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Anesthetic techniques using different substances with anesthetic properties have been used since antiquity. However, even today, it is not known exactly how these anesthetics work, if there is a common site or if their action is multi-level. In this article we present the latest theories regarding the mechanism of anesthetics' action. Thus we will refer to the action of anesthetics on neural networks, we will discuss the fractal theory of consciousness as well as the soliton model and the theory of the action of anesthetics on the cellular cytoskeleton.
Current Anaesthesia & Critical Care, 2000
The mystery behind the possibility of inducing sleep, coma or an anaesthetic state with drugs, stimulated scientists in the 19 th century, and even earlier, to postulate hypotheses about the working mechanisms of these agents. Substantial progress was made at the beginning of the 20 th century when the correlation between lipid solubility of a drug and its anaesthetic potency was shown. This correlation gave rise to many theories based on the disturbance of the lipid bilayer by anaesthetics. This all changed in the 1980s when it became clear that proteins were also affected. The refinement of electrophysiological techniques allowed the properties of different proteins at a cellular level to be studied and directed research towards finding the receptor that could explain the clinically observed phenomena. In the 1990s it gradually became clear that the interaction between anaesthetic agents and proteins was a complex one, and that no protein could be pinpointed as the essential target. At the beginning of the millennium, we face a challenge to integrate this knowledge into the network of subcellular, cellular and regional interactions that form the brain. The progress of this project will very much depend, as in the past, on the development of basic research in other fields.
Different anesthetics are known to modulate different types of membrane-bound receptors. Their common mechanism of action is expected to alter the mechanism for consciousness. Consciousness is hypothesized as the integral of all the units of internal sensations induced by reactivation of inter-postsynaptic membrane functional LINKs during mechanisms that lead to oscillating potentials. The thermodynamics of the spontaneous lateral curvature of lipid membranes induced by lipophilic anesthetics can lead to the formation of non-specific inter-postsynaptic membrane functional LINKs by different mechanisms. These include direct membrane contact by excluding the inter-membrane hydrophilic region and readily reversible partial membrane hemifusion. The constant reorganization of the lipid membranes at the lateral edges of the postsynaptic terminals (dendritic spines) resulting from AMPA receptor-subunit vesicle exocytosis and endocytosis can favor the effect of anesthetic molecules on lipid membranes at this location. Induction of a large number of non-specific LINKs can alter the conformation of the integral of the units of internal sensations that maintain consciousness. Anesthetic requirement is reduced in the presence of dopamine that causes enlargement of dendritic spines. Externally applied pressure can transduce from the middle ear through the perilymph, cerebrospinal fluid, and the recently discovered glymphatic pathway to the extracellular matrix space, and finally to the paravenular space. The pressure gradient reduce solubility and displace anesthetic molecules from the membranes into the paravenular space, explaining the pressure reversal of anesthesia. Changes in membrane composition and the conversion of membrane hemifusion to fusion due to defects in the checkpoint mechanisms can lead to cytoplasmic content mixing between neurons and cause neurodegenerative changes. The common mechanism of anesthetics presented here can operate along with the known specific actions of different anesthetics.
