BRAIN POTENTIALS AND RHYTHMS INTRODUCTION 



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differential properties of its successive segments — the 

 dendrites, cell body, axon hillock, myelinated axon, 

 naked branches and endings — are determining factors 

 of its electrical behavior. One of the more important 

 contributions of contemporary research is the un- 

 veiling of the distinctive electrogenic properties of 

 dendrites at least in certain specialized neurons ijy 

 studies such as those of Buser (15), Chang (i 7), Clare 

 & Bishop (18), Grundfest & Purpura (30), Lorente 

 de No (41) and Roitbak (47). Dependency of recorded 

 potentials upon morphological characteristics of the 

 neurons was initially recognized by Lorente de No 

 (40) when he made a distinction between neurons 

 generating open fields and those generating closed or 

 semiclosed fields. 



The problems attacked by the microphysiological 

 technique involve the most fundamental operations 

 taking place in the brain, the neuron being considered 

 as a relay, as a focus of integration or as a source of 

 rhythmic activity. All three cases pose the common 

 question of the way in which slow waves — i.e. post- 

 synaptic potentials, after-potentials, autogenic local 

 variations of the resting potential — and spikes or 

 trains of spikes interact with each other. 



Bombardinent by afferent impulses leads to the 

 build-up of slow variations, negative in the case of 

 excitation and po.sitive with inhibition. These slow 

 waves in turn produce, accelerate, slow or suppress 

 efferent impulses. Through these two closely allied 

 reciprocal processes, the neuron performs its ele- 

 mentary functions. From this rather monotonous 

 theme of action, almost infinite varieties of neural 

 behavior arise, determined partly by the intrinsic 

 properties of the neuron and partly by those of its 

 environment, including its connections with other 

 neurons. 



This last consideration draws attention to the notion 

 that in the central nervous system, and especially in 

 the brain, unit activity described in isolation would 

 be nonsense. Simultaneous recordings from single 

 units in different parts of the brain with a inultitude 

 of microelectrodes is a technical achievement that 

 cannot go very far relatively to the number of neurons 

 involved in the simplest operations carried out by the 

 cerebral structures. This brings us to examine the 

 resources of the macrophysiological approach. 



MACROPHYSIOLOGICAL STUDIES 



A priori, the macrophysiological approach can give 

 significant results only when a large mass of neurons 



working in approximate synchrony is activated. 

 Fortunately this can be experimentally induced by 

 application of brief stimuli to nerves and central 

 tracts leading to the brain, or by local stimulation of 

 the cerebral structure themselves. On the other hand, 

 spontaneous synchronizations often occur, which are 

 imperfect and of limited extent in normal conditions 

 but exaggerated and widespread in convulsive states. 

 In any case, synchrony is essentially a feature of the 

 slow components of neuronal activity. Spikes usually 

 appear in complete asynchrony and remain prac- 

 tically undetectable with macroelectrodes, whereas 

 the microelectrode technique is well fitted for spike 

 recording. Thus these two approaches are more or 

 less compleinentary. 



In addition to the basic factors which determine 

 the course of elementary electrogenic processes at the 

 neuronal level, many others come into play and com- 

 bine in various ways to generate the different forms 

 of complex brain potentials, transitory evoked po- 

 tentials, periodic waves, or steady gradients. All the 

 characteristics of a multiplicity of elements — number, 

 density, internal organization and extrinsic relations 

 — take part in the final result but cannot always safely 

 be inferred froin it. 



Chance distribution of elementary properties, such 

 as latencies, excitability levels, is the familiar statistical 

 aspect first to be considered here. Then corne the 

 problems related to the physical conditions of recep- 

 tion: recording may be superficial or deep; electrodes 

 are of various types, numbers and placements; dis- 

 tribution of potentials in a volume conductor of 

 limited extent has its intangible laws which can be 

 applied here only with very crude approximation. 



A further step considers the role of architectonic 

 organization, a factor of primary importance here, 

 for physical as well as for physiological reasons. 

 Laminar, nuclear or reticular structures cannot 

 produce similar electric fields, and the field configura- 

 tion in each particular case depends upon the way in 

 which neurons of different kinds are distributed and 

 oriented within the structure. For instance, surface 

 potentials derived from the cerebral cortex can be 

 thought of as engendered by polarized leaflets, the 

 unit components of which are parallel dipoles formed 

 by the long pyramidal neurons. Synchrony itself is 

 favored by such regularity of internal organization, as 

 a result of a certain congruence between the spatial 

 order existing in the neural structures and that dis- 

 played by the total electric field produced by the 

 active elements of these structures. However, this 

 assumption of a field effect, although very likely in 



