796 



Comparative Animal Physiology 



Many invertebrate ganglia are likewise initially stimulated by high potas- 

 sium or low calcium (see Table 69, Ch. 15, for heart ganglia). In crayfish 

 abdominal ganglia in situ the effect of high potassium is to depress spon- 

 taneous activity, whereas low potassium increases it. After a half hour of 

 bathing in saline the reverse effects of potassium are observed.^^^^- ^^^' ^^ Ad- 

 dition of acetylcholine to the bathing solution prevents the reversal in potas- 

 sium action and suggests that differences in behavior of intact and isolated 

 ganglia may result from loss of ACh on isolation.'^^^ Transmission across 

 synapses of the cockroach is remarkably unaffected by changes in potassium 

 and calcium.^^^ 



tgc-:-_-_--^ 



(b) 



Fig. 298. Current flow at a synaptic junction; presynaptic fiber represented by vertical 

 block and postsynaptic neurone by horizontal line. Current flow with presynaptic 

 impulse {a) approaching synapse and (h) at synapse. Focal anode (Ai) followed by focal 

 cathode (Ca). Cross hatching, region of the presynaptic impulse. Current flow repre- 

 sented as toward region of negativity. From Eccles."" 



ELECTRIC CURRENT. The action potential wave of the discharging axons 

 may itself be an adequate stimulus for the receiving neurones. An electric 

 shock of time relations similar to the action potential is an adequate nerve 

 stimulus. In the oculomotor nucleus of the rabbit a volley of impulses in 

 ascending axons, subthreshold for the motoneurones, can sum with a sub- 

 hminal electric shock applied directly to the motor cells during the rising 

 phase of the action potential.-^- When two nerves are placed in proper con- 

 tact side by side, an impulse can be transmitted from fibers of one ner\'e to 

 the other. Transmission in such an artificial synapse, or ephapse, occurs if 

 the impulse in the pre-ephaptic fiber stops at the junction so that the final 

 phase of its potential change is negative or catelectTotonic.^^ Delays vary 



