microtubules (MT) are constructed of thousands of tubulin proteins, which are dimeric proteins which orient into helical tubes held together at the seems by MT-associated proteins (MAPs). the MT of any system - amoeba or neuron - make up a network which is generally acknowledged as being the skeleton of the cell, and is, in my opinion, extraordinarily underrated in that role.
in the 1980's, Stuart Hameroff proposed that MT also compute. this was based on both the predisposition of MT to self-assemble and disassemble via the aid of MAPs, and collective network oscillation based solely on nearest neighbor interactions (these being the swapping of a free electron between alpha and beta subunits of the tubulin dimers). Hameroff's theory is primarily dependent on nearest neighbor interaction. to be brief, this interaction refers to the electric dipole created between the alpha and beta monomers of each tubulin protein: this is created by the 18 calcium ions bound to the beta subunit, creating a slightly negative orientation toward the alpha monomer. namely, as an energetic wave passes through a given MT region it would excite the free electron on the beta subunit and shift it toward the alpha, thereby changing the molecular conformation of the dimer. this change in conformation would subsequently excite all its neighbor tubulins to exercise the same behavior, resulting in an electric wave being propagated down the length of the MT. and they did some badass computational modeling of this theory which showed that by said rules, MT could indeed be stable, signal-perpetuating structures...
one of the immediate problems with the Hameroff model is that it was designed at a temperature of zero... such that signal propagation was an artifact of design, and not explicitly reminiscent of those rules governing biological signals. [to note, this actually doesn't seem problematic to me because it just implies even a the level of the zero point field this mechanism is probable, but...] because physiological conditions are what they are, and due to the patterns governing tubulin subunit interaction as well as the helical nature of the MT themselves, a new theory of signal propagation potential was proposed by Jack Tuszynski (Canada gets a brownie for that one). the stipulation was that MT could be electromechanical signal transmitters only helically about their circumference... in the manner of a spin glass.
the spin glass behavior, put simply means that because of its geometrical orientation a system can't minimize interaction energies across its area simultaneously (a.k.a. magnetic frustration). so for MT, that means propagating a signal in somewhat of a spiral manner along its length which yields a similarly patterned signal transmission between neighboring MT. because they are hexagonal sheets rolled into tubes, MT have a seam - this is where magnetic frustration occurs as the propagating signal tries to navigate the tubule circumference. despite this obstacle, the tubule length allows for propagation. the equations for this type of system are exceedingly complex and just glancing at them makes my heart sink in defeat... but i trust Dmitri Nanopoulos (who is my superhero of the day) and the modeling enough to be profoundly inspired by the idea that my poor neglected, favorite organelle might have such a role in intelligence (i'll get there...)...
so what does this model mean to the microtubule network as a transmitter of signals?
this is bitchin - hang onto your pants. the spin glass model allows MT networks the capacity to adapt to the demands of neuronal signaling.
if MT are in and of themselves an adaptive network, as suggested by Nanopoulos' and the nature of spin glass perpetuation, that potentially translates to every individual neuron being an adaptive network. during the process of neuronal plasticity, such as learning and memory, this is a conceivable explanation of how particular proteins are recruited to synapses to strengthen them after a particular pattern of neurotransmitter activity (an otherwise mechanistically elusive characteristic of long term potentiation).
let's take a step back and see if i can do this anywhere near coherently... i know i'm verbose as hell, bear with me.
i think... that when Daniel Dennet and David Chalmers look for physical structures on which consciousness and intelligence might supervene, they are not looking small enough. namely, the structures they are looking for (as well as Francis Crick with the claustrum and everyone else with the pineal gland) are neural pathways and specific brain regions. it makes little to no sense to me to suspect that such a comprehensive force as consciousness would supervene on a single structure or originate from a single neural circuit. i like the notion that said phenomena arise from the translation of electromagnetic wave collapse by microtubules... which are everywhere.
in addition to all the computer modeling and quantum computational theory, there are the correlational studies. colchicine, a tubulin binding inhibitor, causes retrograde amnesia in goldfish. tubulin production skyrockets as soon as baby rats first open their eyes to EM radiation, and correlates with peak learning and memory in chicks. on the human level, there is Alzheimer's disease, in which the most prominent biochemical and physiological markers are amyloid plaques and neurofibrillary tangles - the latter being comprised of MT stripped of MAPs and left to aggregate into useless clumps. so here we have examples of the effects of dysfunctional MT on multiple levels of evolutionary development and cognitive function.
this organelle, to my thinking, is the most beautiful candidate for housing intelligence. it is the beginning and the end of life, from mitosis to decay - defining nucleation of a cell by aligning and separating chromosomes, propagating EM signals, responding to IR light and dissociating upon cell death. they are intrinsic and critical to the very existence of every cell.
okay. if you've made it this far, prepare to be rewarded by my coming around to making a fucking point.
take, for instance, a paramecium - a single cell with no nervous system, only MT networks. a paramecium, if sucked into a capillary tube, will find an escape route... and when sucked up again, will find its way out quicker and quicker with each subsequent trial. this study, lead by Joseph Huber in the 1970's describes a mechanism for learning and memory in this single cell system with no neurons. intelligence without a brain [or... the brain of single cell systems is represented by the centrosome! but we won't go there yet...]
if learning can be achieved in a paramecium, what sense is there in attributing intelligence in more complex organisms to solely the strengthening of synapses?
what is to say that intelligence doesn't supervene on MT networks and that consciousness is a separate beast emerging from the larger complication of neural networks?
[on which note, this is why i don't think computers will achieve consciousness in the near future - or i hope not, rather - because neurons are so very much more complex than on-and-off switches, or binary decoders, or all-or-noners as implied by the direction of A.I. research]
to wrap this up... and in the interest of circulating back to not only intelligence and possibly consciousness but the evolution of these phenomena (as well as others), i think of this: microtubules bind to DNA - regardless of chromatin and histone orientation, which has profound implications for the quantum processes of epigenetics - through MT associated proteins, and affects its synthesis by facilitation or inhibition.
through MT perpetuation of EM signaling, or whatever the hell wave collapse results in, DNA may have an intelligence of its own. screw morphogenetic fields... evolutionary process may very well be directed by the very biophysical matter that also genetically codes for it all. what if it's really all in the DNA?
it's entirely conceivable to me that DNA contains the code and the energetic blueprint - that its interaction with microtubules allows it to translate this into initial interaction with the explicate world - that this explicate interaction subsequently feeds back through the nervous system into MT networks which then propagate new signals from which intelligence and consciousness emerge.
i love microtubules.