4 : 5/ The Conduction of Impulses by Nerves 85 



"synaptic conduction" is interpreted in this section to include the trans- 

 mission of spike potentials from nerve to muscle. 



Two different modes of synaptic conduction occur: electrical and 

 chemical. These are illustrated in Figure 9. The electrical conduction 

 may be very rare; it has been demonstrated positively only for the "giant 

 synapse" in the crayfish. At this synapse, impulses travel electrically 

 from one axon to another with negligible time delay. Conduction can 

 occur only in one direction. Similar giant synapses in the squid, 

 however, exhibit appreciable time delay and no electrical transfer of 

 charge. Conduction across the squid giant synapses, just as across all 

 vertebrate synapses studied, is mediated by a specific chemical. 



There is no reason to believe that the same chemical is involved at all 

 the synapses lacking direct electrical transfer. On the contrary, there 

 is considerable evidence to indicate that different substances act at 

 different synapses. Most nerve fiber terminals are so small that it is 

 impossible to make direct observations and hence, determine the trans- 

 mitter substance. As a result of the small size, only very small amounts 

 of the transmitting chemicals are necessary. At the neuromuscular 

 junction in vertebrates about 10" 18 moles of acetylcholine (ACh) produce 

 a spike potential. ACh is the only synaptic transmitter substance 

 which has been definitely confirmed. 



In chemical transmission, the substance, as ACh, is released when a 

 spike potential reaches the appropriate nerve fiber terminal. It then 

 diffuses across the synapse. This distance is of the order of a micron 

 or two, and diffusion can occur in a millisecond or two. The diffusing 

 substance is then absorbed at the receiving terminal or motor end plate, 

 where it changes the ionic permeability of the membrane. Finally, the 

 absorbed molecules are enzymatically destroyed. 



Experiments with vertebrate motor end plates have shown that the 

 area sensitive to ACh is extremely small ; it is confined specifically to the 

 outer surface of the motor end plate nearest to the nerve endings. This 

 outer surface may be regarded as a chemoreceptor. Furthermore, these 

 studies showed that ACh does not produce a depolarization of the 

 membrane but increases its permeability to all small cations such as 

 Li + , Na + , and K + . Finally, the ACh is destroyed by a specific protein 

 catalyst, acetylcholinesterase, located in the end plate. 



The response across the synapse is a local response. The spike 

 potential may originate near there as in the case of vertebrate muscle 

 fibers and giant synapses in squid. In contrast, in sensory neurons 

 (whose axon runs towards the cell body) the spike is formed at the distal 

 end of the axon where several fiber terminals join together. In motor 

 neurons, where the local response occurs in dendrites, the transmitted 

 spike potential is formed at or past the nerve cell body. 



