Basic question about neurons

Theophilus Samuels theophilus.samuels at btinternet.com
Mon Nov 26 13:52:16 EST 2001


> It doesn't make too much sense to count number of synaptic inputs per
> second against number of action potentials per second as a measure of
> inputs vs outputs for a cell.

It makes sense if you look past the question.

> You also don't realize the possibility that a tremendous number of
> neurons in the vertebrate CNS are "local" and don't send axons out any
> distance, if they have axons at all.

And you don't realise the philosophical implications of all-or-nothing
events - i.e. who cares whether the axon travels 1m or 1 mm, the key event
is the action potential itself.

> Neurons can work perfectly well
> without making action potentials at all -- graded potentials in axons
> can cause graded release of transmitter.

They may work just as well, but how do you know they're serving the same
"function" as an action potential?

> But it still always comes down to the fact that the number of
> post-synaptic endings (cell inputs) must equal the number of
> pre-synaptic endings (cell outputs).

This is confusing. According to this, if neuron A had x inputs
('post-synaptic endings') then it must also have x outputs ('pre-synaptic
endings'), i.e. if there were 100,000 inputs, then would a single neuron
would have 100,000 outputs?! Perhaps you meant something else.

I agree with you that the original question was probably a spatial question,
I was merely offering another perspective on the problem.

T.L.S.

Richard Norman <rsnorman at mediaone.net> wrote in message
news:g9720uk3nd8gpao041tskbs4q917evjm62 at 4ax.com...
> On Sun, 25 Nov 2001 13:03:20 -0000, "Theophilus Samuels"
> <theophilus.samuels at btinternet.com> wrote:
> >
> >chris ackerman <cma1114 at home.com> wrote in message
> >news:lumL7.44668$Ze5.25808688 at news1.rdc1.md.home.com...
> >> I am a neophyte so forgive the simpleness of my question, but why is it
> >> that I always read descriptions and see pictures of neurons as having
> >> many dendrites and few axon terminals? If this is representative of
most
> >> neurons in the brain, how is it possible to have so many inputs and so
> >> few outputs?
> >>
> >  The question you ask is interesting. Physically speaking, neuronal
input
> >far exceeds its output (i.e. number of dendrites (+ other axons synapsing
on
> >the neuron) >>> single axon). However, looking a little deeper, this is
not
> >entirely the case when considering input and output with respect to the
> >number of electrical signals recieved and transmitted per unit time.
> >  Dendrites are almost never myelinated, and they respond with graded
> >depolarisation or hyperpolarisation that decrements spatially and
temporally
> >(some may actually generate action potentials). Firstly, if we consider
the
> >time it takes for an AP to fire and the refractory period (say 5-10 ms)
then
> >theoretically the output (electrical signals or action potentials) of an
> >axon ranges in the hundreds per second. Alright, considering that the
number
> >of synaptic inputs to a single axon can range into the 100,000s, the
output
> >number is still relatively small (although it is still 100 times more
than
> >you might have first expected). Secondly, even though it is highly
> >debatable, as the AP is an all-or-nothing response, one might argue that
> >they are all that matters since they're the absolute and final product of
> >the culminated input signals. An analogy would be as follows: consider an
> >election process. Many thousands of individuals will vote for a single
> >candidate to win and represent the majority. It is this individual that
> >represents the collective voice of the many and is therefore the most
> >important consequence of the entire original electorial process. It is
this
> >person that is the first representative met by other elected persons and
> >influences other decisions. So, if we now revert back to the neuron, we
can
> >see that the really important consequence of all input signals is the
final
> >action potential. And therefore, even though we might think that the
output
> >profile of a neuron is severely limited, in reality it must be adequate
> >(perhaps even more than adequate!), otherwise we wouldn't be here to ask
> >such questions.
> >
> It doesn't make too much sense to count number of synaptic inputs per
> second against number of action potentials per second as a measure of
> inputs vs outputs for a cell.  I believe the original question was
> really comparing degree of convergence with degree of divergence in
> the wiring pattern of the nervous system -- a purely spatial question.
>
> You also don't realize the possibility that a tremendous number of
> neurons in the vertebrate CNS are "local" and don't send axons out any
> distance, if they have axons at all.  Neurons can work perfectly well
> without making action potentials at all -- graded potentials in axons
> can cause graded release of transmitter.  And there are many synapses
> in "microcircuits" with a small cluster of cell processes (dendrites?
> axons? -- whatever, it doesn't really matter) forming mutually
> reciprocal synapses on each other.
>
> But it still always comes down to the fact that the number of
> post-synaptic endings (cell inputs) must equal the number of
> pre-synaptic endings (cell outputs).
>





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