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Historical view on the attempts to understand the function of sleep in the school of Ivan Pavlov and his Russian forerunners and followers

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We reviewed the ideas of Ivan Pavlov and his Russian forerunners (Ivan Tarkhanov and Maria Manaseina) and followers (Nikolai Rozjanskiy and Konstantin Bykov) on the functional role of sleep. This analysis led to the conclusion that the state of sleep is connected with functional operations that have not been considered in the past and are also not being investigated in present neuroscience. Thus, a real understanding of the function of sleep may only come with a new neurophysiological paradigm.
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Article
Historical view on the attempts
to understand the function of sleep
in the school of Ivan Pavlov and his
Russian forerunners and followers
Ivan N Pigarev
1
and Marina L Pigareva
2
Abstract
We reviewed the ideas of Ivan Pavlov and his Russian forerunners (Ivan Tarkhanov and Maria Manaseina) and followers (Nikolai
Rozjanskiy and Konstantin Bykov) on the functional role of sleep. This analysis led to the conclusion that the state of sleep is
connected with functional operations that have not been considered in the past and are also not being investigated in present
neuroscience. Thus, a real understanding of the function of sleep may only come with a new neurophysiological paradigm.
Keywords
Sleep, sleep deprivation, Tarkhanov, Manaseina, Pavlov, Rozjanskiy, Bykov
Despite the fact that sleep occurs in all organisms with strict
periodicity and occupies an essential part of their lives, the
functional purpose of sleep remains a matter of controversy.
Thousands of studies performed over the past century have
not been able to solve this problem. Undoubtedly, there must
be a reason for this. It is very likely that sleep is associated
with the manifestation of certain key mechanisms that were
not previously encountered in physiological practice and
therefore not considered by modern neuroscience.
We propose that by remembering the “well-forgotten
old,” referring to the history of the development of hypoth-
eses about the phenomenon of sleep, it would be possible to
better assess trends in the development of scientists’
assumptions regarding the functional purpose of this state
of organisms. This may also indicate the likely direction of
further progress toward solving this problem.
In this article, we offer a brief overview of the works and
ideas of the Russian scientist Ivan Pavlov and his prede-
cessors and followers. In undertaking this review, we rea-
lized that these important studies were practically unknown
at present, even in Russia.
Ivan R Tarkhanov—the forerunner of
Russian somnology
Although few of Tarkhanov’s publications survive, in 1879
he posed the question, “Why should a person sleep if the
brain continues to work even more intensely in sleep than
in wakefulness?”
1
If we take into account that at that time
nothing was known about the functions of neurons, and that
the first penetration of microelectrodes into the brain of a
sleeping animal, which confirmed the validity of the state-
ment about the brain’s work during sleep, did not occur for
another 70 years, this idea looks prophetic. From another
source, it becomes clear what I Tarkhanov actually had in
mind. He (Figure 1) wrote,
The centers of breathing and circulation located in the brain do
not sleep; the centers of speech do not sleep, because in dreams
one may talk; the centers of attention do not sleep; hearing,
smell, do not sleep. Finally, the cerebellum doesn’t sleep, as
evidenced by the wonders of balancing acts manifested by
sleepwalkers. So then, what is sleep? Only the centers in which
1
Institute for Information Transmission Problems, Kharkevich Institute,
Russian Academy of Sciences, Moscow, Russia
2
Institute of Higher Nervous Activity and Neurophysiology, Russian
Academy of Sciences, Moscow, Russia
Corresponding author:
Ivan N Pigarev, Institute for Information Transmission Problems,
Kharkevich Institute, Russian Academy of Sciences, B. Karetniy side
street, 19, Moscow. 127994, Russia.
Email: pigarev@iitp.ru
Clinical & Tra nslational Neuroscience
January–June 2019: 1–6
ªThe Author(s) 2019
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our consciousness exists are inactive during sleep. All other
parts of the brain work and even more intensively than during
the day. (cf. Shoyfert
2
)
In a subsequent publication, I Tarkhanov wrote that it is
quite possible that the weakening of a number of functions
during normal sleep is due to the depressing effect of the
brain on the activity of automatic or reflex centers regulat-
ing these movements.
