Learning about learning
Yearning for learning
New neurons
Brains 'n sweat
Sprouting brain cells
Stem cells


The hippocampus plays an essential role in processing memories. Why is it one of the only parts of the brain to grow new neurons?

Check out the animated brain slices at the Whole Brain Atlas, which kindly supplied this image.


The jock's brain
By the mid 1990s, the long debate over the formation of neurons in adult mammals seemed to be nearing the optimistic conclusion that in some limited areas, at least, neurons did reproduce. Two important questions remained. Did people gain brain cells, too, or were rats the only lucky ones? And what was causing the neurons to grow?

One approach was to look at the "Club Med" rats -- the ones that gained neurons in the hippocampus -- and try to figure out exactly what was responsible for the effect (scientists, after all, tend to be atomizers who prefer precise causes over general ones).

In an effort to pin down which particular element of the Club Med treatment was responsible for the improvement, Henriette van Praag and her colleagues set up an experiment in which some mice swam, others performed a learning task, and some got access to an exercise wheel. Oddly enough, new neurons developed only among the runners. It's unclear whether the effect resulted from the exercise itself, or the voluntary nature of the task, but the finding seems sure to wind up on the billboard of an athletic club (see "Running increases cell proliferation... " in the bibliography). Want more sports science?

Rats! What about Homo sapiens?
The question of neuronal growth in people was more thorny. Only hard evidence would prove the case, but nobody proposed to experiment on people, since only an autopsy can prove that growth occurred. Then, four years ago, researchers realized that the drug bromodeoxyuridine (BrdU) that was used to track the cell division in rat brains was also given to cancer patients to track the growth of their tumors.

Peter Ericksson, a brain researcher at Sahlgrenska University Hospital in Goteborg, Sweden, and who had trained with Gage, arranged to study neural regeneration among throat cancer patients who were receiving BrdU. As those patients died, their brains were autopsied for the tell-tale signs of newly-formed cells.

After analyzing five brains from patients aged 57 to 72, a pattern emerged: New cells were growing in part of the hippocampus, but not elsewhere. "It was only in the same area as in the rodent, and they looked identical to the new rodent neurons," says Gage. In one patient, the new neurons were still alive 780 days after administration of the tracer drug (see "Neurogenesis in the Adult Human... " in the bibliography).

Into the brain repair shop
By now, neuronal growth has also been found in marmosets, other non-human primates, tree shrews and rats, proving that it is not exactly an oddity, although it is found only in two small regions of the brain -- the olfactory bulb and the hippocampus.

Still, in the interest of complete labeling, we must warn that while the new neurons located in various studies seem to be healthy, there is no proof that they're working . "They have branched to the right place and are receiving communication from other places," says Gage, but only after ongoing experiments are done will it be possible "to see if they're wired correctly and actually function like real neurons."

I remember my question...
hippo Why would the hippocampus -- a memory-processing area -- be one of only two parts of the brain that get a new supply of neurons? Don't we want to add neurons in the memory storage areas of the brain. Actually, that's a tricky proposition. Spraying an extra layer of iron oxide on a hard disk that's already in use would likely convert your copies of Quake and Doom into random noise. It's vastly preferable to build the memory capacity before the memories are laid down.

But the hippocampus is not a storage shed. Instead, it's role is to receive partly-processed sensory input from the cortex and process it into morsels digestible by the memory-storage regions. The hippocampus "takes information and packages it in a way that can be used as a memory, and sends it out to a storage area," as Gage explains it.

So why bother putting new neurons in the hippocampus? Gage figures that these neurons "need to be fresh, clear and able to process a high-intensity volume of information." It's even possible, he says, that the hippocampus "requires baby cells" because they are more responsive. If that's true -- and new research could tell us before long -- the structure could even require a regular resupply of neurons.

You are not required to read about neural stem cells, but they're pretty cool!

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