A critical assessment of the information processing capabilities of neuronal microtubules using coherent excitations

Abstract

Evidence for signaling, communication, and conductivity in microtubules (MTs) has been shown through both direct and indirect means, and theoretical models predict their potential use in both classical and quantum information processing in neurons. The notion of quantum information processing within neurons has been implicated in the phenomena of consciousness, although controversies have arisen in regards to adverse physiological temperature effects on these capabilities. To investigate the possibility of quantum processes in relation to information processing in MTs, a biophysical MT model is used based on the electrostatic interior of the tubulin protein. The interior is taken to constitute a double-well potential structure within which a mobile electron is considered capable of occupying at least two distinct quantum states. These excitonic states together with MT lattice vibrations determine the state space of individual tubulin dimers within the MT lattice. Tubulin dimers are taken as quantum well structures containing an electron that can exist in either its ground state or first excited state. Following previous models involving the mechanisms of exciton energy propagation, we estimate the strength of exciton and phonon interactions and their effect on the formation and dynamics of coherent exciton domains within MTs. Also, estimates of energy and timescales for excitons, phonons, their interactions, and thermal effects are presented. Our conclusions cast doubt on the possibility of sufficiently long-lived coherent exciton/phonon structures existing at physiological temperatures in the absence of thermal isolation mechanisms. These results are discussed in comparison with previous models based on quantum effects in non-polar hydrophobic regions, which have yet to be disproved.

On resistance switching and oscillations in tubulin microtubule droplets

“…Those faster oscillations could be the result of the much higher stiffness of MT bundles in Taxol-stabilised solutions. Further work is required to evaluate the possibility of integration of nano-seconds scale spiking in MT network into milli-seconds range spiking of neurons to understand better how exactly the MT oscillation can contribute to neuronal activity and information processing [16,49].…”

Two emerging topics regarding long-range physical signaling in neurosystems: Membrane nanotubes and electromagnetic fields

“…Further aspects for future research related to the topics discussed in this review paper would be, for example, investigating the link between the discussed electrical/electromagnetic processes in the brain and possible electromagnetic and quantum physical processes in components of the cytoskeleton (Craddock and Tuszynski, 2010;Craddock et al, 2012;Saha et al, 2012;Pothos and Busemeyer, 2013;Hameroff, 2014;Havelka et al, 2014). Regarding the propagation of electromagnetic fields inside subcellular and cellular structures, mitochondria might facilitate cable-like connection as already proposed by Thar and Kühl (Thar and Kühl, 2004).…”

Nonlinear dynamics of dipoles in microtubules: Pseudospin model

“…Parameters The radius of the MT can be estimated as, R ≈ 11.2 nm [19,22]. The unit cell shown in Fig.…”

“…(Color online) Tubulin neighborhood in the hexagonal unit cell of the microtubule. The distance between dimers is d. The heterodimer helix direction is defined by the height, h. The typical values of parameters are: a = 8 nm, b = 5.87 nm, c = 7.02 nm, d = 5 nm, h = 4.9 nm, θ1 = 0, θ2 = 58.2 o , θ3 = 45.58 o[7,19,20,22] .…”