HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



closely packed and orderly arranged populations of 

 neurons, has not been universally accepted. Inter- 

 actions can of course also occur through synaptic 

 connections at short distance and these must be taken 

 into account. Most frequently, synchrony in a popu- 

 lation of neurons can be explained by the triggering 

 action of a common pacemaker to which these neurons 

 are linked; it thus depends on the gross connections 

 within the brain or ' tractology'. But the problem 

 remains of how the units in a pacemaker are them- 

 selves synchronized. 



This leads us to a major aspect of brain potentials, 

 their frequent appearance in regularly rhythmic se- 

 quences. The origin of these periodic activities has 

 been the object of discussions and controversies. A 

 pluralistic attitude seems to be the wisest, the rhythmic 

 state being only a formal appearance that may be 

 produced by diverse causes. 



It may first be nothing else than the amplified ex- 

 pression of an elementary autorhythmic property of 

 some neurons whereby they emit pulses or become 

 the site of local oscillatory states. This is so commonly 

 encountered in microphysiological experiments with 

 isolated elements that one can hardly douljt that this 

 mechanism operates at times in the central nervous 

 system. But neurons in the brain never work in isola- 

 tion and the factors, synaptic, electrotonic or ephaptic, 

 intervening in synchronization must aflPect the proper- 

 ties of the autorhythmic generators. It may even be 

 that rhythmicity owes its existence, in many cases, to 

 some particular arrangement of the neuronal connec- 

 tions in the grey matter. Several mechanisms have 

 been proposed which are possible but not definitively 

 demonstrated. For instance, alternating states of 

 excitation and inhibition, with their corresponding 

 opposite electric signs, may appear by virtue of 

 reciprocal connections between the generating 

 neurons, as proposed by Jung (33); or closed chains 

 of neurons, which have been traced through central 

 structures by Lorente de No (41), may open the way 

 to recurring pulses activating a homogeneous pool of 

 neurons. 



It seems more sound to many neurophysiologists to 

 replace these postulated effects of a rigid circuitry by 

 others attributable to the properties of diffuse net- 

 works. Neural nets finelv wo\en with short, inter- 



connected neurons are present almost everywhere in 

 the grey matter. A certain average level of intrinsic 

 activity may be maintained within these structures 

 by an incessant and random circular reactivation of 

 their elements. This results in an asynchronous 

 bombardment of the neurons responsible for re- 

 cordable potentials. The determining factor of 

 periodicity is then the recovery cycle of these neurons. 

 This mechanism, first postulated by Eccles (22, 23), 

 has received strong support from the experiments 

 of Burns on isolated slabs of cortex (11, 12). 



Finally, steady potential gradients within large 

 assemblies of neurons and their slow modifications 

 under certain conditions appear to be correlated with 

 spontaneous or e\oked activities in the grey matter, 

 according to the views of O'Leary and his collabora- 

 tors (28; see Chaper XIII). The correlations may 

 express cause-effect relationships in either direction. 

 For instance, long-lasting after-bursts in isolated 

 slabs of cerebral cortex have been related by Burns 

 (13, 14) to gradients that appear as the consequence 

 of different recovery rates of the resting membrane 

 potentials at the two ends of particular neurons. One 

 more factor capable of inducing rhythmic states has 

 thus been revealed. But, since the pioneer studies of 

 Dusser de Barenne et al. (20, 21), Libet & Gerard 

 (39)' Jasper & Erickson (32), Leao (37) and the 

 recent investigators just mentioned, very few workers 

 have been tempted by the delicate techniques in- 

 volved in direct current recordings. These may how- 

 ever represent the next fruitful ad\ance of brain 

 electrophysiology. 



Brain potentials and their rhythms are the net 

 result of a conjunction of many heterogeneous factors 

 — physical conditions, anatomical organization, statis- 

 tical effects and the differential properties of the 

 neuron segments — implicated in different ways. 

 Consequently, brain potentials are able only to reveal 

 a limited aspect of cerebral activity and must always 

 be suspected of giving a distorted picture of the real 

 events. This is why it is so important to arrive at a 

 better understanding of their elaboration, for, cor- 

 rectly interpreted, they remain the unrivalled signs 

 of what occurs in the intimacy of cerebral tissue and 

 the main basis for explaining brain functions. 



REFERENCES 



I. .\drian, E. D. J. Physiol. 88: uy, 1936. 



■2. .Adrian, E. D. and B. H. C. Matthews. J. Physiol. 30: 



I. 1933- 

 3. Albe-Fessard, D. and P. Buser. J. phsiol., Paris 45: 14, 



1953- 



4. Albe-Fessard, D. and P. Buser. J. physiol., Paris 47: 67, 



'955- 



5. Amassi.an, V. E. Eleclroencephalog. & Clin, .\europhysiol. 5: 



4 "5. 1953- 



6. Bartlev, S. H. Psychol, .\lonogr. 94: 30, 1933. 



