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Science is hard

I have two sweeties in school right now pursuing postgraduate degrees related in some way to neuroscience, brain mapping, or brain modeling.

Brain mapping is hard. Really, really, really hard.

It's not just that there's a lot of neurons in the brain (though there are--about 100 billion1 or so). It's not just that they're wired together in beastly complicated ways, though that, too, is true.

It's that "beastly complicated" doesn't even begin to cover it.

This is a drawing of a type of brain cell called a Purkinje cell, taken from a 1918 copy of Gray's Anatomy. 1918! We've known about these things for a long time:



There are a lot of these in your brain, mostly in your voluntary motor control areas. A single Purkinje cell has one axon, which is basically a nerve cell's output, and as many as 200,000 dendrites, which are basically a nerve cell's input. Purkinje cells regulate motor control, primarily by inhibiting other neurons from firing. All your motor control is mediated by these brain cells. They're also hooked into "climbing fibers," axons from other neurons which pass from the center parts of your brain outward.

At rest, these guys fire regularly, sending inhibitory signals to neurons deeper down. When activated, they fire much more rapidly, more strongly inhibiting downstream neurons. All well and good, but...

...a single Purkinje cell can have two hundred thousand inputs. Read that again so that the pure horror has time to sink in. A single Purkinje cell can have two hundred thousand inputs.

So, if you were to, say, want to map a person's brain, that would basically mean recording each brain cell and a list of all the other brain cells it links to. If you had 100 brain cells and each one could link to one other cell, you'd have, potentially, 100 links to record. If you had 100 brain cells and each one could link to 10 other cells, you'd have 100 times 10, or 1,000, links to record. If you had 100 brain cells and each could link to 20 other cells, you'd have 100 times 20 links to record. Makes sense, right?

And if you have 100 billion cells, and each cell can link to 200,000 other cells, you have 100,000,000,000 times 200,000 links to record.

This is a really, really, really big number. This is the kind of number that's within the same order of magnitude as the number of grains of sand on the entire freaking planet. Imagine tagging, isolating, and recording the relative position of every freaking grain of sand on the entire freaking planet and you'll start to gain an appreciation of the magnitude of the challenge involved in mapping a human brain.

Even your own DNA doesn't record this information--it can't. If you were to dedicate the entire information storage capacity of the entire human genome just to mapping the connections between all your brain cells, you'd fall short by several orders of magnitude. The process of building a brain is dynamic; your DNA only describes the gross physical structure, and then as your brain forms it wires itself up more or less randomly2. That's why it takes such a long time to make a human brain--a process that isn't really finished 'til you're out of puberty3.

Which is very depressing, when you consider just how valuable that model will be. And makes my sweeties all the more amazing, I think.



1 American billion (1x109), not British billion (1x1012).

2 Well, not really randomly,, but not deterministically according to a blueprint either. Each nerve cell sends out dendrites, which hook up with whatever nearby nerve cells they happen to hook up with--a neuron that fails to hook up to any other neurons typically dies. The direction and number of dendrites are determined, in general ways, by your genes, but the specific connections that get made are not. And these connections remain dynamic throughout your entire life; long term memory, for example, appears to be encoded in patterns of connections.

3 Interestingly, most of the late-stage development, that takes place during and just after puberty, is inhibitory. Kinda explains a lot, doncha think?

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Comments

( 19 comments — Leave a comment )
mantic_angel
Jul. 17th, 2009 07:59 am (UTC)
"you'd have 100^10, or 1,000, links to record."

I think you meant to say 100x10, not 100 to the 10th power? 100^10 is going to be a lot more than 1K :)

The latter math is also a lot more reasonable when it's done as multiplication rather than exponents.... Admittedly 20 quadrillion is still a big number, but I would have assumed more grains of sand in the world than that.
tacit
Jul. 17th, 2009 08:03 am (UTC)
Right you are! Fixed.
mantic_angel
Jul. 17th, 2009 08:51 am (UTC)
http://www.astro.utu.fi/~cflynn/sand.html

The order of magnitude would seem to rather differ. 10^17 vs 10^24. Even assuming margins of error, you're still probably 3-4 magnitudes different.

