Microtubules are protein structures found within cells. They have diameter of ~ 24 nm and varying length from several micrometers to possible millimeters in axons of nerve cells.

Organization

Microtubules are polymers of tubulin. They are hollow cylinders, which can be thought of as 13 protofilaments of tubulin arranged into a cylinder, or as a single spiral with 13 subunits in one helical turn of the spiral. Microtubules have a polarity: each microtubule has a (+) and a (-) end. Gain or loss of tubulin subunits from the (+) end can change the length of a microtubule.

Microtubules are part of a structural network (the cytoskeleton) within the cell's cytoplasm, but in addition to structural support microtubules are used in many other processes as well. They are capable of growing and shrinking in order to generate force, and there are also motor proteins that move along the microtubule. A notable structure involving microtubules is the mitotic spindle used by eukaryotic cells to segregate their chromosomes correctly during cell division. Microtubules are also responsible for the flagella of eukaryotic cells (prokaryote flagella are entirely different).

Dynamic Instability

Tubulin binds GTP in order to assemble onto the (+) end of a microtubule. Shortly after assembly, the GTP is hydrolyzed to GDP. A GDP-bound tubulin subunit at the tip of a microtubule will fall off, though a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out. Since tubulin adds onto the end of the microtubule only in the GTP-bound state, there is generally a cap of GTP-bound tubulin at the tip of the microtubule, protecting it from disassembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe. GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as rescue.

The drug taxol, used in the treatment of cancer, blocks dynamic instability by stabilizing GDP-bound tubulin in the microtubule. Thus, even when hydrolysis of GTP reaches the tip of the microtubule, there is no depolymerization and the microtubule does not shrink back.

Colchicine has the opposite effect: it blocks the polymerization of tubulin into microtubules.

Motor Proteins

In addition to movement generated by the dynamic instability of the microtubule itself, the fibers are substrates along which motor proteins can move. The major microtubule motor proteins are kinesin and dynein.

Microtubules and theory of consciousness

Stuart Hameroff and Roger Penrose have proposed a theory of the quantum mind in which the hollow cores of microtubules inside neurons form an environment capable of supporting quantum coherence and quantum-scale information processing and conscious awareness. The motivation for doing so arises from the fact that neurons in the brain which most scientists believe are involved in cognitive processing are too large for quantum effects to be significant, and Hameroff's research into the action of anesthetics on microtubules.

This proposal has met with considerable resistance, for a number of reasons, such as the fact that it fails to explain how chemicals and physical processes which alter the behavior of neurons affect consciousness (although anesthetics are reported to interact with microtubules, according to Hameroff), and that microtubules are not confined to the brain or to neurons, leading to the question of why microtubules in the brain lead to consciousness while those in the foot do not. Another criticism, proposed by Max Tegmark and others, is that the brain's thermodynamic environment is too warm to allow quantum coherence to be sustained for a sufficient time, according to the conventional understanding. Hameroff and Penrose have replied to this criticism, citing several misunderstandings of their theory.