244 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



nism for the transmitter. Spontaneous miniature 

 e.p.p.'s have not, however, been oljserved (see Chap- 

 ter VII). Facihtation, which at the vertebrate endplate 

 is also a property of the transmitter-release mecha- 

 nism (i6), is usually present and often striking at crus- 

 tacean nerve-muscle junctions (42, 83). 



The chemical identity of the crustacean neuro- 

 muscular transmitter(s) is unknown. That it is not 

 acetylcholine or a closely related compound seems 

 \ ery likely from a number of experiments (3, 1 7, 40). 

 Wright (88) was able to obtain blocking in the cray- 

 fish closer-muscle with curare and dihydroerythroidin, 

 but perfusion with solutions containing approximately 

 io~^ and 5 X lo""* gm per ml, respectively, was neces- 

 sary. Other physiologically active compounds that 

 have been found to have little or no effect on crus- 

 tacean neuromuscular junctions are trimethylamine, 

 trimethylaminoxide, tyraminc (40), choline (see, 

 however, 20 and below), mecholsl, carbachol, musca- 

 rine, strychnine, pilocarpine, dioitalin, epinephrine, 

 nicotine, caffeine and rotenone (17). Although a num- 

 ber of drugs, such as local anesthetics and veratrine, 

 were found to affect the response of the nerve-muscle 

 system, their action was probably on the nerves for 

 the most part. 



THE MUSCLE SPIKE. As mentioned above, an e.p.p. of 

 sufficient size will, at least in some fibers, evoke an 

 additional membrane response, the spike potential. 

 This spike differs in several aspects from, for example, 

 the conducted action potential of frog twitch muscle 

 fibers. Although it may overshoot the resting poten- 

 tial, the inside of the muscle fiber becoming rela- 

 tively positive during its peak, often this overshoot is 

 absent. The spike is commonly not all-or-nothing, its 

 height varying continuously with the size of the de- 

 polarization (20) or e.p.p. (26) evoking it. It may be 

 nonpropagated and, when conduction occurs, the 

 velocity is low (25 to 40 cm per sec). A striking 

 property of the spike was discovered (20) during an 

 attempt to replace the sodium ions of the external 

 fluid by an 'inert' cation. Substitution of choline for 

 the sodium unexpectedh- resulted in an increase in 

 the size and duration of the spike. Fibers which had 

 previously shown only local spikes exhibited large 

 conducted ones in the choline medium. Even more 

 striking increases were oijtained with other quar- 

 ternary ammonium compounds (e.g. tetraethyl- and 

 tetrabutylammonium). With the latter (TBA) in- 

 creases in spike duration of several hundred times were 

 common and action potentials lasting up to 18 sec. 

 were observed. The TBA effect was irreversible and. 



surprisingly, even after a preparation which had 

 been exposed to this drug was washed with a solution 

 containing no .sodium and no TBA (but with sucrose 

 and excess magnesium) large long-lasting spikes were 

 still observed. Experiments to determine if it were 

 the ammonium compounds themselves which were 

 carrying the current during the spike were incon- 

 clusive. It was noticed that during prolonged ex- 

 posures to TBA the resting potential decreased, ac- 

 companied, however, by an increase in membrane 

 resistance. The possibility was therefore considered 

 that TBA reduces potassium conductance. Although 

 such an assumption could help to explain the enor- 

 mous prolongation of the spike, the identity of the ion 

 carrying the inward current during the action po- 

 tential is still unknown. - 



PERiPHER.-^L INHIBITION. It is now known that the 

 inhibition described above is mediated by separate 

 peripheral axons which run and branch with the 

 motor axons. A direct demonstration of this was pro- 

 \ided b>- van Harreveld & Wiersma (75) who were 

 able to isolate, as single functioning axons, the mcJtor 

 and inhibitory nerve fibers to several muscles in the 

 crayfish cheliped. It was found that contraction 

 evoked by the first type of axon could be reduced or 

 abolished by concomitant stimulation of the second. 

 The inhibitor was more effective in suppressing slow 

 than fast contractions, and in some muscles the 

 latter were unaffected by inhibitory stimulation (53). 

 A study of the comparative effectiveness of the in- 

 hibitors to various muscles in a number of species 

 was made (81). The results were expressed in terms 

 of the ratio of the frequency of inhibitory-axon to 

 that of excitatory-axon stimulation when the former 

 was just sufficient to suppress all contraction. The 

 most effectively inhibited motor system found was 

 the slow contraction of the bender-muscle of Pachy- 

 grapsis crassipes in which this ratio was about one- 

 third. Fast systems most usually had values of this 

 ratio ai:>ove, slow systems below, unity. 



A surprising result was obtained when the 

 electrical, as well as the mechanical, events were re- 

 corded during inhibition. It was found (44, 53) that 

 contraction could be completely suppressed, while the 

 muscle potentials might be reduced by a variable 

 amount or apparently not at all. The extent to which 

 the e.p.p.'s were affected depended upon the relative 



- Evidence that calcium ions carry this current has recently 

 been obtained by P. Fatt & B. Ginsborg (J. Physiol. 142: 516, 

 >958)- 



