EFFECTOR MECHANISMS 



385 



Peripheral Facilitation. In the motor-unit type of response the nerve 

 impulse is transmitted with equal facility to all muscle fibres in the unit. 

 The muscles of many invertebrates, however, have a facilitated type of 

 response. The contraction of muscle systems showing peripheral facilitation 

 is brought about by a train of successive impulses, which become progres- 

 sively more effective by local summation at peripheral synapses. The rate 



Fig. 9.9. Isometric Contractions, a, of Branchiomma Muscle, 

 b, of Squid {Loligo) Mantle Muscle 

 a Stimulation frequencies: a, 34/min, b, 64/min, c, 116/min, d, 13/sec. Time trace 

 a-c, 1/sec; d, 13/sec. (From Nicol (87).) 



b Numbers indicate frequencies/sec. of stimulation. (From Prosser and Young (105).) 



and strength of contractions are delicately regulated by varying the num- 

 ber and frequency of nerve impulses. Peripheral facilitation occurs in the 

 muscle systems of certain coelenterates, and is widespread among higher 

 crustaceans. 



Coelenterates. Peripheral facilitation of muscular contractions is re- 

 ported for Hydromedusae, Scyphomedusae and Actiniaria. When the 

 column of the anemone Calliactis is stimulated electrically with a series of 



M.A — 13 



