Microtubule · Grey Matter

A microtubule is a hollow tube about 25 nanometres wide assembled from the protein tubulin, and in a neuron microtubules are the long rails that hold axons and dendrites in shape and carry cargo between the cell body and the synapses.

Microtubule. A microtubule is a hollow tube about 25 nanometres wide assembled from the protein tubulin, and in a neuron microtubules are the long rails that hold axons and dendrites in shape and carry cargo between the cell body and the synapses.

Each tube is usually 13 strands of paired alpha and beta tubulin. The tube has a direction: it grows fastest at its plus end, and it can switch from growing to shrinking when the GTP bound to new tubulin is spent, a cycling called dynamic instability. In axons every plus end points away from the cell body; in mature dendrites about half point each way. Motor proteins walk along them: kinesins mostly out toward the terminals, dynein back to the cell body. Vesicles travel at 50 to 400 mm a day, so cargo needs days to reach the end of a motor axon a metre long.

Tau keeps axonal microtubules bundled. In Alzheimer's disease tau carries too many phosphate groups, comes off the tubes and aggregates into the neurofibrillary tangles found inside neurons.

They reach into synapses. Growing plus ends, marked by the protein EB3, enter dendritic spines and change their shape through the actin inside them (Jaworski and colleagues, 2009), tying microtubules to structural plasticity; activating NMDA receptors with a protocol that induces LTD suppresses their entry.

A speculation places consciousness in them. Roger Penrose and Stuart Hameroff proposed in the 1990s that quantum states in microtubules give rise to consciousness. Tegmark estimated in 2000 that they would decay in 10−1310^{-13}10−13 to 10−2010^{-20}10−20 s, against the milliseconds of neural events, and no experiment has shown quantum effects in microtubules taking part in cognition; a rat study of 2024 found that a drug stabilising microtubules delayed loss of the righting reflex under isoflurane by 69 s, a result about how anaesthetics act that leaves the quantum claim untested.

A neuron's shape is held up and supplied by tubes that are always being rebuilt.

The same polymer that carries cargo a metre down an axon also enters single spines, so its failures reach from transport to memory.

Questions: Do microtubules take part in reshaping a synapse? They do, briefly and from the outside. Spines are built of actin, but in cultured hippocampal neurons the growing tips of microtubules, marked by the protein EB3, make short visits into spines and deliver partners that tune the actin inside; blocking microtubule growth or removing EB3 changed spine shape. Activity steers the visits: activating NMDA receptors with a protocol that weakens synapses suppressed microtubule entry into spines, and keeping EB3 high prevented the spines from shrinking. Is axonal transport the same thing as the nerve impulse? No, they are two different traffics on the same axon. The impulse is a wave of voltage across the membrane, carried by ion channels at up to about 120 m/s in thick myelinated fibres. Transport is physical cargo, vesicles, mitochondria and proteins, carried along microtubules by motor proteins at 50 to 400 mm a day for the fastest cargo and under 8 mm a day for the slowest. A signal crosses a metre of axon in about a hundredth of a second; a vesicle takes days.