Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C18
Alpha band synchronisation in the reticular activating system and globus pallidus during anaesthesia-induced loss of consciousness in humans
J. A. Hyam1,2, J. S. Brittain3,1, S. P. Murray4, T. Z. Aziz1,2, A. L. Green1,2
1. Department of Physiology, Anatomy & Genetics, University of Oxford, Oxford, United Kingdom. 2. Department of Neurosurgery, John Radcliffe Hospital, Oxford, United Kingdom. 3. Centre of Excellence in Personalised Healthcare, University of Oxford, Oxford, United Kingdom. 4. Nuffield Department of Anaesthetics, University of Oxford, Oxford, United Kingdom.
Introduction: The state of unconsciousness produced by general anaesthesia is associated with alpha frequency oscillations in the thalamus. Anaesthesia is further associated with a reduction in thalamic metabolic activity in imaging studies and this is also seen within the basal ganglia and reticular activating system (RAS). We therefore hypothesised that a similar alpha synchronisation occurs during induction of anaesthesia in the basal ganglia and RAS. Methods: We recorded local field potentials (LFPs) from implanted deep brain electrodes during induction of anaesthesia in patients receiving therapy for dystonia or Parkinson’s disease. Electrodes were located in the globus pallidus interna of the basal ganglia and the pedunculopontine nucleus of the RAS. Anaesthesia was achieved using remifentanyl and propofol. Indices of attainment of general anaesthesia were abolition of the blink reflex and fall in electroencephalographic bispectral index. LFPs were transformed into the frequency-power domain using fast Fourier Transform and the time-frequency domain using optimal spectral tracking (reference 1). The dominant alpha frequency and its power was recorded and compared during twenty-second periods prior to administration of anaesthetic agent and after attainment of anaesthesia. Results: We studied 4 GPi and 5 PPN cases. Kolmogorov-Smirnov test demonstrated that the data was not normally-distributed and the Wilcoxon signed rank test was employed. Alpha (12Hz) power synchronisation was greater in all cases after anaesthesia onset compared to prior to anaesthetic administration (median power 41.314 µV2/Hz (SD+43.4 µV2/Hz) and 0.727 µV2/Hz (SD+3.4 µV2/Hz), respectively, where z=-2.666, df=9, p=0.008). There was a delay of alpha synchronisation of LFP power after administration of anaesthetic was initiated however the synchronisation was sudden and rapid when it occurred and always preceded loss of blink reflex. Conclusions: Therefore, during anaesthesia-induced loss of consciousness, the RAS and GPi both became synchronised to the same alpha band reported in the thalamus. This continuous alpha activity located proximal to the thalamus supports the suggestion that it provides the electrical substrate for the blockade of inputs encoded at other frequencies from being transmitted via the ascending networks to the thalamus and, ultimately, the cortex.
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