Basic question about neurons

Theophilus Samuels theophilus.samuels at btinternet.com
Mon Nov 26 17:37:00 EST 2001


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.

> 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".

I understand what you're saying here and I do agree with you, it's a clever
way of making sense of the physical side of things.

T.L.S.


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
event
> >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".
>





More information about the Neur-sci mailing list