CHAPTER II 



Neuron physiology — introduction 



J. C. ECCLES I Department of Physiology^ Australian National University, Canberra, Australia 



CHAPTER CONTENTS 



Morphological Features of the Neuron 



Physiological Properties of Surface Membranes of Neurons 



Transmission Between Neurons 



Excitatory Synaptic Action 



Inhibitory Synaptic Action 



Factors Controlling Impulse Generation 



Central Inhibitory Pathways 



Inhibitory and Excitatory Transmitter Substances 



MORPHOLOGICAL FEATURES OF THE NEURON 



THE CONCEPT that the nervous system is composed of 

 discrete units or nerve cells was first proposed in 

 1886-7 tiy His and Forel, later it was strongly sup- 

 ported by van Gehuchten and Cajal, and finally in 

 1 89 1 it was given an appropriate nomenclature, 

 ' neuron' and ' neuron-theory', by Waldeyer. Al- 

 though all the great neurohistologists of that classical 

 era were ranged for or against the neuron theory, it 

 was pre-eminently the achievement of Cajal to estab- 

 lish the fact that the functional connections between 

 individual nerve cells, or neurons, are effected by 

 close contacts and not by continuity in a syncytial 

 network, as was proposed in the rival reticular theory 

 of Gerlach and Golgi. Appropriately, Cajal's last 

 great contribution (11) was devoted to a critical sur- 

 vey of the evidence for and against the neuron theory, 

 which has not been seriously challenged since that 

 time, at least for the vertebrate nervous system. 



Neurons have the most diverse forms, yet there are 

 certain features that are common to all. The nucleus 

 always lies in an expanded part, the soma or cell 

 body, from which the axon takes origin and often runs 

 for long distances before breaking up into the synaptic 



terminals that make contact either with other neurons 

 or with effector cells such as muscles, glands or elec- 

 tric organs. Under physiological conditions of opera- 

 tion, axons (with the exception of primary afferent 

 axons) transmit impulses only in the centrifugal 

 direction and thus constitute the effector apparatus of 

 the nerve cell. The different types of nerve cells show 

 much more variation in their other branches, the 

 dendrites, which normally share with the soma the 

 receptive function for the nerve cell. Pyramidal cells 

 of the cerebral cortex and the Purkinje cells of the 

 cerebellum have the most extensively branched den- 

 drites, but most neurons of the central nervous system 

 have fairly elaborate dendritic structures. By contrast, 

 in the dorsal root ganglion cells the receptive structure 

 is remotely located in the receptor organs which are 

 connected to the soma by a long axon-like fiber that 

 normally conducts in the centripetal direction, and 

 which we may call the primary afferent axon. 



\'ery great functional significance is attached to the 

 surface membrane of the neuron. This membrane 

 must not be confused with the fibrous, glial and 

 myelin structures which contribute a sheath to 

 neurons, providing them with mechanical strength 

 and electrical insulation. Until the advent of electron- 

 microscopy the surface membrane had not been ob- 

 served directly; yet it was an essential postulate in 

 explanations of the electrical properties of the surface 

 of the neuron and of the manner in which its interior 

 was maintained at a very different compo.sition from 

 the exterior, particularly in respect to such ionic 

 species as sodium, potassium and chloride. It also 

 provided a structural basis for explaining such funda- 

 mental processes as the conduction of the impulse and 

 the operation of excitatory and inhibitory synaptic 

 junctions. Recently, numerous electronmicroscopic 



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