Volume 1, Issue 4 
4th Quarter, 2006

The Dynamics of C-termini of Microtubules in Dendrites: A Possible Clue for the Role of Neural Cytoskeleton in the Functioning of the Brain

Jack Tuszynski, Ph.D.

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I will discuss some numbers that are actually a common place for several objects with that many members. For example, the number of possible DNA sequences in the nuclear DNA is several billion base pairs, which is enormous.

The protein we will consider in this article is tubulin. Tubulin is composed of 450 amino acids, with twenty choices in each position. The sequence of a protein of that magnitude is beyond immense. The possible number of nerve cells is also an example of an immense set. I sometimes jokingly worry about the number of tunes that people can compose, which is another example of an immense set. We will never run out of music, which is good news. Thus, there are the different types of morphologies.

Next, I will take you on a tour inside a neuron. To some degree, I think it is an amazing tour because you are looking into components of cells, down to individual atoms, and this is made available only because of computational capabilities.

Image 1: Microtubules in Neurons

The microtubules are the most important structures doing work in the cells. Image 1 shows a nerve cell with microtubules inside the axon. Microtubules also exist in the soma and dendrites. Not only do they provide the mechanical stability of the cell, but they also interact with many other substructures including the membrane ion channels and motor proteins. They are actually indispensable for all traffic going on in the cell.

One thing that I want to mention as a side remark is that, in my opinion, there is no precise definition of life. Yet one criterion that is very important for living systems is intentionality of motion. If you can make an intentional motion, then I would say you are probably alive. What is the substrate of intentional motion? I think it is microtubules and motor proteins. These are the most important proteins that define living systems. 

I will take you on a guided tour through the moving components of the cell. I will not spend too much time on quantum mechanics of microtubules a la Hammeroff and Penrose[1] orchestrated objective reduction theory, although it is interesting and it does bring physics into the picture. I will try to demonstrate that even within the classical realm, there is more than enough computational power inside the neuron to amaze. 

Image 2 shows a dendritic structure with synaptic connections and microtubules inside the dendrites and tubuli in dimers, which are the building blocks of microtubules.

Image 2: Cytoskeletal Structures in Dendtrites

This is another most dramatic representation. The solid bars are microtubules that are interconnected by microtubule-associated proteins. Keep in mind that this is not static. There is a lot of dynamic motion. 

This is the scheme of the tour that I offer to you. From the level of the neurons, we go inside and see microtubules interconnected by other protein structures. We then go inside microtubules to see the building blocks, the tubulins, and we go inside tubulins and see every atom.

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1. Hameroff & Penrose - Orch OR (“Orchestrated Objective Reduction”) is a theory of consciousness put forth in the mid-1990s by British theoretical physicist Sir Roger Penrose and American anesthesiologist Stuart Hameroff. Whereas some theories assume consciousness emerges from the brain, and among these some assume that mind emerges from complex computation at the level of synapses among brain neurons, Orch OR involves a specific form of quantum computation that underlies these neuronal synaptic activities. The proposed quantum computations occur in structures inside the brain’s neurons called microtubules. Wikipedia.com Nov. 7, 2006 12:09 PM EST
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