84 



The Conduction of Impulses by Nerves /4 : 5 



animal it is only used in one direction. This limitation is imposed by 

 the synapses between neurons and by their junctions with the sensory 

 receptors. 4 Similar limitations exist for muscle fibers which conduct 

 spike potentials in either direction along the fiber but are only stimulated 

 in life at the junction between the nerve terminals and the muscle. At 



Synapse 



Presynaptic 

 Fiber 



Vesicle Containing 

 Transmitter Molecules 



Postsynaptic 

 Fiber 



Sensitive Area 

 (a) 



Figure 9. (a) Chemical transmission. The incoming spike re- 

 leases packets of molecules which diffuse across synapse to 

 produce local excitation at sensitive areas. Diagrammatic 

 representation, (b) Electrical transmission. With suitable 

 geometry, a large field strength can be created in synapse by a 

 spike potential on fiber A, thereby exciting B. The geometry 

 and synaptic rectification prohibits conduction in the other 

 direction. Diagrammatic representation. 



this point, the muscle fiber has a special structure called an end plate. 

 The neuromuscular junction is homologous to the synapses between 

 neurons ; much of our knowledge of neural synaptic conduction is based 

 on studies of the neuromuscular junction. Accordingly, the term 



4 Synapses are probably not polarized in some invertebrates. 



