Basic question about neurons

Richard Norman rsnorman at mediaone.net
Mon Nov 26 19:50:08 EST 2001


On Mon, 26 Nov 2001 22:37:00 -0000, "Theophilus Samuels"
<theophilus.samuels at btinternet.com> wrote:

>Yet again, you and I are are trying our best to understand one another. We
>are both going off on a tangent from the main problem, but what the hell.
>
>> 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.
>
>I am well aware of the properties of the neurons situated in the retina.
>However, you are still missing the point. You are correct in that most of
>the retinal cells (except some amacrine cells) respond to stimulation with
>graded changes in membrane potential. In fact, only the 'ganglion cells'
>fire action potentials, that is the final neuron in the retinal pathway, the
>one that sends the information to the forebrain. It is almost impossible to
>describe the level of processing that occurs in the retinal cells in this
>short message, but, surely you cannot deny the following fact: despite all
>of the retinal processing taking place, the end product of all this
>processing are still action potentials, or in other words, all-or-nothing
>responses. Can you not see this? Or am I not making any sense? Let's sit
>back for a moment and contemplate the following - why are action potentials
>all-or-nothing events? Surely during your career, you must have asked
>yourself the both scientific and philosophical question 'why are APs
>all-or-nothing events?' Given that some microareas of the brain may not even
>use AP for information processing, nevertheless depending on what level you
>think conscious processing occurs (i.e. at graded potentials, action
>potentials or both) makes the question even more intriguing.
>So, how does all this banter relate to the original question, well to
>reiterate, depending on how you view the importance of a single neurons
>inputs and outputs, then perhaps the difference is not as large as we might
>first imagine.
>
Yes, action potentials do really matter to us because, in the last
analysis, everything we do is controlled by action potentials sent out
motor neurons to our muscles.  But the original question was about
numbers of "inputs" and "outputs" to and from nerve cells.  It may
well be that most of the synapses in the CNS are between neurons (or
portions of neurons) that work with graded potentials.  So action
potentials are really quite irrelevant to the question of whether a
cell has more synaptic inputs or synaptic outputs.

The all-or-none action potential is necessary to overcome the "spreads
with decay" local nature of the graded signals.  Developing linear
amplifiers that can amplify the signal down a long axon and still
accurately retain the graded nature is probably quite an impossible
task for evolution to come up with.  So the all-or-none action
potential was a good alternative.  




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