3
This idea found its development in
the theory of “diffuse cortical inhibition” proposed later by
Ivan Pavlov.
4
It is very likely that I Tarkhanov was the first experi-
menter to study sleep mechanisms in insects. He illustrated
the above idea of the depressing effect of the brain on the
activity of the underlying centers by experiments on the
light-emitting apparatus of fireflies. During a trip to Italy,
he noticed that fireflies glowed during active behavior, and
when sleep was observed, the glow stopped. However, if
the heads of the fireflies were removed, the glow did not
stop, but continued for several hours. According to I
Tarkhanov, the reason lay in the removal of “central inhibi-
tion, suppressing luminescence.”
5
However, we should keep
it in mind that in these observations, I Tarhanov did not dis-
tinguish the probable sleep and immobile quiet wakefulness.
The described state of animal was “presumed” to be sleep,
and Tarkhanov’s hypothesis was based on this supposition.
The study of sleep was continued by the pupil of Ivan
Tarkhanov, Maria M Manaseina (Figure 2). Her research,
which laid the foundation for somnology as an independent
branch of physiological science, was described in detail by
V Kovalzon
6
and M Bentivoglio.
7
Experiments of Maria Manaseina—The
first direct demonstration of the functional
role of sleep
As already mentioned, for many researchers, the function
of sleep still remains a mystery, in spite of the fact that a
direct and definite answer to this question was given by
Manaseina
8,9
at the end of the 19th century. She deprived
animals (puppies) of sleep to find out the consequences of
sleep loss. For sleep deprivation, she used a method, which
now we call “gentle handling.” The effect was extremely
clear. Within 5 days, all the puppies that were not allowed
to sleep died. In comparison, other puppies deprived of
food survived up to 3 weeks.
It became clear that a period of sleep was needed pri-
marily for ensuring the viability of an animal’s body.
However, this conclusion was not made at the time,
because M Manaseina herself
8,9
as well as her followers,
Italian researchers M Daddi
10
and G Tarozzi,
11
who
repeated her experiments, and confirmed her results, intui-
tively believed that sleep was primarily necessary to ensure
the functioning of the brain. As a result, they expected to
see the effects of sleep deprivation in changes to brain
structure, and they actually described such changes in the
morphology of some nervous cells.
However, today the methodological approaches, which
were used in their studies, allow us to doubt the reliability
of their observations. This doubt was especially strength-
ened after a more recent study performed in rats. It was
found that the rats also died after total sleep deprivation,
but there were no obvious morphological changes to the
cortical neurons after sleep deprivation.
12
Although I Pavlov and scientists of his school most
likely knew about the data obtained by M Manaseina, the
main conclusion concerning the importance of sleep first of
all for viability of the body, but not the brain’s functioning,
Figure 1. Member of the Russian Academy of Sciences, Prince
Ivan Romanovich (Ramazovich) Tarkhanov (1846–1908).
Figure 2. Maria Mikhailovna Manaseina (born Korkunova)
(1841–1903).
2Clinical & Translational Neuroscience
was not done by them. Intuitive belief in the importance of
sleep to ensure brain performance continued to dominate
the thinking of physiologists. This gives good support to the
idea that scientific reality is often counterintuitive (see e.g.
other examples in a recent theoretical analysis of the stud-
ies of local field potentials.
13
).
The views on sleep function in the time of
Ivan P Pavlov
It is known that in the 19th century, physiology was mainly
aimed at studying visceral functions, and data were accumu-
lated during acute experiments on isolated organs, or later on
animals under anesthesia. Naturally, in this work, no atten-
tion was paid to sleep. In the laboratory of Pavlov (Figure 3),
due to a number of new experimental techniques, the inves-
tigation of the digestive system in behaviorally active ani-
mals was undertaken. A study of digestion, and particularly
the role of the nervous system in this process, brought Ivan
Pavlov to worldwide recognition. In the course of these
studies, the phenomenon of conditioned reflex was discov-
ered. His classical studies of salivation in dogs became a
powerful method in studies of the nervous system.
