Propofol is commonly used as anesthesia agent in operating theatres and intensive care units is characterized as artificially putting a patient to sleep or making him /her unconscious for a shorter or longer time to perform certain invasive operations/ procedures. Although ven offering acute amnesia and anterograde confusion when administered by an anesthesiologist, the various brain processes related to this unconscious state are interesting for the study of consciousness research.
To date, experts in the University of Michigan, abbreviated as U-M, has progressed in establishing the geometry of the human brain during the state of unconsciousness, and given a response to the basic understanding of consciousness. By using optogenetics to pace and stop different cells in the brains of mice, the researchers led by Zirui Huang, Ph. D., George Mashour, M. D., Ph. D., and Anthony Hudetz, Ph. D., from U-M’s Center for Consciousness Science established how propofol altered the connectivity between the neurons of the thalamus and cortex, two of the brain regions critical to The authors published the work in Nature Communications under the title, “Propofol Disrupts the Functional Core-Matrix Architecture of the Thalamus in Humans.”
Anesthetics have been widely used and are known to produce effects that result in loss of consciousness but the ways in which they cause this, or which brain areas are significantly affected have often been a question for discussion. In particular, questions arose about what extent anesthetics such as propofol affect the thalamus which processes sensory stimuli or the cerebral cortex which integrates these stimuli. The U-M team applied functional magnetic resonance imaging (fMRI) to track the network organization in the brain of healthy volunteers while they went through wakefulness and sleep under propofol sedation. This let the researchers intervene and observe the alterations of blood flow to various areas of the brain that occurred before, during, and after their proceeding sedation.
At least at minimum, the thalamus displays equal proportions of specialized nuclei that perform unimodal relay functions or conveying selective sensory information to discrete cortical regions or areas and those that execute trans modal activities or distributing information to the higher coical layers. Yet, the study revealed that in deep sedation there is a significant reduction of Transmodal processing and, therefore, an increase of unimodal processing. In other words, the person is still perceiving stimuli through the body’s sensory organs, but the signals are not being fully processed, so consciousness is lost.
The researchers also found out some of the cell types in thalamus which are very essential in this process. The thalamus contains two types of cells: The pyramidal neurons are grouped into two classes; the core cells that have connections with the deeper cortical layers as well as the matrix cells that have loose connections with the higher cortical layers. The team also found out that while conducting mRNA expression that matrix cells, which are associated with connections to higher order cortical areas, are more important in the loss of consciousness than core cells. This means that earlier conjectures regarding the performance of GABA, which is an inhibitory neurotransmitter mediator of propofol, should have been Worlds Apart. Surprisingly, there is evidence that contribution of GABA was not as significant as expected of an inhibitory neurotransmitter.
Overall, the present work contributes to the current knowledge about modulation of consciousness by anesthetics including propofol. This proves that severe sedation essentially constitutes of loss of consciousness arising from functional inactivation of thalamic matrix cells that are globally involved with the integration of information from the sensory organs across the cerebral cortex. These results pave the way for further research of consciousness and its neurobiological substrate.
Reference
Huang, Z., et al. Propofol disrupts the functional core-matrix architecture of the thalamus in humans. Nature Communications (2024).
