Remarks on the number of tubulin dimers per neuron and implications for Hameroff-Penrose Orch OR
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- Kanazawa University
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- Received 13 October 2009 09:35 UTC; Posted 14 October 2009
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- Molecular Cell Biology, Neuroscience
- Abstract:
Stuart Hameroff has wrongly estimated that a typical brain neuron has 107 tubulin dimers and wrongly attributed this result to Yu and Baas, J. Neurosci. 1994; 14: 2818-2829. In this letter we show that Hameroff’s estimate is based on misunderstanding of the results provided by Yu and Baas, who actually measured the total microtubule length in a single axonal projection with length of 56 μm in a differentiating in vitro stage 3 embryonic hippocampal neuron. In order to visualize how big Hameroff’s error is, we have reconstructed two of the studied by Yu and Baas embryonic hippocampal neurons with Neuromantic v1.6.3 and compared them with previously published reconstructions of adult hippocampal neurons. Correct calculations show that an adult differentiated pyramidal neuron in vivo has approximately 1.3×109 tubulin dimers incorporated in cytoskeletal microtubules. This estimate has profound implications for the Hameroff-Penrose Orch OR model, because it sets limitations on the number of quantum coherent neurons and implies that if 100% of the neuronal microtubules are quantum coherent for 25 ms then Hameroff-Penrose Orch OR conscious events should involve only 15 pyramidal neurons.
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4 comments
Here are several more works that cite Yu and Baas (1994) for the value of 10^7 tubulin dimers per neuron:
Smith CUM. 2009. The ‘hard problem’ and the quantum physicists. Part 2: Modern times. Brain and Cognition 71(2):54-63. http://dx.doi.org/10.1016/j.bandc.2007.09.004
Kaivarainen A. 2008. The Hierarchic Theory of Liquids and Solids: Computerized Applications for Ice, Water and Biosystems. Nova Science Publishers.
Ivancevic VG, and Ivancevic TT. 2008. Quantum Leap: From Dirac And Feynman, Across The Universe, To Human Body And Mind: World Scientific Publishing Company.
Ivancevic VG, and Ivancevic TT. 2007. Computational Mind: A Complex Dynamics Perspective. Studies in Computational Intelligence Vol. 60, Springer.
And several works that directly use the estimate of 10^7 tubulin dimers per neuron without reference to any source:
Tuszynski JA, and Woolf NJ. 2006. The Path Ahead. In: The emerging Physics of Consciousness, Tuszynski JA (Editor). The Frontiers Collection, Springer, pp.1-26.
Roy S, and Kafatos M. 2003. Quantum processes, space-time representation and brain dynamics. http://arxiv.org/abs/quant-ph/0304137
Roy S, and Kafatos M. 2004. Quantum Processes and Functional Geometry: New Perspectives in Brain Dynamics. Forma 19: 69–84.
The Yu and Bass number of tubulins per pyramidal neuron (10 million) was the best available when Orch OR was put forth in 1995. Neurons involved in a particular conscious moment (e.g. a 25 msec gamma synchrony conscious moment) would require 10 billion tubulins. So, as Danko says, if the number of tubulins per pyramidal neurons is 100 times larger, only a few pyramidal neurons would be required if that was the whole story. But 10 billion tubulins for a 25 msec event includes tubulins not only in pyramidal neurons, but also in cortical interneurons (and glia) connected to pyramidal neurons by gap junctions (as described in more recent updated versions of Orch OR, e.g. the Conscious pilot paper available on my website www.quantumconsciousness.org). So the number of quantum coherent tubulins in a pyramidal neuron is not the critical factor. How many tubulins in an inter-neuron? One would have to average that with the relatively few pyramidal neurons in a dendritic web, or hyper-neuron which has been used in updated versions of the Orch OR model. Finally, even in pyramidal neurons we are talking about tubulins only in dendrites, and only some dendrites per neuron. And only some tubulins in microtubules isolated from decoherence. The variablitiies in these factors negate Danko’s objections.
