably so. Each normal 

 swimming contraction of the medusa bell (Aurelid) is evoked by a single 

 impulse in the nerve net. Quick contractions concerned with escape res- 

 ponses sometimes depend upon a single nervous impulse. Thus it is in the 

 sabellid Myxicola, where a tactile stimulus evokes a single impulse in the 

 giant axon — the final common path to the longitudinal muscles of the 

 body — and a quick synergic twitch (Fig. 10.11) (55, 87). 



The tension developed during tetanus is usually greater than that pro- 

 duced by a single twitch, the effect increasing with frequency of stimulation, 

 e.g. tonic muscles of lamellibranchs. Varying the frequency of excitation, 

 therefore, is one method of regulating the strength of muscular contraction. 

 Not all muscles react in this manner to increased frequency of stimulation, 

 however. The mantle circular muscles of Loligo (cephalopod) and the 

 body longitudinal muscles of Myxicola (polychaete) are motor units 

 served by giant axons: in both instances the tension developed during 

 tetanic contraction is no greater than that produced by a single twitch, 

 i.e. tension remains constant with increase in frequency (Fig. 9.9). These 

 muscles are specialized to develop maximal response to a single impulse, 

 and mechanical summation and facilitation are non-operative (105). 



Regulation of Speed and Strength of Contraction: Motor-unit 

 Response. In the two systems just described the whole muscle consists of 

 one or a few motor units. In the polychaete Myxicola the whole longi- 

 tudinal muscle is served by a single giant axon, and in the squid each 

 tertiary giant axon supplies a large area of mantle muscle (p. 436). A single 

 nerve impulse in one of these units excites all the muscle fibres and pro- 

 duces a maximal, all-or-nothing contraction. The triggered responses of 

 these units are particularly well suited for an all-out effort in which there is 

 no need for economy or accurate adjustment. 



Vertebrate muscle provides the classic type of motor-unit : here a motor 

 axon innervates a fixed group of muscle fibres and the whole muscle 

 consists of several such groups. An impulse in one of the motor axons 

 produces a synchronous contraction of all the muscle fibres in the motor 

 unit which it supplies. The magnitude of response of the whole muscle is 

 determined by the number of motor units activated. Regulation of res- 

 ponse occurs in the spinal cord and, once an impulse has left the latter, 

 further action is a non-stop process. 