Science, 2008
When we are anesthetized, we expect consciousness to vanish. But does it always? Although anesthesia undoubtedly induces unresponsiveness and amnesia, the extent to which it causes unconsciousness is harder to establish. For instance, certain anesthetics act on areas of the brain's cortex near the midline and abolish behavioral responsiveness, but not necessarily consciousness. Unconsciousness is likely to ensue when a complex of brain regions in the posterior parietal area is inactivated. Consciousness vanishes when anesthetics produce functional disconnection in this posterior complex, interrupting cortical communication and causing a loss of integration; or when they lead to bistable, stereotypic responses, causing a loss of information capacity. Thus, anesthetics seem to cause unconsciousness when they block the brain's ability to integrate information. How consciousness arises in the brain remains unknown. Yet, for nearly two centuries our ignorance has not hampered the use of general anesthesia for routinely extinguishing consciousness during surgery. Unfortunately, once in every 1000-2000 operations a patient may temporarily regain consciousness or even remain conscious during surgery (1). Such intraoperative awareness arises in part because our ability to evaluate levels of consciousness remains limited. Nevertheless, progress is being made in identifying general principles that underlie how anesthetics bring about unconsciousness (2-6) and how, occasionally, they may fail to do so. Cellular actions of anesthetics The cellular and molecular pharmacology of anesthetics has been reviewed extensively (6-8). General anesthetics fall into two main classes: intravenous agents used to induce anesthesia, generally administered together with sedatives or narcotics; and volatile agents, generally used for anesthesia maintenance (Table 1). Anesthetics are thought to work by interacting with ion channels that regulate synaptic transmission and membrane potentials in key regions of the brain and spinal cord. These ion channel targets are differentially sensitive to various anesthetic agents (Table 1). Anesthetics hyperpolarize neurons by increasing inhibition or decreasing excitation (9) and alter neuronal activity: The sustained firing typical of the aroused brain changes to a bistable burst-pause pattern (10) that is also observed in non-rapid-eye-movement (NREM) sleep. At intermediate anesthetic concentrations, neurons begin oscillating, roughly once a second, between a depolarized up-state and a hyperpolarized down-state (11). The up-state is similar to the sustained depolarization of wakefulness. The down-state shows complete cessation of
Anaesthesia and intensive care, 2009
The challenge to achieve a gestalt understanding of general anaesthesia is really dependent upon an understanding of the elusive concept of consciousness. Until very recently, anaesthesia has been understood to depend fundamentally on the lipid solubility of anaesthetic agents, unsurprisingly a misleading view which has followed from the greater simplicity of lipid chemistry compared with protein chemistry and, it is contended here, from a serious misunderstanding of the older experimental data. Nonetheless, because an over-simplistic view of lipids pertains in much pharmacological thinking about anaesthesia, this paper devotes some attention to potentially relevant aspects of lipid function and also to concepts of anaesthesia which are based on the properties of intracellular and extracellular water. It is argued that the more correct pharmacological explanation is likely to be action at hydrophobic sites of crucial functional molecules, most plausibly protein molecules: empirical ...
Anesthesiology, 2006
Journal of mathematical neuroscience, 2015
With the advances in biochemistry, molecular biology, and neurochemistry there has been impressive progress in understanding the molecular properties of anesthetic agents. However, there has been little focus on how the molecular properties of anesthetic agents lead to the observed macroscopic property that defines the anesthetic state, that is, lack of responsiveness to noxious stimuli. In this paper, we use dynamical system theory to develop a mechanistic mean field model for neural activity to study the abrupt transition from consciousness to unconsciousness as the concentration of the anesthetic agent increases. The proposed synaptic drive firing-rate model predicts the conscious-unconscious transition as the applied anesthetic concentration increases, where excitatory neural activity is characterized by a Poincaré-Andronov-Hopf bifurcation with the awake state transitioning to a stable limit cycle and then subsequently to an asymptotically stable unconscious equilibrium state. ...
Anesthesia and consciousness represent 2 mysteries not only for biology but also for physics and philosophy. Although anesthesia was introduced to medicine more than 160 y ago, our understanding of how it works still remains a mystery. The most prevalent view is that the human brain and its neurons are necessary to impose the effects of anesthetics. However, the fact is that all life can be anesthesized. Numerous theories have been generated trying to explain the major impact of anesthetics on our human-specific consciousness; switching it off so rapidly, but no single theory resolves this enduring mystery. The speed of anesthetic actions precludes any direct involvement of genes. Lipid bilayers, cellular membranes, and critical proteins emerge as the most probable primary targets of anesthetics. Recent findings suggest, rather surprisingly, that physical forces underlie both the anesthetic actions on living organisms as well as on consciousness in general.
