Thursday, April 30, 2009

There She Be

With all the press lately about piracy, I hope nobody thinks it’s too insensitive of me to relate the story of my short career as a buccaneer.

The place was Floodwood Mountain Scout Reservation in New York state; or, to be more specific, that summer camp’s West Pine Pond. Three of us patrol leaders, our members off doing whatever they did when we weren’t tormenting them, got bored of an afternoon and decided to take a sailboat onto the lake. We checked out a Sunfish, piled on, and tacked out onto the water.

Well, three young teenagers are too many for a little Sunny. We were having trouble merely hanging on and staying afloat, let alone maneuvering a craft with which we were only a couple of lessons away from pure novicehood. Perhaps it was inevitabile, given nobody else on the pond was much better at it; perhaps a little miscreance on the part of the other kids played a role — but the next thing we knew we were being rammed, broadside.

The bow of the other Sunny skipped atop our deck, slamming into Skip Remington — I swear to God, that was his name. Skip was skinny as a rail, looked like he would blow away in a stiff wind; but he was also a berserker. Hurt and angry, giving an animalistic bellow, he grabbed their bow with both hands and, impossibly given the wind resistance of a sail and the water resistance of a keel, flipped them over. We sailed away, hooting at them as they scrambled to de-capsize.

We took the moment to take stock, and realized that the encounter was not only not traumatic — it had actually been kind of fun.

“Let’s sink another one,” I said.

So we picked out another sailboat — this one, shame on us, crewed by a single, small boy — and bore down on it. I don’t know how I wound up the unofficial skipper, but at this point I said, “Pull up on his windward side — that way, we’ll becalm him.”

For yes, little geek that I was, I’d read up on windjammer battle tactics, and new the secret of the weather gauge — the fact that, when one sailing vessel puts another downwind of it, the upwind craft literally steals the wind from the other’s sails. The hapless downwind vessel is rendered therefore unable to maneuver, and vulnerable to the upwind craft’s cannon.

Well, we didn’t have any cannon, and I’m not sure what my plan was supposed to be at that point — because events overtook the pace even of my fevered little brain. Inexperts that we were, we’d jibed — turned our stern to the wind, which in a fore-and-aft rigged boat like our little Sunny, meant that our sail suddenly caught the breeze and swung around violently. We had the luck of the unrighteous, though, since given the 50-50 chance of either pitching us off our deck or slamming into the other boat’s mast, it did the latter.

The force of the hit neatly flipped him over. A second Ship Taken, Sunk, or Burned: we were pretty obviously the undisputed scourge of West Pine Pond.

Sailing away, we felt much of ourselves — so much that our next choice of encounter was with a rowboat, which resulted in a lot of splashing and invective but nothing by the way of a conclusive result. At that point a camp counselor on a motorboat — the U.S. Navy of Floodwood Mountain, in effect — came out to arrest us and banish us from the waters for the rest of the day.

I bring this story up because our entry this week has to do with the crucial question asked by all pirates — as well as all dog handlers: Where be she?

I’ve burned the Webwaves on at least one dog-handler list regarding my conceptual difficulty understanding how in the world a dog can tell the direction of a trail of scent on the ground. I know they can do it; I just know enough about the difficulty of the task to wonder how.

An airscent dog, picking up the smell of a search subject on the wind, encounters not a uniform cloud of scent but a discontinuous, complex cloud of intense filaments of scent, with little or nothing to smell in between. The task of figuring out the direction it’s coming from would be a killer, if it weren’t for the obvious clue: the wind direction. Now, wind directions change, and so the filamentous cloud isn’t always a straight shot upwind. But that’s a refinement of a strategy that basically depends on there being at least a little air movement.

Trailing dogs have no such easy directional clue. Because the scent in the air is discontinuous, I think it’s probably safe to say that a ground trail is essentially a “footprint” of discontinuous filaments on the earth and vegetation. But regardless, you’ve got a trail, possibly hours or even days old, in which a dog, to tell which direction she has to move, presumably has to detect an increase of intensity between pieces of scent that fell from the subject’s body seconds apart.

Now, there’s always the possibility that a dog’s nose is sensitive enough to discern the one-gazillionth difference in intensity between the two bits of smell. But I’m leery of any argument that ends with, “because dogs’ noses are so awesomely sensitive.” I want to know how, and when it seems impossible to get from point A to point B — even when I can see it happen — I get the feeling that we’re missing something important.

