NEURON PHYSIOLOGY INTRODUCTION 6 1 



FIG. I . Drawings showing dimensions and form of various types of synaptic junctions as revealed 

 by electronmicroscopy. In all transverse sections the presynaptic terminals are sho%vn above and the 

 postsynaptic element below. In addition the presynaptic terminals can be identified by the contained 

 synaptic vesicles. The synaptic cleft is seen as the narrow space between the juxtaposed presynaptic 

 and subsynaptic membranes and is shown communicating at the sides of the synapse with the inter- 

 stitial spaces. A, A large synapse on a motoneuron of the abducens nucleus. [From Palay (67).] B. 

 Synapse in the ventral acoustic ganglion of the guinea pig. [From de Robertis (21).] C. Synapse 

 between red receptor and postsynaptic cell in the rabbit retina. [From de Robertis & Franchi (23).] 

 D, E. Elongated nerve terminal of amphibian muscle as seen from above (Z)) and in transverse 

 section (£). The naturally occurring irregularities of the junctional folds are neglected in order to 

 give a regular geometrical diagram with approximately equivalent dimensions. A junctional fold is 

 shown by a broken line in E. [From data and figures of Couteaux & Taxi (19) and Robertson 

 (70).] 



nals contain the characteristic synaptic vesicles which 



o 



are 300 to 500 A across and which are often ckistered 

 close to the synaptic region. 



The word synapse, as proposed by Sherrington 

 (71), may be applied to the presynaptic terminal with 

 its contained synaptic vesicles, the synaptic cleft of 

 150 to 500 A, and the subsynaptic membrane with its 

 special receptive and reactive mechanism. Later, 

 when the mode of operation of synapses is discussed, 

 it will appear that much of the old morphological 

 characterization of synaptic endings is of little signifi- 

 cance, at least for many types of neurons. Thus the 

 various localizations designated axosomatic, axoden- 

 dritic and axoaxonic would be almost equipotent in 

 their action except for those neurons that have very 

 elongated dendrites, as for example the pyramidal 

 cells of the cortex. Furthermore, there can be little 



significance in the detailed form of synapses as de- 

 scribed bv such terms as hautons lerminaux and en 

 passant, giant club endings, basket-type endings, etc. 

 [cf. Bodian (3)]. 



PHYSIOLOGICAL PROPERTIES OF SURF.iiCE 

 MEMBRANES OF NEURONS 



By inserting an electrode within a nerve fiber or 

 the soma of a neuron and analyzing the potential 

 changes produced by current pulses, it has been 

 shown that the surface membrane has a high electrical 

 resistance, corresponding to its low ionic permeability, 

 and a high electrical capacity, as would be expected 

 for a membrane no more than 50 A thick. The elec- 

 trical resistance shows wide variations with different 