Stuart
I am glad to have this reply, because I was expecting it. I did not wanted to discuss it myself before Prof. Hameroff is engaged in a clear position. So, now it seems that what he is suggesting is to make a web of interneurons and glia, and to exclude pyramidal neurons, which is strange because exactly the pyramidal neurons are those who receive the major sensory input, and importantly they are the sole output from the cortex! Also, I strongly advice Prof. Hameroff to study the reconstruction of the pyramidal CA1 neuron – what one sees is a bunch of apical dendrites, and a bunch of basal dendrites, the axon is a sole projection going out throgh the bunch of basal dendrites, and its terminals are actually not reconstructed at all. Indeed what is estimated by my calculation is actually the volume contributed by apical and basal dendrites, so one cannot reduce numbers as he likes (my estimate is essentially considering > 95% dendrite volume and thus dendritic MTs).
Taking into consideration this latter clarification, the only way to have thousands of neurons in coherence will be to drastically reduce the number of Orch OR conscious MTs per neuron, so that from 100% they will be say 0.001%. However such a drastic reduction by 5 orders of magnitude (or more) is physically impossible, because the Orch OR figures provided by Hameroff et al. require continuous in space interaction of the coherent microtubules via MAP molecules. Reducing the number of conscious MTs per neuron to 0.001% will make it impossible for one to have a continuous Orch OR microtubule network, instead there will be disconnected coherent MTs “here and there”.
Moreover, I would like to see how the continuous microtubule network extends through glial cells, which are 10:1 more numerous compared to neurons and thus inclusion of glial cells in Orch OR means exclusion of pyramidal neurons (which are only 15). My estimate is crucial because it shows the old ideas in Orch OR do not work, and shifting the focus to glia is unserious – glial cells do not receive sensory input from thalamus, nor they can restore the lost function due to injured neurons!
Also, I have never seen into the Orch OR calculations to enter the MAP2 mass! What I have shown is only the tip of the iceberg. Including the MAP2 mass will reduce the number of coherent tubulin dimers even further – compare tubulin dimer mass of ~110 KDa (55 KDa + 55KDa) with MAP2b (adult neurons) mass of ~280 KDa. It would be nice to see how Hameroff-Penrose model works and this better be detailed calculations and properly cited sources.
And last but not least, the claim “Yu and Bass number of tubulins per pyramidal neuron (10 million) was the best available when Orch OR was put forth in 1995” means that Prof. Hameroff did not read at all my article. My estimate is based exclusively upon Yu and Baas data. I am amazed that this is called the best available estimate in 1995. Wasn’t it obvious that Yu and Baas say that they have measured the total MT length in a single axonal projection with length of 56 micrometers? Who extrapolated the result of 10^7 tubulin dimers per 56 micrometer long axon to a whole adult neuron with fully developped dendritic arborizations?
I think it is more noble to publicly admit the mistake (which by the way inflated the citation of Yu and Baas 1994 article for a false statement by several dozens and also the impact factor of J. Neurosci.), instead of saying that this estimate was the best one. After all who calculated the number 10^7, since it is evident that Yu and Baas did not do such a calculation. - (Login to share with a colleague)
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Georgiev, Danko. Remarks on the number of tubulin dimers per neuron and implications for Hameroff-Penrose Orch OR. Available from Nature Precedings <http://dx.doi.org/10.1038/npre.2009.3860.1> (2009)
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Danko Georgiev on 19 October 2009 13:28 UTC
Yet another article that boldly states that there are 10^7 tubulins per neuron. The authors don’t even cite source for this claim, though they cite 9 articles by Hameroff.
Craddock T, and Tuszynski J. 2009. A critical assessment of the information processing capabilities of neuronal microtubules using coherent excitations. Journal of Biological Physics. http://dx.doi.org/10.1007/s10867-009-9158-8