The central nervous system occupies the centre of study of the dynamics of the disintegration and restoration of various physiological functions. In all forms of dying, we know, this system is affected first and most seriously. The general pattern, as mentioned above, with few exceptions, is consecutive discontinuation of the functioning of its various systems, beginning with the newest and most complex, to the older and simpler. In this case the philogenetically increased vulnerability of young formations is particularly obvious.
During the earliest stage of the extinction of vital functions, for example during haemorrhage or anoxia, there is a marked activation of the cerebral cortex, expressed in desynchronization of the EEG, and the simultaneous development of motor excitement, agitation and panting, quickening of the pulse, a gradual rise of blood pressure, spasm of the peripheral vessels and dilation of the capillary system of the brain. The reflex that begins in the sinocarotid receptor zone and closes through the brain stem and its reticular formation, in particular, plays an essential role in activation of the cortex at this point.
The reaction has a compensatory nature, its purpose being to preserve the functions of the higher sectors of the brain.
In the process of dying, however, it is not solely the increase in functions that is of basic importance, but rather their inhibition that follows. Indications of the earlier discontinuation of higher nervous activity are the total disappearance of conditioned motor reflexes in dogs and of consciousness in humans. The second signalling system is particularly sensitive to hypoxia; it is disturbed more severely from pathological effects of any kind and inhibition develops in it earliest of all.
The early discontinuation of cortical functions during dying is confirmed by the EEG data, which indicate the absence of cortical bioelectric activity while subcortical formations still retain their functions.
As the higher sectors of the brain cease to function decerebrate rigidity, tonic spasms, tension of the limbs, and involuntary defaecation and urination often develop.
The extinction of the cortex, and subsequently of the subcortical nuclei and of the diencephalon, leads to liberation of brain stem systems that had previously been subordinated to the higher sectors of the brain. As a result of this liberation and the preservation of reflex and direct influences on the stem, excitement within it increases sharply. The excitation of the reticular formation of the medulla oblongata and of the nuclear formations intimately associated with it is particularly marked during agony in dying due to severe loss of blood.
At this time blood pressure and cardiac activity increase, the previously extinguished EEG is restored and even some conditioned reflexes, and in man occasionally consciousness, all this in the presence of agonal breathing. But the continuing action of the fatal factors leads in the long run to inhibition of the processes of central regulation and of the compensatory mechanisms in the central nervous system.
Thus, the pattern of the extinction of brain functions is basically determined by their relative complexity and dependence on sectors of the central nervous system more or less sensitive to hypoxia, and by the form of anoxia of the brain and its rate of development. It has been established that, for man, and apparently for the higher animals as well, the cerebral cortex primarily ensures the most complete inclusion of all the compensatory mechanisms to combat the developing process of dying. As the more complex and vulnerable systems are consecutively extinguished, liberation of the lower, normally partially suppressed, functional systems becomes possible.
In that connection extinction of the functions of the central nervous system is not only a phenomenon of lapsing, but also a temporary pathological stimulation, most often expressed in spasms of a decerebrate character and an increase in the activity of one sector or another of the vegetative nervous system. In all cases, however, consciousness disappears first, and the functions of the brain stem last of all.
Some authors point out that the weak electrical activity of the cerebral cortex sometimes observed, and which seems to have been restored during agony, after a spell of total electric silence, is an artefact. The oscillations sometimes observed during this period on the EEG, with the same rhythm as respiration, can, it should be noted, be of a physiological nature. On the pathophysiological plane this apparently incomprehensible phenomenon can be explained by irradiation of stimulation from the medulla oblongata to the subcortical formations and cortex of the brain. Such an ‘artefact’ can be considered as a regular phenomenon, developing in cases of rapid agony, when the medulla oblongata is occasionally able as if to arouse the cerebral cortex.
The dynamics of the restoration of central nervous function depends on the functional relations of the various sectors of the brain suffering from anoxia, the relative degree of damage to them, and the character of the pathophysiological factors affecting the brain in the restorative period. When no large foci of damage develop in the brain, the stem systems are restored first of all and then the subcortical formations and cortex. Last of all psychic functions are restored: thought, emotional control, work habits, and speech. In unfavourable conditions a picture of postanoxial encephalopathia develops, with widely varying forms of expression from slight emotional instability to extreme coma.
The possibility of a relapse in the neurological state after apparently complete restoration of functioning of the organism must be kept in mind. Such a relapse usually occurs after the end of the third day, and is usually either fatal or causes lasting psychoneurological invalidism. Control of the dynamics of the extinction and restoration of functions, and prognostication, are based on detailed neurological and electroencephalographic examinations, which usually, according to the findings of our Laboratory, give reliable results.
The dynamics of cerebral electric activity during dying of the organism and its resuscitation is a complex and urgent problem of great practical as well as theoretical interest. It is closely linked to that of the effect of hypoxia on cerebral biopotentials, revealed and studied in detail by a number of authors. Further investigation along these lines brought out a need for deeper study of several concrete problems, primarily discovery of the forms and stages of the changes in bioelectric activity during dying and resuscitation and their comparison with the dynamics of metabolic disturbances in the brain, and investigation of the role of various subcortical structures in the genesis of disturbances of cerebral electrical activity.
Study of these problems in our Laboratory enabled us to accumulate and systematize much factual material and to draw a series of conclusions. It was shown that, under the influence of severe hypoxia during dying, the EEG generally reflected regular stadial changes, namely: (1) latent period; (2) a stage of activation of electrical activity; (3) a stage of progressive slowing down and synchronization of biopotentials; and finally (4) a stage of the gradual disappearance of all forms of electrical activity, at first in the cortex and subcortical formations, and then in the stem sectors of the brain.