Muscle and Electric Organs 601 



The efficiency of this complicated neuromuscular system is high. Bronk^"* 

 showed that the efficiency in terms of tension developed for heat produced in- 

 creases as stimulation continues. The slow system is more efficient than the 

 fast one in that its ratio of heat to mechanical work is lower.'^^ Only a few 

 impulses need leave the nervous system to maintain contraction, and a shift 

 in nerve fiber or a slight change in frequency from the nervous system 

 changes the whole character of the muscular response. 



Multiple innervation of muscles occurs also in insects. ''•'• ''■'- One of the 

 nerve fibers going to a leg muscle in a cockroach causes a tonic (slow) con- 

 traction; it discharges, when attached to the nervous system, at 30/sec., whereas 

 the other shows reflex bursts of large spikes at 75/sec. and causes a quick 

 high-level contraction.''" The slow system shows much facilitation, the fast 

 system little or none. No inhibitors were found in the cockroach but there 

 is evidence for {peripheral inhibition in leg muscles in Dixippus.^^^ 



In the clam Mya arenar'ia, the adductor muscles and mantle retractor mus- 

 cles receive two types of nerve fiber. "*^ One type discharges reflexly at high 

 frequency and causes rapid contractions; the other discharges slowly and 

 causes a prolonged tonic type of contraction with low potential electrical re- 

 sponse. Such a system is efficient when a low level of contraction must be 

 maintained and occasional quick contractions superposed. The retractor of 

 the buccal mass of Helix appears not to be doubly innervated.^'*" In Pecten 

 the adductor muscle is composed of a striated portion for rapid swimming 

 movements and a smooth portion for maintained contraction.''** During swim- 

 ming the smooth muscle is inhibited and part of this inhibition may be pe- 

 ripheral.-^^ No evidence is available as to whether multiple innervation exists 

 among annelids or echinoderms. 



As comparative studies of neuromuscular junctions are extended, it be- 

 comes apparent that these junctions serve many of the functions usually 

 ascribed in vertebrates to the synapses of the central nervous system. 



RELAXATION RATES AND TONUS; 

 MECHANICAL PROPERTIES OF MUSCLE 



The preceding sections show that muscles diffei in their time constants 

 of contraction and in their dependence on neuromuscular facilitation and 

 inhibition. Muscles diflfer also in rates of relaxation and in maintenance of 

 tension. Table 71 shows that relaxation times differ more than do contrac- 

 tion times and that a muscle can contract fairly rapidly and relax extremely 

 slowly (e.g., clam adductor). Contraction and relaxation are active processes, 

 both requiring energv; vet the rates of contraction and relaxation are de- 

 termined in part by the mechanical properties of the muscle as a whole- 

 connective tissue elements, sarcolemma, and muscle proteins. These mechani- 

 cal properties, particularly those of the contractile proteins, change during 

 contraction. To what extent can differences in relaxation and maintenance 

 of tension be accounted for by mechanical properties of muscle? 



Tonus is the maintenance of tension under extension, or change in length 

 without change in tension. It has been attributed to: (1) the mechanical 

 "set" of contractile elements, (2) continued nervous bombardment, and 

 (3) continued stimulation by chemical agents. 