But, as often happens in science, the new method was
ahead of the level of understanding of the possible princi-
ples underlying brain functioning. The general approach to
brain function was largely based on ideas obtained during
investigations of the neuromuscular apparatus. The pattern
of activity of entire structures of the brain could be
described in terms of tonic or phasic activation or inhibi-
tion, and these conclusions were made by observations of
complex behavioral acts.
Decades separated these first experiments from the cre-
ation of information theory and the development of
principles of universal computers. As a result, the under-
standing of the brain as an organ for information processing
by physiologists was delayed.
The mechanism of the conditioned reflex was consid-
ered at that time as a mechanism of a huge “logical” com-
plexity, which required a large “brain” mass. On the other
hand, it was proposed at that time that this mechanism
alone would be able to provide all forms of complex beha-
vior, including consciousness and memory. As a result, it
was natural to accept that it was in the cerebral cortex
where mechanisms of conditioned reflexes related to con-
sciousness were localized and that unconditioned reflexes
or instincts and the sphere of the subconscious were asso-
ciated with subcortical structures.
Soon after the beginning of intensive work on behavio-
rally active animals, experimenters at the Pavlov laboratory
encountered a phenomenon that became a serious obstacle
to research: as a result of prolonged monotonous experi-
ments, animals began to fall asleep, and the realization of
conditioned reflexes ceased.
14
Pavlov entrusted the study
of this developing sleep, which interfered with their work,
to his graduate student Nikolai Rozjanskiy (Figure 4). In
1913, Rozjanskiy defended a thesis, “Materials for the phy-
siology of sleep,” which in 1954 was included in a book.
15
In the first chapter of the thesis, N Rozjanskiy compares
three rather similar states, natural sleep, hypnosis, and
hibernation. It is interesting that in the early work on the
study of hibernation, an increase in body mass of the
experimental animals was noticed, despite the fact that
animals did not eat during the entire period. This fact was
explained by the transition of fat to glycogen with the
inclusion of oxygen molecules from the air.
In the third chapter, Rozjanskiy formulated his point of
view on the mechanism of sleep. He recognized the cortical
Figure 3. Member of the Russian Academy of Sciences, Ivan
Petrovich Pavlov (1849–1936).
Figure 4. Member of the academy of medical sciences of the
USSR, Nikolai Appolinarievich Rozjanskiy.
Pigarev and Pigareva 3
inhibitory mechanism of sleep and rejected the idea of a
special sleep center. I Pavlov also always sharply opposed
the idea of the existence of such a center.
4
It should be noted
that I Pavlov and also N Rozjanskiy were aware of the
anatomical studies of von Economo, who described local
degenerations in the brain stem and hypothalamus in patients
who died from lethargic encephalitis.
16
But they believed
that as a result of such degenerations, the afferent pathways
reaching the cortex were broken. This, in turn, disrupted the
cortical inhibitory processes responsible for the onset of
sleep. In this explanation, one can see the prototype of the
idea of the brainstem-activating system that arose later. The
variety of subcortical structures that have been discovered
since that time, the activation of which leads to falling asleep
or awakening,
17
rather confirms the idea of Pavlov, shared
by Rozjanskiy, about the absence of a single center of sleep.
Further, N Rozjanskiy recognized that not all observa-
tions at the time could be explained from the position of
diffuse cortical inhibition and concluded:
The question of sleep and wakefulness needs to be discussed as
a specific question of physiology. Its analysis must be based on
three aspects:
1. The recognition that states of sleep and wakefulness
are both active.
2. The basis of the sleep–wakefulness behavior as an
adaptation to daily changes in illumination.
3. The use of Pavlov’s doctrine about diffuse cortical
inhibition.
In subsequent chapters, N Rozjanskiy describes the
results of experiments made in 1913 to study the influence
of sleep on the realization of conditioned reflexes. The
thesis had 36 conclusions. Some of them gave recommen-
dations on how to deal with animals’ sleep during the
course of experiments. The final conclusion was, “the con-
ditions that underlie the periodicity of sleep, still cannot be
considered as clarified.”
N Rozjanskiy made another important observation. He
noticed that during development of sleep conditioned
reflexes to somatosensory stimuli disappeared first while
reflexes to auditory and visual stimuli continued to work.