Well, Akihiko Nishikimi and the Crazy 88 of Kyushu University and elsewhere have helped make my life even more complicated; they figured out the internal mechanism by which neutrophils — white blood cells that ooze their way toward infections by following a trail of chemical signals called chemoattractants — create a pseudopod that allows them to crawl in the direction of the infection. What caught my eye, though, was a comment in an accompanying commentary that mentions that the needed chemical structure “accumulates at the site of the plasma membrane that senses the highest concentration of the extracellular stimulant.”

Now hold on a second: a neutrophil is only 12 to 15 micrometers across — about 1,700 of them lined up side by side would make an inch [1]. Assuming a concentration swing like that of the chemoattractant interleukin-6, which can vary by 1,000-fold in the bloodstream, that means that in order to find its way from, say, the entrance of my femoral vein to an infection in my toe — about a meter distance — that white blood cell would have to detect an average concentration difference, if my math is right, of 2 percent or less from one end of the cell to the other. Offhand it may not be hard to imagine a cell telling the difference between 102 and 100 from one end to the other — but remember, while it’s doing that it’s also sensing a level between 102 and 100 on all sides. And it doesn’t produce 102 pseudopods on the “hot” side for every 100 on the “cold” side. It just makes a pseudopod in the direction it wants to move: somehow, it's translating 102 vs. 100 into 1 vs. 0.

If you assume the gradient is uneven — say, the cell gets hit with a sudden increase when the blood flow brings a wave of attractant to it — it could make the difference across the cell much larger. But that would only explain how it starts moving, not how it keeps moving once that big increase is over and it has even less of a difference to detect. [2]

Except — and I’m guessing here — maybe the neutrophil isn’t a trailer, but an airscenter. Maybe the direction of the blood flow is the cue it needs to make sense of the complex series of attractant pulses that come its way, as the chemoattractant mixes turbulently in the blood and forms filaments of scent. Alternatively, the cells just ride the bloodstream, excited by the surge of attractant, until they’re right on top of the infection, and then the local concentration gets really high — but the impression I got was that the concentrations in the blood were enough to get them moving, which argues against that.

Well, either I’ve gotten something figured wrong or there’s more to the story. I’m going to see if I can learn more. If I figure it out — or find out if anybody else figured it out — I’ll give you all a hail.

[1] Which is actually pretty big, considering we’re talking about single cells here.
[2] Of course this is another unrealistic end of the spectrum. Probably the concentration falls off rather sharply from the source, then begins to level out to a low level farther out. This gives us the opposite problem: easy to find source from close in, even harder from far away.

7 comments:

smartdogs said...

What about quantum entanglement?

Ken Chiacchia said...

Everybody picks on me when I'm confused.

smartdogs said...

A recent article in PhysOrg.com discusses how quantum entanglement or spooky action at a distance may be important in connecting mental processes (like framing and word association). Entanglement is a phenomenon in quantum mechanics where separate quantum systems behave in like they are a single, connected system.

There are some interesting articles around the web on how quantum entanglement may be important in cognition, music, computers and more. (I'd just read the PhysOrg article ref'd above before I read yours so my connection was classical, not quantum.)

Anyway - I just thought that quantum entanglement might provide an interesting theory for how things involving seemingly impossible concentration gradients (like intercellular communication and air-scenting) occur.

Ken Chiacchia said...

Yeah, but remember, we're not talking about two cells communicating -- we're talking about a site of infection communicating with thousands (millions?) of neutrophils. Quantum entanglement links two particles with each other.

Besides, we've still got the same problem: entanglement would allow two cells to communicate, but unless one of them knows where the other one is it can't communicate any locational information. And that's where we circle back on ourselves -- somebody has to be reading a directional cue.

Finally, it seems clear from all the machinery that the attractants really do cause the cells to move in a particular direction -- what an instantaneous link between the two can add to that isn't clear to me.

I'll have to check out the physorg.com article -- I'm also puzzled how a life system can create entangled particles in the first place, it's quite a trick.

Ken Chiacchia said...

OK, I saw the article -- they aren't talking about actual quantum links in the brain. They're talking about using the mathematics that describe quantum links as a way of explaining how the brain handles words that associate with each other.

smartdogs said...

I don't know - I think it's possible that the ideas of quantum mechanics will provide a more than an interesting metaphor for biological processes. For instance - check out this article on photosynthesis.

Ken Chiacchia said...

Oh, at the protein-chemistry level there's no doubt that quantum effects are important; we know that some enzymes depend on quantum tunnelling to get between a substrate and a product when the intermediate is way too energy-expensive to reach. You simply tunnel your way through, and the enzyme helps make that happen.