Basic question about neurons

Richard Norman rsnorman at
Mon Nov 26 19:44:00 EST 2001

On Mon, 26 Nov 2001 21:35:31 GMT, "chris ackerman" <cma1114 at>

>"Richard Norman" <rsnorman at> wrote in message
>news:rm950u8c0vh58lvnvd2o2hi7q3r87q7e6m at
>> 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.

My impression is that sensory input far exceeds motor output in
numbers of cells.  The visual system of most organisms contains an
enormous number of receptors.  The olfactory system is usually also
extremely complex.  Contrast the entire motor system controlling, say,
a lobster leg with only about a dozen motor neurons for all the joints
in one leg.  An insect leg similarly has very few motor neurons.  At
the same time, each type of leg is coated or tipped with an enormous
number of sensory sensilla.  We mammals use a couple of orders of
magnitude more motor cells to control our muscle, but that still pales
in comparison with the number of sensory cells.

But you are exactly right about the fact that the cartoon images of
neurons in textbooks almost never looks like a real neuron.  It is
even worse when they try to show myelin.  Text draw an axon looking
like a bunch of link sausages joined end-to-end.  In reality, a
myelinated axon some 10 micrometers in diameter may have Nodes of
Ranvier perhaps a millimeter apart. The ratio of length to width is
perhaps 100 to 1.  I have never seen a drawing of a myelinated axon
that shows the reality.

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