EFFECTOR MECHANISMS 389 



Despite the paucity of motor-axon supply, crustacean muscles show 

 much versatility of response. Considerable variation occurs in the degree 

 of facilitation exhibited by different muscles. Many crustacean muscles 

 receive two motor axons, one providing fast, the other slow, facilitation. 

 In such muscles the response is regulated, not only by the number and 



Fig. 9.12. Contractions of Limb Muscles (Flexor of Carpopodite) 

 of Rock Lobster Pamdirus 

 {Above) the four contractions elicited by stimulation of the four motor fibres at a 

 frequency of 30/sec. From left to right, axon of diameter 89//, second quickest contrac- 

 tion; axon 91//, third quickest contraction; axon 122//, slowest contraction; axon 129//, 

 quickest contraction. {Below) inhibition of the four contractions. Upper signal indicates 

 electrical stimulation of the inhibitor, lower signal stimulation of the motor fibre. 

 Records of contractions are graded, left to right, according to their speed. Diameter of 

 motor fibres, in same order, are, 126/*, 74//, 81// and 110//. Diameter of the inhibitor 

 fibre is 32//. In the left hand record inhibition is only partial and shows temporary 

 escape at the arrow. Time trace in sec. (From van Harreveld and Wiersma (53).) 



frequency of nerve impulses, but also by switching of axons. In the Rep- 

 tantia the flexor muscles of the limbs receive as many as four motor axons, 

 whereas the claw opener has a single motor axon. The claw opener of the 

 hermit crab Eupagarus receives only one motor axon, yet exhibits both 

 slow and fast contractions. It seems that the single axon makes two kinds 

 of synapses on the muscle, namely fast end-plates activated by high- 

 frequency stimulation, and slow end-plates activated by low-frequency 