2011
The complexity, entropy and other non-linear measures of the electroencephalogram (EEG), such as Higuchi fractal dimension (FD), have been recently proposed as the measures of anesthesia depth and sedation. We hypothesized that during unconciousness in rats induced by the general anesthetics with opposite mechanism of action, behaviorally and poligraphically controlled as appropriately achieved stable anesthesia, we can detect distinct inter-structure brain dynamic using mean FDs. We used the surrogate data test for nonlinearity in order to establish the existence of nonlinear dynamics, and to justify the use of FD as a nonlinear measure in the ‡, § Corresponding authors. 113 114 S. Spasic et al. time series analysis. The surrogate data of predefined probability distribution and autocorrelation properties have been generated using the algorithm of statically transformed autoregressive process (STAP). FD then is applied to quantify EEG signal complexity at the cortical, hippocampal and pontine level during stable general anesthesia (ketamine/xylazine or nembutal anesthesia).
2014
Advances in neuroscience have been closely linked to mathematical modeling beginning with the integrate-and-fire model of Lapicque and proceeding through the modeling of the action potential by Hodgkin and Huxley to the current era. The fundamental building block of the central nervous system, the neuron, may be thought of as a dynamic element that is "excitable", and can generate a pulse or spike whenever the electrochemical potential across the cell membrane of the neuron exceeds a threshold. A key application of nonlinear dynamical systems theory to the neurosciences is to study phenomena of the central nervous system that exhibit nearly discontinuous transitions between macroscopic states. A very challenging and clinically important problem exhibiting this phenomenon is the induction of general anesthesia. In any specific patient, the transition from consciousness to unconsciousness as the concentration of anesthetic drugs increases is very sharp, resembling a thermodynamic phase transition. This paper focuses on multistability theory for continuous and discontinuous dynamical systems having a set of multiple isolated equilibria and/or a continuum of equilibria. Multistability is the property whereby the solutions of a dynamical system can alternate between two or more mutually exclusive Lyapunov stable and convergent equilibrium states under asymptotically slowly changing inputs or system parameters. In this paper, we extend the theory of multistability to continuous, discontinuous, and stochastic nonlinear dynamical systems. In particular, Lyapunov-based tests for
Neuropsychologia, 1995
Al~traet-A theory of anaesthesia is presented. It consists of four hypotheses: (1) The occurrence of states of consciousness causally depends on the formation of transient higher-order, selfreferential mental representations, The occurrence of such states is identical with the appearance of conscious phenomena. Loss of consciousness will occur, if and only if the brain's representational activity falls below a critical threshold. (2) Mental representations are instantiated by neural cell assemblies. (3) The formation of assemblies involves the activation of the NMDA receptor channel complex. The activation state of this receptor determines the rate at which assemblies are generated. (4) General anaesthetics have a common operative mechanism: they directly or indirectly affect the function of the NMDA system.
European Journal of Anaesthesiology, 2009
General anaesthesia is administered each day to thousands of patients worldwide. Although more than 160 years have passed since the first successful public demonstration of anaesthesia, a detailed understanding of the anaesthetic mechanism of action of these drugs is still lacking. An important early observation was the Meyer-Overton correlation, which associated the potency of an anaesthetic with its lipid solubility. This work focuses attention on the lipid membrane as a likely location for anaesthetic action. With the advent of cellular electrophysiology and molecular biology techniques, tools to dissect the components of the lipid membrane have led, in recent years, to the widespread acceptance of proteins, namely receptors and ion channels, as more likely targets for the anaesthetic effect. Yet these accumulated data have not produced a comprehensive explanation for how these drugs produce CNS depression. In this review, we follow the story of anaesthesia mechanisms research from its historical roots to the intensely neurophysiologic inquiries regarding it today. We will also describe recent findings that identify specific neuroanatomical locations mediating the actions of some anaesthetic agents. Keywords anaesthetic mechanisms; anaesthetic targets; anaesthetics; ion channels; receptors Introduction`H ealthy discontent is a prelude to progress'-Mahatma Gandhi Over time humankind has employed an array of natural medicines and physical methods to alleviate pain and suffering. Ancient Indian and Chinese texts record the beneficial analgesic effects of cannabis and henbane. In Egypt around 3000 B.C., the opium poppy, hellebore, beer, and the legendary mandrake were used for similar purposes [1]. Other approaches to deal with surgical trauma and pain relied on physical methods such as cold, nerve compression, carotid artery occlusion or infliction of a cerebral concussion. The effectiveness of these historical agents is unknown but allude to an ever present need.
