question: job of a neuron

Richard Norman rsnorman at mediaone.net
Mon Sep 3 14:19:37 EST 2001


On 3 Sep 2001 11:53:03 -0700, jonesmat at physiology.wisc.edu (Matt
Jones) wrote:

>remove_this!helbrecht at gmx.net ("Wolfram") wrote in message news:<9mroi3$3pcpu$1 at ID-40201.news.dfncis.de>...
>> Hello everyone,
>> 
>> in the field of neuroscience i am layman. I have no clinical
>> background nor nothing else. Just a person (a bit) interested in
>> the field.
>> 
>> Everywhere on the web i can read pages about _how_ neurons work,
>> _how_ they interact with others, _how_ the whole brain works (some
>> pages assert this).
>> 
>> Has yet someone figured out _what_ a neuron does, i.e. what its
>> task is?
>
>
>Boy, didn't -this- question become controversial all of a sudden!
>
>I see a lot of people don't think it's very meaningful to ask what a
>single neuron does, and draw the analogy with asking what a particular
>wire or electronic component does.
>
>Me, I think it's a perfectly reasonable question. For example, what
>does a wire do? It does the following (ideally):
>
>1) Conducts current with zero resistance.
>2) Acts as an isopotential element (i.e., causes anything connected to
>its two ends to be at the same potential.
>3) In non-ideal cases, it may get hot, or even give off light. This
>can sometimes be useful.
>
>How 'bout a transistor?
>
>1) It acts as a variable conductance, allowing current to flow if
>certain conditions are met at its base (or gate).
>
>An op-amp?
>
>1) It produces an output that rises until its two inputs attain
>equivalent potentials.
>
>That doesn't seem so hard. Why not a neuron?
>
>1) It takes some number of inputs, each of which occurs at a
>particular time and  has a particular shape and a particular size, and
>performs -some- sort of operation on those inputs. Then it produces
>-some- sort of output (including the case of no output at all).
>
>That's -what- a neuron does, at least in the most general sense. The
>details include such worries as -what sort- or operation, and -what
>sort- of output. These details are currently in dispute among
>neurophysiologists. For example, some proposed operations are:
>
>1) Summing, or integrating, or integrating in a leaky fashion. This
>operation is most often supposed to operate on the membrane potential
>(i.e., summing IPSPs and EPSPs), but could just as easily work on
>membrane current or conductance.
>
>2) Detecting coincidences.
>
>3) Multiplication (actually, division).
>
>4) Nonlinear operations, such as detecting a threshold-crossing in the
>input.
>
>Some of the proposed outputs are:
>
>1) A certain rate of spiking.
>
>2) A certain pattern of spiking.
>
>3) In the case of both of the above, as well as for nonspiking
>neurons, a release of some number of packets of neurotransmitter.
>These can be either "excitatory" or "inhibitory", but in either case
>they contribute either to electrical or chemical changes in
>postsynaptic cells.
>
>So these are a brief, definitely nonexhaustive list of things that a
>single neuron is -capable- of doing. Just as with a wire, or an op
>amp, they can act as building blocks for more complicated tasks.
>Nonetheless, whatever the task, or whatever the context, the final
>circuitry is made up of things that do some combination of the above.
>
>I think it makes perfect sense to ask about this. If we didn't -know-
>the answer to this question for a wire, how much success would we have
>figuring out how a toaster works? If we can't answer it for a neuron,
>how are we ever going to figure out how the -brain- works?
>
>
>Cheers,
>
>Matt

I think if you look back through the history of this thread that a
number of reasonable physiological definitions of neuron have been
posted (wavelet theory notwithstanding).  The original poster was very
non-technical and wouldn't understand the subtleties involved.  Those
of us who do have some technical expertise understand all too well the
subtleties and the non-meaning of the question.  The problem is that,
beyond being a "computational" or "informational" element, the neuron
is also a living cell, and does all the things necessary to sustain
"life' (whatever that is).  Furthermore, in many (but not all) cases,
the act of living interacts significantly with the process of
processing information.  The metabolism, biochemical synthesis,
genetic control, etc of a neuron can all too easily interact with the
signal processing which involves second transmitters, calcium influx,
etc.  And  up/down regulation of membrane proteins strongly modifies
information processing.

So we know a lot about how neurons work.  But we know little about the
job of any particular neuron.  Perhaps the Mauthner cell of a goldfish
or the VD cell in the stomatogastric ganglion of a lobster.  But point
to a random arbitrary spot of the cerebral cortex and select one of
the ten thousand neurons in any cubic millimeter and try to say just
what the "job" of that one cell is.  Almost certainly, if you kill it,
no one (not even the owner of that particular brain) will ever know
the difference.




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