He concluded that sleep spread from the somatosensory
areas of the cortex. It seems that in Pavlov’s lab, the
dynamics of sleep and the possibility of localized sleep in
separate cortical zones were considered as entirely natural
phenomena, and not worthy of special discussion or
description.
In Pavlov’s clinical observations,
18
we found a case
where, after the presentation of a patient with narcolepsy,
who during cataplexy, could not see anything, but heard
everything, I Pavlov noted that the patient was different
from normal in that he could not see. This means that the
work of the large hemispheres is limited and only certain
regions continue to work. Partial drowsiness develops.
The phenomena of sleep dynamics and the possibility of
local or partial sleep were reopened at a new technical level
in the 1990s and have now become one of the popular areas
of research. The overview of local sleep studies was pre-
sented in our recent article.
19
Unfortunately, at the time of
preparing the previously published review, we did not
know that this phenomenon was first described at the very
beginning of the 20th century.
Last behavioral study of sleep before the
era of electrophysiology
It seems that in the school of I Pavlov, it was understood
quite well that the theory of sleep as a diffuse inhibition has
a number of contradictions and that this theory could not
explain many of the phenomena observed both in life and in
experiments. But nothing better could be offered at that
time. In the late 1930s, in the laboratory of Konstantin
Bykov (Figure 5) in Leningrad, another attempt was made
to study sleep experimentally.
20
However, the authors of this work did not formulate any
new hypotheses, which they would like to test. In the intro-
duction, they declared,
It seemed to us important to continue the collection of material
characterizing the state of the animal during prolonged sleep
deprivation. We were particularly interested in changes in the
activity of the central nervous system during prolonged insom-
nia and during the return of the animal to normal conditions.
The term “insomnia” used by the authors is equivalent to
the modern term “sleep deprivation,” that is, deprivation of
sleep caused by artificial methods.
Figure 5. Member of the academy of sciences of the USSR,
Konstantin Mikhailovich Bykov (1886–1959).
4Clinical & Translational Neuroscience
In essence, their work was a repetition of the experi-
ments of M Manaseina.
9
However, the study, on adult dogs,
in the laboratory of K Bykov was carried out using new
methods. Five dogs were used in this study. A large number
of experimenters took part in the experiments, and special
attention was paid to ensuring animals did not remain unob-
served and could not fall asleep. Sleep deprivation was
maintained “to the last opportunity” either until it was not
possible to wake up the dog or when problems with the
gastrointestinal tract, heart, or breathing reached critical
levels for survival. It was noted that continued deprivation
of sleep would certainly lead to the death of animals within
the next few hours. But before this happened, the dogs were
allowed to fall asleep and experimenters carefully moni-
tored the dynamics of recovery.
In the course of sleep deprivation, unnatural postures
appeared in the animals. Their legs became stiff, and ani-
mals often fell down. Dogs could take food in their mouths
and then, without chewing, spit it out. Salivation in
response to food was not observed; in the late stages of
deprivation, dogs did not react even to sausages. Growing
aggressiveness was reported, especially in relation to other
dogs. With the cessation of deprivation, dogs immediately
fell asleep in any position of the body and only with addi-
tional time did they adopt a regular sleeping position. Sleep
was extremely deep, and the animals did not respond even
to very strong pain stimuli. After a while, reactions to pain-
ful stimuli appeared, but the localization of the pain stimu-
lus was disrupted. For example, when the front paw was
irritated, the dog could start licking a back. This extremely
important observation was completely forgotten, though it
revealed a possible mechanism for the frequent motor
errors of operators deprived of sleep. It was noted that the
pattern of behavior of all dogs in these experiments was
quite similar.
The process of sleep deprivation caused dramatic
changes in physiology. On the second and less often on the
third day of sleep deprivation, a decrease in appetite, diar-
rhea, sometimes appearance of vomiting, profuse saliva-
tion, and “foam from the mouth” were noted. The
temperature of the body and heart rate decreased during
the first two days of sleep deprivation, and the rhythm of
breathing was disturbed. At the end of sleep deprivation,
body temperature started to rise, pulse rate decreased, and
respiration could change to a Cheyne-Stokes pattern, with
pauses in breathing of up to 10 s. After the end of sleep
deprivation, all these changes passed very quickly, and on
following days, the appearance and behavior of dogs did
not differ from normal.
