Hi Dr. Jones, Thank You for the work you put into your information-generous
post.
in the perspective in which i work, i 'translate' such 'time' courses into
their underpinning energy-flow dynamics [eliminating 'time'], thereby
mapping everything into energy-flow directionalities, which, further, map
(directly :-) directoinalities inherent in to 'behavior'.
no one's ever agreed with me on doing this [at least not in anything
communicated to me], but doing it this way significantly-simplifies
everything.
Cheers, Dr. Jones, ken
Matt Jones wrote in message ...
>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
