Basic question about neurons
cma1114 at home.com
Mon Nov 26 16:35:31 EST 2001
"Richard Norman" <rsnorman at mediaone.net> wrote in message
news:rm950u8c0vh58lvnvd2o2hi7q3r87q7e6m at 4ax.com...
> On Mon, 26 Nov 2001 18:52:16 -0000, "Theophilus Samuels"
> <theophilus.samuels at btinternet.com> wrote:
> >> 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
> >is the action potential itself.
> Sorry -- see below, the key event is NOT an action potential.
> >> 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?
> You are obviously not familiar with the many neurons that do complex
> information processing completely without action potentials. The
> retina of the eye is perhaps the best example -- the rods and cones,
> the horizontal cells, and the bipolar cells all function without
> action potentials.
> >> 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.
> What I mean is that every synapse has both a presynaptic side and
> a postsynaptic side. The presynaptic side is an "output" from one
> cell and the postsynaptic side is an "input" to another (or possibly
> the same) cell. So the totality of "outputs" in the entire CNS must
> equal the totality of "inputs".
My initial question presupposed this, but now I am wondering if there might
be more CNS inputs, of external sensory and proprioceptive information, than
CNS outputs to muscles. To a non-expert like myself some imbalance seems
plausible, although not a many-orders-of-magnitude difference. What I am
getting from this discussion and from other readings I have done is that a
(at least rough) balance exists because all neurons do not look like the
prototypical image of many inputs converging to a single output, but in fact
1) some neurons have more outputs (through many axon terminals) than inputs,
2) some axons terminals reach multiple dendrites either through direct
connections or through release of neurotransmitter not at a synapse but into
an extracellular region where it can be picked up my multiple dendrites, and
3) some dendrites synapse with other dendrites.
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