NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



245 



times of arrival of the inhibitory and motor nerve 

 impulses. Maximum reduction (to aljout 20 per cent) 

 occurred when the former slightly preceded the 

 latter; and no effect on the e.p.p.'s was seen if the 

 inhibitory impulse arrived much after the excitatory. 

 Thus, it was suggested that inhibition could act in 

 two places: on some process a) between nerve im- 

 pulse and muscle potential, and 6) between muscle 

 potential and contraction. 



More recently, changes in the muscle membrane 

 during inhibition have been studied by Fatt & Katz 

 (21) using intracellular electrodes. They confirmed 

 previous results that the e.p.p. can be greatly reduced 

 during inhibition and that the extent of the reduc- 

 tion depends upon the relative timing of the inhibitory 

 and motor nerve impulses. In order to test for other 

 postjunctional effects of inhibitory impulses, two 

 microelectrodes were inserted into the same muscle 

 fiber, one in a recording circuit for measuring mem- 

 brane potential and the other connected to a current 

 generator for the purpose of altering the level of the 

 membrane potential. Then it was found that inhibi- 

 tory nerve impulses did not result in any detectable 

 postjunctional potential changes if the membrane 

 potential was at a certain level, usually at or near the 

 resting potential; but if the membrane potential were 

 displaced, by passing current through the other intra- 

 cellular electrode, inhibitory nerve impulses were fol- 

 lowed by transient muscle potentials, similar to, but 

 slower than, e.p.p.'s. They were referred to as I-po- 

 tentials and could appear either as transient hyper- 

 polarizations or depolarizations depending upon 

 whether the resting membrane potential had been 

 decreased or increased, respectively, by the current 

 passed through the second microelectrode. That is, 

 the I-potentials were seen as reductions of any dis- 

 placement of the membrane potential from some 

 equilibrium level, usually close to the resting po- 

 tential. These are the effects which would be expected 

 if the event underlying the I-potentials was a tran- 

 sient increase in some fraction of membrane conduct- 

 ance and, in particular, the conductance of some 

 species of ions having an equilibrium potential equal 

 to the membrane potential at which no I-potential 

 appears. K+ or Cl~ might, therefore, be the ions in- 

 volved. Although the conductance change underlying 

 the I-potential does tend to reduce any deviation of 

 the membrane potential (including an e.p.p.) from 

 an equilibrium potential near to the resting potential 

 and would thus serve as an inhibitory mechanism, the 

 effect was found to be insufficient to account quanti- 

 tatively for all of the inhibition actually observed. 



Another mechanism was therefore suggested in which 

 the inhibitory and excitatory transmitter substances 

 would specifically antagonize one another at the 

 receptor sites on the muscle membrane. The original 

 observation that contraction can be abolished without 

 any reduction in the size of the e.p.p.'s still awaits con- 

 firmation using intracellular recording of potential 

 while observing contraction of the same fiber (cf. 18). 



Insects 



There have been fewer physiological studies of in- 

 sect than of crustacean motor systems, but there seem 

 to be many resemblances between the neuromuscular 

 mechanisms of these two groups, as well as some inter- 

 esting differences. Among the latter, one should note 

 the apparent lack of peripheral inhibitory axons in the 

 insects. Also, the histological appearance of the motor 

 ner\e endings can be, at least superficially, different 

 from that in the crustaceans; for in many insect 

 species, rather than continuously tapering to sub- 

 microscopic diiTiensions, the nerve terminals present 

 an enlarged bulbous or conical appearance. Accord- 

 ing to Marcu (52), however, these structures, re- 

 ferred to as Doyere's cones (cf. 25), may only repre- 

 sent a sudden and profuse branching of the nerve 

 ending in which the individual twiglets are not always 

 seen. Marcu also studied a species of orthopteran in 

 which the manner of branching of the nerve was more 

 similar to the situation in crustaceans. Hoyle (35), 

 working with the locust, has observed what may have 

 been the terminal apparatus still attached to the final 

 nerve branch after pulling the latter free from the 

 muscle fiber. The axons, probably beyond the place 

 at which they had entered the sarcolemma, were con- 

 tinuous with a branched claw-shaped structure which 

 spread over an area 20 to 30 y. in diameter. 



The insects also show a difference from the crus- 

 taceans in the gross organization of the muscle. For 

 example, Hoyle (35) observed that the muscle fibers 

 of the locust were organized into muscle bundles 

 each of which received separate nerve and tracheal 

 branches. He referred to these bundles as 'muscle 

 units'. In some muscles this type of arrangement did 

 not seem to signify any fundamental difference from 

 the crustacean situation. For example, the extensor 

 tibia is innervated by three efferent axons and the 

 branches from two or all three of the axons supply 

 each muscle unit. But in the flexor tibia, each of the 

 five or six muscle units was supplied by separate 

 neurons. The fibers of a unit may receive, however, 

 more than one axon. Working with the same muscle 