Elsevier eBooks, 2016
Cognitive Neurodynamics, 2010
The neuronal mechanisms of general anesthesia are still poorly understood. Besides several characteristic features of anesthesia observed in experiments, a prominent effect is the bi-phasic change of power in the observed electroencephalogram (EEG), i.e. the initial increase and subsequent decrease of the EEG-power in several frequency bands while increasing the concentration of the anaesthetic agent. The present work aims to derive analytical conditions for this bi-phasic spectral behavior by the study of a neural population model. This model describes mathematically the effective membrane potential and involves excitatory and inhibitory synapses, excitatory and inhibitory cells, nonlocal spatial interactions and a finite axonal conduction speed. The work derives conditions for synaptic time constants based on experimental results and gives conditions on the resting state stability. Further the power spectrum of Local Field Potentials and EEG generated by the neural activity is derived analytically and allow for the detailed study of bi-spectral power changes. We find bi-phasic power changes both in monostable and bistable system regime, affirming the omnipresence of bi-spectral power changes in anesthesia. Further the work gives conditions for the strong increase of power in the d-frequency band for large propofol concentrations as observed in experiments.
Toxicology Letters, 1998
Anesthetic gas molecules are recognized to act by van der Waals (London dispersion) forces in hydrophobic pockets of select brain proteins to ablate consciousness. Enigmatic features of consciousness have defied conventional neurophysiological exp lanations and prompted suggestions for supplemental occurrence of macroscopic quantum coherent states and quantum computation in the brain. Are these feasible? During conscious (non-anesthetic) conditions, endogenous Van der Waals London dispersion forces occur among non-polar amino acid groups in hydrophobic pockets of neural proteins and help regulate their conformation/function. London forces are weak instantaneous couplings between pairs of electron induced dipoles (e.g. between adjacent nonpolar amino acid groups), and are quantum mechanical effects capable of supporting quantum superposition/computation and macroscopic quantum coherence. Quantum effects mediated by endogenous London forces in hydrophobic pockets of select neural proteins may be necessary for consciousness. The mechanism of anesthetics may be to inhibit (by exogenous London forces) the necessary quantum states.
Journal of Biological Physics, 2008
The present work derives the spatiotemporal field equation of neural populations considering two types of neurons. The model considers pyramidal cells, which may excite or inhibit other neurons, and GABAergic interneurons inhibiting terminal neurons. Additionally, taking into account excitatory and inhibitory synapses, the neural population obeys a vector-field equation involving nonlocal spatial interactions. The work studies the effect of the anesthetic agent propofol, which increases the decay time of inhibitory synapses. In addition, it explains the bifurcation mechanism in some detail and finds a saddle-node bifurcation subject to the propofol concentration. This bifurcation may model the transition between consciousness and nonconsciousness and vice versa during the administration of general anesthetics in medicine.
