82 



INVERTEBRATE PHYSIOLOGY 



Fig. 4. In each ganglion the nerve trunks concerned are the branches N3b 

 and N5. They are probably the homologues of the branches similarly desig- 

 nated in the metathoracic ganglion of Periplaneta by Pringle (1939). In 

 Albrecht's (1953) account of the Locusta nervous system the trunks are 

 designated N2 and N3, the actual second and fourth nerves not being men- 

 tioned in his description. The pro- and the mesothoracic N3b nerves carry 

 the single "fast" axons of the extensor tibiae muscles. The metathoracic 

 N3b carries a single "slow" axon of the metathoracic extensor tibiae, the 

 Si axon (Hoyle, 1955). The pro- and mesothoracic "slow" axons travel 

 in the N5 trunks whilst the metathoracic N5 carries the single metathoracic 

 extensor tibiae "fast" axon. This divided origin of the "slow" and "fast" 

 axons supplying the same muscle is extremely convenient, for it means that 

 the neuromuscular mechanisms of the muscles can be studied during sep- 

 arate stimulation of the innervating axons without the risk of damaging 

 them which is encountered when axons have to be separated by dissection. 

 The metathoracic N3b does, however, contain an additional axon supply- 

 ing the metathoracic extensor tibiae muscle. This axon (So) evokes no 

 mechanical response when stimulated alone, but efforts must be made to 

 avoid stimulating it if the effects of Si alone are to be observed. 



2 /^P 



rvi. e.t 



Fig. 4. Diagram of the locust thoracic nerve ganglion chain showing the origins 

 of the "slow" and "fast" axons supplying the extensor tibiae muscles. F= "fast" axon ; 

 Sj^, S.„ "slow" axons; N^ a,b,c, N^, nerve trunks, m.e.t. = metathoracic extensor 

 tibiae, ms.e.t. = mesothoracic extensor tibiae, p.e.t. = prothoracic extensor tibiae. 



Stimulation of the metathoracic extensor tibiae "slow" axon at fre- 

 quencies up to 10 per second produces no mechanical response except in 

 a few cases where a minute extension of the tibia may be seen to be asso- 

 ciated with each stimulus. A slow, smooth extension starts at about 15 per 

 second and increases in the rate of extension follow increasing frequency 

 of stimulation up to about 150 per second. The final tension produced dur- 