I was bored and curious if this was easily researched with at least a vaguely respectable answer :)

It is a bit mind-boggling to realize that much data goes in to such a basic aspect of the brain though. Even if that was all it took, we'd be looking at large petabyte drives to store just the connections o.o
tacit
Jul. 17th, 2009 09:02 am (UTC)
Interesting. The Web page I found claimed that the number of grains of sand was about 7x1017, which is in the right ballpark in terms of orders of magnitude.
mantic_angel
Jul. 18th, 2009 05:49 am (UTC)
I suppose this is the problem with trying to ballpark something like that, although I'm curious what website you found now. The ones I checked tended to be 10^20 - 10^24 in magnitude.
alumiere
Jul. 17th, 2009 08:31 am (UTC)
i hope they succeed; a better understanding of precisely how the brain works is necessary if science wants to better treat and eventually cure illnesses like mine or ms or parkinsons or alzheimers
sarahmichigan
Jul. 17th, 2009 11:31 am (UTC)
Cool. I have a friend who has a cat named Purkinje and hadn't known where the name came from. BTW, minor nitpick, you'll be wanting to add a "b" to the "rain cell" in your fifth paragraph.
tacit
Jul. 17th, 2009 08:52 pm (UTC)
Indeed! Im a fast but lousy typist. Fixed!
ellindsey
Jul. 17th, 2009 01:22 pm (UTC)
I am simultaneously filled with horror at the sheer astonishing complexity and the daunting task of ever building an artificial brain that can even approach that, and awe and wonder that such a staggeringly complex system can basically self-assemble based on a set of starting conditions and rules. It leads me to think that the important part to understand is not the overall final wiring pattern, but the rules and physics which dictate how the system develops.
suzmonster
Jul. 17th, 2009 01:35 pm (UTC)
Your sweeties are pursuing my dream, too. I want a degree in cognitive neuroscience. Then I can be called Dr. Suz, and that sounds enough like Dr. Seuss to be funny. =)
spiralflames
Jul. 17th, 2009 03:00 pm (UTC)
fascinating. she says spockily.
big_sarah_yeti
Jul. 17th, 2009 09:46 pm (UTC)
Hallo,

I know that this has exactly nothing to do with your post, but I would like to express my dismay that I am going to be visiting the US (NY, San Fran and LA) next summer, and yet you are going to be nowhere near. I would have loved to meet you, if only to discuss my BDSM based PhD thesis. I just thought I would express that frustration.

Grrr
tacit
Jul. 18th, 2009 07:31 pm (UTC)
When will you be around? I'm only about 8 hours by car or an hour by plane from SF.
big_sarah_yeti
Jul. 18th, 2009 07:54 pm (UTC)
Wow, 8 hours is a long time in UK land...distances all seem bigger when you come from a pokey island I guess!
I'll be there in June most probably, though it depends on the people I'm visiting and when cheap flights are. I've never really travelled before so I need to enlist the help of some friends before settling on dates.
tacit
Jul. 18th, 2009 08:07 pm (UTC)
Drop me a line when you have some idea about when you'll be here!
big_sarah_yeti
Jul. 18th, 2009 09:06 pm (UTC)
Will do! :)
fallingupthesky
Jul. 18th, 2009 01:51 am (UTC)
I think that's exaggerated. First of all, not all of the cells in the brain are Purkinje cells. Second, "up to" doesn't mean that this is the normal amount, for all I know the average might be 10,000 (no idea, just making up a number). It's still an awful lot no matter how you figure it, but I think you're too high by a few orders of magnitude.
naxela
Jul. 24th, 2009 12:31 pm (UTC)
> American billion (1x109), not British billion (1x1012)


Incidentally most people in the UK use the American definition nowadays - or try to use the "to the power of" way of writing such numbers.
jonnymoon
Jul. 29th, 2009 09:54 pm (UTC)
A mind boggling number. More connections, even, than the number of dollars that Barak Hussien Obama's wasting on his idiotic ideas.

Which is another mind-numbing number in of itself. 24 trillion dollars.

That's $24,000,000,000,000. And you thought you had a big number.



( 19 comments — Leave a comment )