Laurent <lorseau at ens.insa-rennes.fr> wrote in message news:<3D352376.D1582305 at ens.insa-rennes.fr>...
> Thanks.
>> I read somewhere that some neurotranmitters act only after the axon has stopped
> firing.
Richards answers are right (as usual). There is -no- special
mechanism that prevents release of neurotransmitter, or its action at
postsynaptic receptors, until the action potential has subsided.
> I think this means the transmitters can't act as far as the axon is firing ?
> Is this really possible ?
There is no special built-in delay to ensure a specific waiting time.
> For me, that would mean, by transposing it to a bigger scale, that we are waiting
> for the end of some action to continue the process, which does not seem too stupid
> to me. How often do we not even want to think, as far as the action is not over ?
> But that could be totaly wrong...
I do not think this waiting is necessary or desireable. Most of the
time, the nervous system attempts to do everything as fast as it can,
given the constraints of its architacture, and the constraints imposed
by physics. Typically, it achieves near optimal speed given the
materials it is made of.
However, these constraints -do- introduce delays. Here is a list of
approximate delay times associated with various steps in transmission
between two run-of-the-mill neurons in cortx or hippocampus:
1) Delay from the time the spike (action potential) is fired at the
cell body until the propagating spike reaches the nerve terminal -
about 1-3 msec.
2) Delay from the time the spike reaches the terminal until calcium
channels are fully open - about 100 usec (microsec).
3) NOTE: although calcium channels are open at the peak of the spike,
not very much calcium will enter until the spike starts to fall. This
is because the peak of the spike is close to the calcium reversal
potential (see a good neuroscience textbook if you don't understand
what I just said). Therefore -most- calcium entry occurs on the
falling phase of the spike (perhaps it was something like this that
you read about that made you think the spike needed to be over before
transmitter was released). So, the delay from the initial opening of
Ca channels until the bulk of calcium enters is about the duration of
the spike - about 0.5-1 msec.
4) Time required for activation of the release machinery and fusion of
the vesicle - about 100 usec. NOTE: there are also very delayed
components of release that can occur up to several hundred msec after
calcium entry, but the main portion happens very quickly.
5) Time required for diffusion of neurotransmitter out of the vesicle
and across the synaptic cleft - about 10 usec (that's right, ten
microseconds). As Richard mentioned, diffusion is extremely fast. It
has to be, since its essentially the mechanism underlying evry other
motion in biology. Basically, nothing much can be faster than
diffusion.
6) Time required for binding to, and initial activation of
postsynaptic receptors - about 10-25 usec.
7) Time required for -PEAK- postsynaptic response - if the receptors
are ligand-gated ion channels, their peak opening will occur within
about 0.5-10 msec. If the receptors are G-protein-coulpled, the peak
response will occur about 100 msec later (because of multiple
biochemical events between receptor activation and the final
response).
8) Time required for the decay of the postsynaptic response - for
ligand-gated channels, the currents will decay in 1-200 msec,
depending on channel type. For g-protein coupled responses, usually
last for about 0.5-2 seconds. Also, for really fast ligand-gated
responses, the ultimate duration of the postsynaptic potential change
will be roughly the membrane time constant - about 10-30 msec.
So, the total time from spike firing at the cell body until the peak
of the postsynaptic response is typically on the order of 1-4 msec.
The largest delay by far is the time it takes the spike to propagate
down the axon (which depends on axon length, of course).
Interestingly, the different delays from different axons can be used
to establish timing mechanisms, called delay lines. This is used in
the auditory system to locate sounds horizontally, by exploiting the
interaural time-diffeence (i.e., time between when the sound reaches
one ear, then the other).
Cheers,
Matt
Cheers,
Matt
