Valerie,
I cannot over-emphasize terminology.
Much (all) of our present study is both
language-dependent & concerned with
the development and operation of our
language abilities and processes. You
asked: "Neurotransmitters either create
or inhibit postsynaptic potentials (EPSP's
and IPSP's)...

Much of science is mathmatical modeling
and your inclusion of the parentheses in
your question is a good example of an
algebraic model in word-format. If we
look at it as such it becomes:
Neuroxmttr yields:
(create)(inhibit)(EPSP)(IPSP).
Apply the distributive property of algebra -
(create x excite), (create x create), (inhibit x
create), (inhibit x inhibit). Science is the
art of discovering what is discoverable,
and discovery is the art of phrasing the
proper question. We can't have excite x
ecite, or inhibit x inhibit directly from the
Nxmttr. We can only have all-or-none.

The neurotransmitter itself does not
excite or inhibit an exictation or inhibition.
It, as an all-or-none, either intiates or
does not, a cascade of events in the
post-synpatic membrane (the next soma
or dendrite, etc., in line) that produces
another action potential. It is then the
neural integration - the summation of
these AC's according to their rate of fire,
spatial orientation - their proximity to each
other, and their temporal association -
how close or seperated are they in time?
that produce either an excitatory
(depolarizing), or inhibitory
(hyperpolarizing) potential on the axons of
the receiver, thus "deciding" to signal the
next post-synapse in line or not. Again,
it's unfortunate, but science is saddled
with a lot of terminology "baggage". We
are accustomed to think of de- as "not"
(negative), or "less", and of hyper- as "up"
(positive), or "above", but, because the
"resting" potential is negative, de-polarize
means less negative (more positive -
algebra again: negative x negative =
positive) and hyper-polarize means pos x
neg = (more) neg. Am I helping?

The nature of your questions indicates
that you're giving serious thought to this
stuff. What's your major? (I'm in
Respiratory Therapy - I breathe (no pun
intended) physiology).
more to follow