THE CONDUCTION OF THE NERVOUS IMPULSE 845 



The conductivity of the synapse can be altered not only by the pas- 

 sage of nerve impulses through it, but by many other agents. Certain 

 chemical substances such as strychnine and tetanus toxine have a selec- 

 tive action upon the synapse, lowering the synaptic resistance, and con- 

 verting inhibition into active contraction (see page 941). In those 

 invertebrates which possess an asynaptic nervous system, or nerve net, 

 strychnine is without such effect. Nicotine has a selective action upon 

 the synapses of the sympathetic nervous system, increasing the resist- 

 ance so that impulses are unable to pass (page 895). 



Reflex activity is very easily abolished by lack of oxygen, as will be 

 indicated in the next chapter. In this regard it differs from nerve trunk 

 conduction to a marked degree. Since conduction can be shown to be 

 quite independent of the nerve cell body, it must be the synapses which 

 are rendered impassable by asphyxia. Fatigue occurs readily in re- 

 flex conduction, and must also have its seat in the synapse, since the 

 nerve trunk itself is quite indefatigable. 



The Miyoneural Junction. The synapse is a region of tissue at the 

 junction of two nerve fibers having properties quite distinct from those 

 of the nerve fibers themselves. The myoneural junction, interposed be- 

 tween nerve fiber and muscle is an analogous structure, the properties 

 of which are different from both muscle and nerve. Certain drugs have 

 a selective action upon it, such as curare, which decreases the conductiv- 

 ity of the myoneural junctions of skeletal muscles and thus results in 

 their paralysis. Curare is a poison which is used by certain savages on 

 their arrow heads. Its fatal effects are due to its action in paralyzing 

 the respiratory muscles by blocking the passage of nerve impulses across 

 the myoneural junction. Epinephrine, the secretion of the adrenal glands, 

 has a specific affinity for the myoneural junctions of certain autonomic 

 nerves, exciting the junctional tissue to action similar to that produced by 

 the nerve impulses (see page 776). The most prominent characteristic 

 of the myoneural junction is its resemblance to the synapse. Like it, 

 it is a region in which conductivity is difficult and readily modified, so 

 that it may be the seat of summation, inhibition, and fatigue. 



