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Ask a scientist what memory is, and you'll probably get long-term potentiation (LTP) as an answer. The idea goes something like this: When a nerve impulse reaches the end of an axon, it triggers the release of a neurotransmitter into the gap of the synapse. When the neurotransmitter attaches to the dendrite of the next neuron, it starts an impulse in the second cell. If this happens many times, the signal is strengthened, maybe permanently. In this way, neurons become conditioned to respond strongly to signals they have received many times before. Or, in simpler terms, “Neurons that fire together wire together.”
The mechanism for LTP is chemical. The specifics of the chemical changes that form memories are too complex to get into here—and only a few of them are understood anyway. But what’s important is the formation of memory proteins. The production of one memory protein prompts the formation of another and so on and so on. Together, these proteins operate in the synaptic gap and strengthen a memory. Yet more proteins are thought to be responsible for maintaining a memory once it is formed.
That’s all fine in theory, but we are talking physical processes here. Changes at a synapse last for seconds at most, but memories can last a lifetime. That suggests the need for some “storage lockers”—structures in which the proteins that maintain memory can be housed long-term. As neuroscientists describe the idea, synaptic information must be encoded and hard-wired at a deeper, finer-grained molecular scale. So say physicists Travis Craddock and Jack Tuszynski of the University of Alberta, and anesthesiologist Stuart Hameroff of the University of Arizona in a recent issue of the journal PLoS Computational Biology.
They have been able to demonstrate one way in which synaptic memory might be stored in microtubules. Microtubules are hollow tubes made of proteins collectively called tubulin. They are interconnected by linking proteins into a lattice-like network. Microtubules are to nerve cells what bones are to the human body. They act as a skeleton, providing structural support. But that’s not their only role. They are essential to communication and information processing within a nerve cell. Think of microtubules as comparable to the hardware of a computer that is required for software to operate—that’s not a bad analogy.
This new model from Craddock and his colleagues shows how the spatial dimensions, geometry, and electrical attractions of an enzyme known to be important in memory formation can perfectly match the hexagonal lattices of tubulin proteins in microtubules. The researchers’ calculations show how this match-up results in enormous amounts of information storage at a low energy cost. They also show how changes in the tubulins in microtubules can trigger nerve cell firings and regulate what goes on at the synapse.
While this study is highly technical, its significance cannot be underestimated in my opinion. Understanding how the “living machinery” of memory formation and storage works is key to comprehending both normal brain function and aberrations. “Many neuroscience papers conclude by claiming their findings may help understand how the brain works, and treat Alzheimer’s, brain injury, and various neurological and psychiatric disorders,” said Hameroff, senior author on the study “This study may actually do that. We may have a glimpse of the brain’s biomolecular code for memory.”
For More Information:
Craddock TJA, Tuszynski JA, Hameroff S (2012) Cytoskeletal signaling: Is memory encoded in microtubule lattices by CaMKII phosphorylation?doi10.1371/journal.pcbi.1002421.
