question: job of a neuron

Richard Norman rsnorman at mediaone.net
Wed Sep 12 22:18:10 EST 2001

On Wed, 12 Sep 2001 16:26:35 GMT, "Glen M. Sizemore"
<gmsizemore at triad.rr.com> wrote:

>MJ: It turns out you can get fairly interesting
>behavior out of circuits
>that contain -nothing- but inhibitory neurons,
>because each cell has
>its own native excitability even without excitatory
>synaptic drive.
>Thus, two interneurons coupled to each other can
>produce either
>synchronous or alternating oscillations. Many
>interneurons wired up
>together can create a "pacemaker" circuit, even
>though none of the
>cells themselves are inherently periodic. Interneuron-interneuron
>connections are widespread in cortex, and this may
>be part of their
>GS: Interesting.....I have believed for a long time
>that spontaneous locomotion (spontaneous at the
>level of behavior - i.e., no eliciting stimulus) is close
>to the first kind of behavior in existence. I believe
>that it is more fundamental than the unconditioned
>reflex which almost certainly requires excitatory
>synaptic drive. So, the notion that inhibitory
>neurotransmission is, evloutionarily, the most
>primitive, makes some sense to me, given what you
>write about oscillations which are almost certainly
>the backbone of most central pattern generators.
>BTW, what is the evidence that inhibitory
>connections are the first to arise?

Note that purely inhibitory actions can easily cause positive
activity.   Most simply, disinhibition (inhibition of an inhibitory
connection) can release activity latent in a spontaneously active
cell.  Second, "inhibitory rebound" can stimulate activity in a cell
that is not spontaneously active.  This is a situation that is easily
replicated in even a simple H-H model.  Suddenly stop hyperpolarizing
a cell and it tends to fire.  This seems to be because the
hyperpolarization removes any remnant sodium activation that is
present even at resting potential.  So when the hyperpolarization is
stopped, the cell is hyperexcitable.  When a cell has a propensity to
"burst" (produce a sustained flurry of APs rather than one or two,
perhaps caused by a calcium system) you can easily get complex
rhythmic activity out of purely inhibitory connections.  Hence the
stomatogastric ganglion (where is the "other" Richard -- this is his
area!)  I believe the even simpler (nine neuron) cardiac ganglion
works in a very similar manner. 

As to the "ancient" nature of inhibitory channels (or any neuron
channels), remember that the brain did not really invent much.  It
really just took advantage of signaling machinery that has been part
of the cell repertoire for a very long time.  This is machinery that
allows single celled beasties to respond to external events.

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