Another theory has just been published, and I pass along Science Daily's summary mainly because it reminded me of the mind-numbing complexity of life at the molecular level. The basic idea is that long-term memories are stories via differing arrangements of "microtubules" in nerve cells. Lattice-like arrangements of the protein Tubulin are a big part of what gives neurons their shape. But might they be doing more?
We know that a big part of how neurons work is "Long Term Potentiation" or LTP, which means that the brief electric flash of a passing nerve impulse leaves the neuron highly sensitive to similar impulses in the future. And we know that a hexagonal enzyme called calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key part in LTP. These molecules change shape in response to nerve impulses. In their changed form, they can add a phosphorus atom ("phosphorylate") some other molecule. In the new model, the addition of phosphorus atoms to Tubulin structures acts like turning and off the transistors in a computer, forming bits and bytes of memory. To quote:
The standard experimental model for neuronal memory is long term potentiation (LTP) in which brief pre-synaptic excitation results in prolonged post-synaptic sensitivity. An essential player in LTP is the hexagonal enzyme calcium/calmodulin-dependent protein kinase II (CaMKII). Upon pre-synaptic excitation, calcium ions entering post-synaptic neurons cause the snowflake-shaped CaMKII to transform, extending sets of 6 leg-like kinase domains above and below a central domain, the activated CaMKII resembling a double-sided insect. Each kinase domain can phosphorylate a substrate, and thus encode one bit of synaptic information. Ordered arrays of bits are termed bytes, and 6 kinase domains on one side of each CaMKII can thus phosphorylate and encode calcium-mediated synaptic inputs as 6-bit bytes. But where is the intra-neuronal substrate for memory encoding by CaMKII phosphorylation? Enter microtubules.I find myself very dubious that a bit/byte model derived from electronic computers explains how our brains work, but who knows? Meanwhile, contemplate the dizzying array of structures and processes that make up our cells.
Using molecular modeling, Craddock et al reveal a perfect match among spatial dimensions, geometry and electrostatic binding of the insect-like CaMKII, and hexagonal lattices of tubulin proteins in microtubules. They show how CaMKII kinase domains can collectively bind and phosphorylate 6-bit bytes, resulting in hexagonally-based patterns of phosphorylated tubulins in microtubules. Craddock et al calculate enormous information capacity at low energy cost, demonstrate microtubule-associated protein logic gates, and show how patterns of phosphorylated tubulins in microtubules can control neuronal functions by triggering axonal firings, regulating synapses, and traversing scale.