150 INVERTEBRATE PHYSIOLOGY 



concept is the fact that, when a drug is found effective in a nerve-muscle 

 preparation, it can be shown that in most instances it has a similar effect 

 when applied to the axon alone (Ellis, Thienes, and Wiersma, 1942). 

 According to Florey and Florey (1954) there is evidence that, in double 

 motor-innervated muscles of the crayfish, acetylcholine is the transmitter 

 of the fast system, while 5-hydroxytryptamine would bring the slow sys- 

 tems into action since these drugs caused, on perfusion, contractions of 

 different types. It was, however, not shown that these effects were not due 

 to stimulations of the axon branches outside the muscle fibers. 



Since it is well known that both fast and slow systems can be inhibited 

 by the same inhibitory fibers and that the same holds true for muscles with 

 four motor fibers, a very interesting field of research offers itself. 



Florey (1954) has since withdrawn the claim that 5-hydroxytrypta- 

 mine is the transmitting substance for the slow contractions. This sub- 

 stance causes stimulation of sensory end organs and their sensory nerves, 

 which in turn stimulate in some way the motor nerves. The present re- 

 viewer considers it likely that this stimulation is due to an ephaptic trans- 

 mission process, which takes place in a region where the slow motor axons 

 are hyperexcitable because of an existing demarcation potential. This will 

 be normally near their cut end. 



Investigations in Molluscs 



The physiology of neuromuscular transmission in molluscs still suffers 

 from the uncertainty caused by the fact that histological methods have not 

 yet shown conclusively where peripheral ganglion cells are present, inter- 

 calated between the motor fibers of the main nerves and their endings. A 

 very recent preliminary publication (Bowden and Lowy, 1955) reports the 

 presence of nerve cells in all lamellibranch muscles so far examined, among 

 which are muscles previously believed to be free from such cells. It will 

 have to be proven that these nerve cells and their synapses are situated in 

 the motor pathway ; but it will be necessary to consider this possibility in 

 evaluating the physiological data, until a definite answer is obtained. 



In the adductor muscles of Anodonta, Barnes ( 1955) has found evidence 

 that the fast part of the muscles is innervated by nerve fibers which behave 

 as typical motor axons, causing a relatively quick contraction and relaxa- 

 tion (it is, however, of importance to keep in mind that van Overbeek, 

 1931, observed that the fast part of this muscle could be brought to con- 

 traction by quick stretch after its isolation from the ganglia) . Although the 

 slow part of the muscle could be made to contract by nerve impulses, its 

 relaxation would depend on an active process through the medium of in- 

 hibitory fibers. This view corresponds well with one held for the slow part 

 of the adductor muscle of Pecten by Benson et al. (1942), who showed 