Anesthesiology, 2007
Dynamic action of anesthetic agents was compared at cortical and subcortical levels during induction of anesthesia. Unconsciousness involved the cortical brain but suppression of movement in response to noxious stimuli was mediated through subcortical structures. Twenty-five patients with Parkinson disease, previously implanted with a deep-brain stimulation electrode, were enrolled during the implantation of the definitive pulse generator. During induction of anesthesia with propofol (n = 13) or sevoflurane (n = 12) alone, cortical (EEG) and subcortical (ESCoG) electrogenesis were obtained, respectively, from a frontal montage (F3-C3) and through the deep-brain electrode (p0-p3). In EEG and ESCoG spectral analysis, spectral edge (90%) frequency, median power frequency, and nonlinear analysis dimensional activation calculations were determined. Sevoflurane and propofol decreased EEG and ESCoG activity in a dose-related fashion. EEG values decreased dramatically at loss of consciousness, whereas there was little change in ESCoG values. Quantitative parameters derived from EEG but not from ESCoG were able to predict consciousness versus unconsciousness. Conversely, quantitative parameters derived from ESCoG but not from EEG were able to predict movement in response to laryngoscopy. These data suggest that in humans, unconsciousness mainly involves the cortical brain, but that suppression of movement in response to noxious stimuli is mediated through the effect of anesthetic agents on subcortical structures.
British Journal of Anaesthesia, 2002
General anesthesia reversibly alters consciousness, without shutting down the brain globally. Depending on the anesthetic agent and dose, it may produce different consciousness states including a complete absence of subjective experience (unconsciousness), a conscious experience without perception of the environment (disconnected consciousness, like during dreaming), or episodes of oriented consciousness with awareness of the environment (connected consciousness). Each consciousness state may potentially be followed by explicit or implicit memories after the procedure. In this respect, anesthesia can be considered as a proxy to explore consciousness. During the recent years, progress in the exploration of brain function has allowed a better understanding of the neural correlates of consciousness, and of their alterations during anesthesia. Several changes in functional and effective between-region brain connectivity, consciousness network topology, and spatio-temporal dynamics of between-region interactions have been evidenced during anesthesia. Despite a set of effects that are common to many anesthetic agents, it is still uneasy to draw a comprehensive picture of the precise cascades during general anesthesia. Several questions remain unsolved, including the exact identification of the neural substrate of consciousness and its components, the detection of specific consciousness states in unresponsive patients and their associated memory processes, the processing of sensory information during anesthesia, the pharmacodynamic interactions between anesthetic agents, the direction-dependent hysteresis phenomenon during the transitions between consciousness states, the mechanisms of cognitive alterations that follow an anesthetic procedure, the identification of an eventual unitary mechanism of anesthesia-induced alteration of consciousness, the relationship between network effects and the biochemical or sleep-wake cycle targets of anesthetic agents, as well as the vast between-studies variations in dose and administration mode, leading Frontiers in Systems Neuroscience | www.frontiersin.org 1 August 2019 | Volume 13 | Article 36 Bonhomme et al. Anesthesia and Consciousness
2018
Biography Dr. Rocco J. Gennaro is the Philosophy Department Chairperson and a Professor of Philosophy at the University of Southern Indiana. He received his Ph.D. in 1991 at Syracuse University. Dr. Gennaro’s primary research and teaching interests are in Philosophy of Mind/Cognitive Science (especially consciousness), Metaphysics, Early Modern History of Philosophy, NeuroEthics, and Applied Ethics. He has published ten books (as either sole author or editor) and over fifty articles and book chapters in these areas.
Anesthesiology, 2018
Anesthetic gases block consciousness selectively, sparing nonconscious brain activities , and thus their specific action could unravel the age-old mystery of how the brain generates, or mediates, consciousness. In this issue, Li et al. 1 make a significant contribution to our understanding of both anesthesia and consciousness , showing that an isotope of the anesthetic xenon (129 Xe) with the quantum property of nuclear spin 1/2 is significantly less potent than xenon isotopes without spin, despite identical chemical actions. Li et al. suggest that the xenon nuclear spin antagonizes its own anesthetic action by promoting consciousness, and that consciousness involves quantum brain processes, thus supporting a genre of theories known as "quantum consciousness."
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