For much of the history of brain science, the word “engram” has been a bit of a catch-all term, referring to the hypothetical physical incarnation of memory. If this turned out to be a storm of electrical activity, then that’s what the engram would be; if it turned out that networks of physical neurons were the home of specific memories, then an engram was that, instead.
Lately, though, the word has gotten a lot more specific. We now have the term “memory neurons” to refer to the nerve cells of the hippocampus, which seem to play a crucial role in storing and retrieving memories. Since the original proof that at least some memory is localized to specific physical neurons just a few short years ago, understanding of the physical basis of memory has advanced at a lightning pace.
Even in the past year, scientists have made incredible strides. One amazing study may have found the molecular basis for memory formation — the seemingly harmful breakage of DNA. It turns out an enzyme from the topoisomerase family responds to new stimuli by breaking DNA, which seems to activate the transcription of as many as a dozen quick-acting genes associated with neural development. These genes are inhibited by a system of enzymes, but as the now-broken DNA folds up in response to its new situation, these inhibitory genes are blocked, and the now-uninhibited memory genes are free to go out and direct the development of the brain.