Nervous Systems 797 



with conditions, but may be as brief as 0.1 to 0.3 msec, for certain mam- 

 malian preparations,^'^- or 2 to 5 msec, between squid giant axons."* The 

 excitability of the receiving nerve fiber can be altered by subthreshold im- 

 pulses, and transmission is facilitated when the afferent impulse coincides 

 with oscillations in local graded potentials in the efferent fiber.^^ Similar 

 electrical transmission across points of contact occurs between two separate 

 lobes of the brain of frogs. ^*''^ The current fields set up in nerve centers are 

 such that electrical stimulation could occur at synaptic junctions.^^*' 



Eccles'-^ has analyzed the electrical pattern at a nerve terminal and dem- 

 onstrated its adequacy for stimulation. As an impulse approaches a nerve 

 terminal it sets up an anodal (positive) focus, which, when the peak of the 

 impulse reaches the terminal, reverses (downsweep of potential), causing a 

 cathodal focus with anodal surround on the receiving neurone. Current flow 

 at the cathodal focus causes a local response which depolarizes an area de- 

 pending in size on the current magnitude, and if it is large enough sets off 

 a propagated impulse. 



Fig. 299. Action potenrial from the stellate sympathetic ganglion of a cat (i) before and 

 (it) after curarization. Synaptic potential only remains after curarization. From Eccles.^ 



We are left with numerous agents— acetylcholine, potassium, electric cur- 

 rent—each present and capable of stimulating nerve cells. Whether or not 

 a nerve cell responds is determined by the environment of the cell, and one or 

 several components of this environment may constitute adequate exciting 

 agents.*"- The old distinction between electrical and chemical transmission is 

 superseded by two other hypotheses. One is that axon propagation is elec- 

 trical, involving potassium and other ions, and that at synaptic transmission 

 either chemical agents (ACh or others) are liberated (Dale-Loewi), or that 

 synaptic transmission is also electrical (Eccles). The other theory (Nach- 

 mansohn) states that axon and synaptic transmission are not different, that 

 both are electrical, and that ACh is an essential Hnk in the electrical pic- 

 ture. 



Synaftic and Ganglionic Potentials. The interpretation of potentials re- 

 corded from nerve centers is difficult because of the small dimensions of the 

 separate parts of neurones. Evidence is accumulating that conduction in the 

 different parts of a neurone occurs at different rates. 



SYNAPTIC POTENTIALS. At interneuronic junctions, and presumably asso- 

 ciated with a transmission process, is a synaptic potential analogous to the 

 end-plate potential of muscle (Ch. 16). In sympathetic ganglia, stimula- 

 tion of pre-ganglionic axons sets up a complex propagated electrical response 

 in postganglionic neurones; after curarization this propagated response is 

 replaced by a graded potential which is conducted decrementally away from 



