thom at crayfish.ucsd.edu
Wed Oct 7 19:08:33 EST 1992
Hi all. Regarding the thread on neurotransmitters, and how greater
complexity derives from "inhibition" and "excitation", and how more
than two neurotransmitters are thereby useful...
I won't repeat what others have stated, but just make two points.
First, the ideas of 'excitation' and 'inhibition' refer to the
membrane polarization level of a cell, as you noted; what these
labels obscure is the fact that the voltage across a cell membrane
can change via several different currents. The action potential is
"sodium rushes in (depolarization), then potassium rushes out (re-
polarization)", dogmatically, but in the complex behavior of neurons
studied at the cellular level many different channel types exist. There
are fast sodium currents (inward), fast and slow calcium currents
(inward), fast, voltage-dependent potassium currents, slow voltage
dependent potassium currents, slow, calcim-dependent potassium currents,
"inwardly-rectifying" potassium currents (all outward), hyperpolarizatin-
activated inward currents, and many more. They turn on at different
times, off at different times, at different levels and with different
kinetics and may be dependent on or independent of voltage. Different
currents have different impacts on any number of significant factors--
firing rate, burst frequency, resting potential. They may interact in
complex ways... for instance, calcium entry first activates, then
at higher concentration inactivates a particular potassium current in
some Aplysia neurons.... We are right in the midst of this sort of
sorting-out research at present.
Neurotransmitters and neuromodulators affect specific currents in specific
ways, as others have noted... thus nearly anything, conceiveably, can
be changed. To reduce the effects to "excited" or "inhibited" is like
just stating who won an election--it's certainly pertinent, important
information, but clouds over all the reasons, campaigns, social movements,
national mood, issues, previous records, and all that influenced the
election outcome and whose influences do not cease simply becuase the
election outcome is a boolean result.
I think my second point is in there somewhere. If not, I've forgotten it.
hope this is of use!
tcleland at ucsd.edu
Dept. of Biology, UCSD
Cellular mechanisms of neural central pattern generation in crustacean models.
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