In all dogs, food conditioned reflexes and differentiations
according to the classical method of Pavlov were elaborated.
After the first day of sleep deprivation, all reflexes were
reduced, and the differentiations were disinhibited. From the
second day, all the conditioned reflexes disappeared. The
authors summarize this part of the study as follows:
With prolonged sleep deprivation, there is a sharp drop in
excitability of the cerebral cortex, up to the complete func-
tional shutdown of it. Recovery of excitability of the cortex
does not occur immediately after the cessation of sleep depri-
vation but has a wavelike character.
The authors came to the following conclusion:
At present the above material, as well as all the literature data
does not allow us to state a general and definite view on the
phenomenon of sleep. None of the existing theories can
explain all the facts observed during insomnia. Perhaps it is
impossible to explain all phenomena of sleep by the develop-
ment of inhibition in the cortex. But it seems to us unquestion-
able that it is the primary point.
The experiments in the laboratory of K Bykov were
conducted around the time when electrophysiology was
born. Later, electrophysiological methods made it possible
to elucidate the mechanisms of signal propagation along
nerve fibers and transmission from one nerve cell to
another. It became possible to assess the degree of involve-
ment of a particular brain structure in the analysis of the
incoming signals. Soon it was shown that in the cerebral
cortex during sleep, there is no “diffuse inhibition,” and
cortical neurons during sleep continue to fire actively. But
the obvious conclusion about the crucial role of sleep for
maintaining the viability of organisms, which arose after
the experiments of M Manaseina and the above-described
experiments in the laboratory of K Bykov, again were not
made by scientists.
In the second half of the 20th century, when it became
possible to use a variety of techniques to study the process
of sleep, a huge amount of data were accumulated. How-
ever, we have to admit that, despite the tremendous growth
of factual material concerning the various mechanisms of
sleep at synaptic and molecular level, the general under-
standing of sleep function has not advanced very far since
the time of I Pavlov. But the general view on structural and
functional hierarchy of the brain also remained practically
unchanged. Now, it is becoming clear that most likely
understanding of sleep function is simply impossible within
the framework of old ideas about the organization of the
nervous system and requires their replacement. In turn, the
new results of sleep studies do not fit into the generally
accepted concepts but find a simple and natural explanation
in the framework of other approaches to the organization of
the nervous system.
21
We have plan to discuss all these
questions in our next article.
Conclusion
Historical analysis of opinions concerning the function of
sleep is helping to assess trends in this field of science and
provide the direction of further progress toward the solution
of this problem. This analysis allows us to conclude that
Pigarev and Pigareva 5
sleep is associated with the implementation of functional
operations that have not been examined in the past nor
considered in modern neuroscience. Thus, we can expect
that a real understanding of the function of sleep will only
come with a new neurophysiological paradigm.
Acknowledgements
We are very thankful to Dr Mikhail Poluectov for the suggestion
to write this article, to Irina Surikova for preparation of the illus-
trations, and to Dr Denys Garden for critical reading of the manu-
script and helpful comments.
Declaration of conflicting interests
The author(s) declare no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
work was partly supported by the Russian Foundation of Basic
Research grants no. 19-04-00215 and 18-013-00597.
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6Clinical & Translational Neuroscience
Chapter
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Inability to solve complex problems or errors in decision making is often attributed to poor brain processing, and raises the issue of brain augmentation. Investigation of neuronal activity in the cerebral cortex in the sleep-wake cycle offers insights into the mechanisms underlying the reduction in mental abilities for complex problem solving. Some cortical areas may transit into a sleep state while an organism is still awake. Such local sleep would reduce behavioral ability in the tasks for which the sleeping areas are crucial. The studies of this phenomenon have indicated that local sleep develops in high order cortical areas. This is why complex problem solving is mostly affected by local sleep, and prevention of local sleep might be a potential way of augmentation of brain function. For this approach to brain augmentation not to entail negative consequences for the organism, it is necessary to understand the functional role of sleep. Our studies have given an unexpected answer to this question. It was shown that cortical areas that process signals from extero- and proprioreceptors during wakefulness, switch to the processing of interoceptive information during sleep. It became clear that during sleep all "computational power" of the brain is directed to the restoration of the vital functions of internal organs. These results explain the logic behind the initiation of total and local sleep. Indeed, a mismatch between the current parameters of any visceral system and the genetically determined normal range would provide the feeling of tiredness, or sleep pressure. If an environmental situation allows falling asleep, the organism would transit to a normal total sleep in all cortical areas. However, if it is impossible to go to sleep immediately, partial sleep may develop in some cortical areas in the still behaviorally awake organism. This local sleep may reduce both the "intellectual power" and the restorative function of sleep for visceral organs.
