392 



THE BIOLOGY OF MARINE ANIMALS 



group of motor fibres is present during fast contraction, whereas activity 

 in the other group occurs during tonic contraction (or delay of relaxation). 



A single stimulus produces but a small response; repeated stimulation 

 elicits tetanus and increase of tension. Peripheral facilitation also occurs, 

 as evidenced by the enhanced effect of paired shocks and staircase pro- 

 duced by low-frequency stimulation (cf. Fig. 9.15). Intimate details of 

 peripheral neuromuscular organization are still unknown (17, 48, 103). 



Earlier work suggested that the lamellibranch plain adductor possesses 

 a peculiar catch mechanism which, when set, might maintain tension for 

 long periods with little energy expenditure. This hypothesis postulates an 

 increase in muscle viscosity during tonic contraction, an increase which has 

 not been discovered. Lactic acid accumulates during prolonged contrac- 



Sec ll l ll ll ll I l l l llll l l l l l lll l llll ll llllll l l i mMWt 



Fig. 9.15. Electrical Responses of the Byssus Retractor Muscle of Mytilus 



{Above) responses to stimulation, a, 0-5/sec; b, 1/sec; c, 2/sec; d, 3/sec; e, 5/sec; 

 /, 7/sec. {Below) stimulation, a, 3-5/sec; b, 1 5/sec. Time trace, 01 sec. (From Fletcher, 

 1937, and Prosser et ah (103).) 



tion, revealing energy consumption. Electrical recording from muscles 

 shows that both phasic and tonic contraction are accompanied by muscle 

 action potentials, and during maintained tonus there is continuous 

 electrical discharge (Figs. 9.14, 9.15). The time course of relaxation is pro- 

 longed, and it would appear that maintenance of tonus in the smooth 

 muscle depends upon slow tetanic stimulation and slow decay of tension. 

 Following denervation the adductor muscle remains contracted and 

 continues to exhibit electrical discharge. This tetanically maintained con- 

 traction, of peripheral nature, may be myogenic or originate in peripheral 

 neural elements. The recent discovery of peripheral nerve cells in lamelli- 

 branch adductors and retractors favours the latter explanation (20, 81, 

 82, 83). 



Evidence also exists for peripheral inhibitory control in lamellibranch 

 plain muscle. Stimulation of the anterior byssus retractor muscle of Mytilus 

 by direct current produces a prolonged contraction, which is suppressed 



