148 



HANDBOC1K OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



b) Polarized ephaptic transmission 

 Evolutionary Aspects of Ephaptic Transmission 

 Quasiartificial Synapses 



CONTRACTION OF A MUSCLE when an apparently un- 

 reactive nerve is stimulated, the problem of trans- 

 mission in its most obvious form, challenged the in- 

 genuitv of early physiologists. Electricity quickly 

 became a relatively familiar force after the invention 

 of the Leyden jar and was inxoked in Galvani's 

 theory (84). Electric fluid supplied from the central 

 nervous system, he said, charged the interior of a 

 muscle as the Leyden jar is charged by an electro- 

 static machine. Contraction was cau,sed by discharge 

 of this electrical fluid when the mu.scle and its nerve 

 were connected by a metallic arc. The 'discharge 

 hvpothesis' formulated by Krause and Kiihne in 

 the i86o's encompassed as well the data obtained 

 in the two decades after the foundation of electro- 

 physiology by du Bois-Reymond and others. "A 

 nerve onlv throws a muscle into contraction by 

 means of its currents of action," said Kiihne in his 

 Croonian Lecture of 1888 (133). This electric theory 

 of transmis.sion (fig. i) was dominant until very recent 

 times (98) despite the questions and doubts raised by 

 du Bois-Reymond himself in 1874 (55), and by Bern- 

 stein in 1882 (20). The former suggested that another 

 inechanism, secretion by the nerve of some chemical 

 agent, might be the cause of neuromuscular excita- 

 tion. 



Transmission in the central nervous system hardly 

 off"ered a problem to the physiologists of the nine- 

 teenth century, chiefly for one reason. Nerve and 

 muscle are distinctly diff"erent tissues performing 

 different functions and obviously joined together at a 

 specialized region, the endplate. Connections between 

 nerve cells, however, were thought to be continuous, 

 the neurofibrils of one penetrating into the body of 

 another. This reticular theory of Gerlach was chal- 

 lenged only at the end of the nineteenth century when 

 His, Kolliker and pre-eminently Ramon y Cajal 

 proposed the neuron theory (169), so named by VVald- 

 eyer. Sherrington, in 1897 (181), applied the term 

 synapse to the region of contact or contiguity at which 

 transmission takes place from the presynaptic nerve 

 cell to another, the postsynaptic cell. The present 

 chapter will use these terms in their general context, 

 including in this sense the neuromuscular and neuro- 

 glandular junctions. 



The occurrence of demonstrai)le barriers at the 

 contacts between neurons, different staining qualities 



B 



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FIG. I. Models for electrical transmission. .-1, B. du Bois- 

 Reymond's 'modified discharge hypothesis' of 1874 for the 

 neuromuscular- junction. A: The current loops produced at a 

 large endplate surface, which is itself not part of the muscle 

 fiber, he thought would cause both anodal and cathodal de- 

 polarizations. The current fields, indicated by the arrows, 

 would thus alternate between excitant and depressant actions. 

 B: du Bois-Reymond suggested that a geometrical arrangement 

 which excited the muscle at a point contact would be more 

 effective. [From du Bois-Reymond (55).] C.- Eccles' model of 

 1946 proposed an essentially similar arrangement. Before the 

 impulse of the presynaptic fiber had arri\ed at the synapse 

 (left), there would be a hyperpol arizing (inward) current flow 

 in the synaptic membrane. When the impulse reached its 

 terminus (right) it would cause depolarization and excitation. 

 [From Eccles (57).] D: Electrical model for inhibitory synaptic 

 effects showing interaction of excitatory (E) and inhibitory (I) 

 synapses. The latter were assumed to be the terminals of a short 

 axon, Golgi II cell which developed a nonpropagating spike at 

 its soma. The anodal focus caused by the I knob was supposed 

 to depress the cathodal excitatory effects of the E knobs. Cur- 

 rent flows are simplified in the diagram, loops which are sup- 

 posed to diminish their excitatory effect are shown only at the 

 edges of each E knob. [From Brooks et at. (28).] 



that indicate histochemical differences between pre- 

 and postsynaptic units and the independent existence 

 of the latter after destruction of the former (i.e. 