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Sleep deprivation leads to cognitive impairments in humans and, if sustained for 2–3 weeks in rats, it is invariably fatal. It has been suggested that neural activity associated with waking, if it is not interrupted by periods of sleep, may damage brain cells through excitotoxic or oxidative mechanisms and eventually lead to cell death. To determine whether sustained waking causes brain cell degeneration, three parallel strategies were used. The presence and extent of DNA fragmentation was analyzed with the TUNEL technique on brain sections from rats sleep deprived for various periods of time (from 8 h to 14 days) and from their respective controls. Adjacent sections from the same animals were stained with a newly developed fluorochrome (Fluoro-Jade) specific for degenerating neurons. Finally, total RNA from the cerebral cortex of the same animals was used to determine whether the expression of several stress response genes and apoptosis-related genes is modified after sustained waking. In most long-term sleep deprived rats only a few scattered TUNEL positive nuclei (1–3) were found in any given brain section. The overall number, distribution, and morphology of TUNEL positive cells in long-term sleep deprived rats did not differ significantly from yoked controls, short-term sleep deprived rats, and sleep controls. No evidence of degenerating neurons as detected by Fluoro-Jade was found in any experimental group. mRNA levels of all the stress response genes and apoptosis-related genes tested did not differ between long-term sleep deprived rats and their yoked controls. These results argue against the hypothesis that sustained waking can significantly damage brain cells through excitotoxic or oxidative mechanisms and that massive cell death may explain the fatal consequences of sleep deprivation.
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The experimental studies on sleep deprivation were initiated by the Russian physician and scientist, Marie de Manacéine, who studied sleep-deprived puppies kept in constant activity. She reported in 1894 that the complete absence of sleep was fatal in a few days, pointing out that the most severe lesions occurred in the brain. In 1898, the Italian physiologists Lamberto Daddi and Giulio Tarozzi also kept dogs awake by walking them; the animals died after 9-17 days, and their survival was unrelated to food consumption. In the histological study performed by Daddi, degenerative alterations, mainly represented by chromatolytic changes, were observed in neurons of the spinal ganglia, Purkinje cells of the cerebellum, and neurons of the frontal cortex. Daddi ascribed these changes to a state of autointoxication of the brain during insomnia. In 1898, the psychiatrist Cesare Agostini, interested in the psychic phenomena caused by prolonged insomnia in humans, sleep deprived dogs by keeping them in a metallic cage in order to avoid fatigue. The dogs survived about 2 weeks, and degenerative changes were observed in their brains. In these experimental paradigms, the effect of sleep loss was confounded by motor exhaustion and/or intense sensory stimulation. In spite of the absence of adequate controls, the pioneering studies performed at the end of the 19th century represented the first experimental attempts to relate sleep with neural centers and suggested that sleep is a vital function and that the brain may be affected by insomnia.
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At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.
Observations and experiments on the light emitted apparatus of the Italian lighting beetles. Tr. ob-va rus. vrachey, S.Pb
  • I R Tarkhanov
Tarkhanov IR. Observations and experiments on the light emitted apparatus of the Italian lighting beetles. Tr. ob-va rus. vrachey, S.-Pb. Proceedings of Russian Medical Society, 1894-1895, pp. 104-112 (Russian).
Experiments on naturally sleeping animals
  • I R Tarkhanov
Tarkhanov IR. Experiments on naturally sleeping animals. Zdorovie 1879; 121: 371-372 (Russian).