On Tue, 16 Jul 2002 18:39:45 +0200, Laurent
<lorseau at ens.insa-rennes.fr> wrote:
>Ok ! thanks, Richard and Brian !
>>So it can take quite a while for the transmitters to go accross the synaptic
>cleft.
>It is that delay I was 'thinking' about.
>For a single synapse, could this delay vary a lot with time ? Or does it just
>attempt to match the right delay to work properly for that synapse's function ?
>>Is there a relation between that delay and the time period during which the
>transmitters are caught by the target neuron ? That might depend on the amount
>of transmitters, no ?
>And does the type of transmitters affect the time it takes to go accross the
>synaptic cleft ?
>Actually, the time it takes to diffuse across the synaptic gap is very
tiny. That is only a very small part of the synaptic delay.
What you want to know, apparently, is:
Can a cell fire action potentials and then stop.
Then after some substantial delay a synaptic potential is
produced.
That scenario is unlikely. When the axon produces the action
potential, the release machinery starts to work immediately. At
least, the trigger is immediate. Most of the synaptic delay seems to
be in the time it takes to open the Ca channels and get the Ca inside
to activate the transmitter release. The released transmitter really
crosses the synaptic gap quickly. It then activates the post-synaptic
receptors. Fred's response refers to thea binding step between
transmitter and post-synaptic receptor. But it is unlikely that this
whole process would take more than a couple of milliseconds.
Another possibility involves the notion of "metabotropic" receptors.
In this case, the binding between nerve transmitter and receptor first
activates a G protein which then activates a protein kinase which then
phosphorylates (activates) some other protein in the cell. Eventually,
some biochemical process in the cell activates a membrane channel to
produce a synaptic potential. This certainly could be delayed. But
usually this process starts relatively quickly after the original
axonal activation and continues for a substantial time period
afterwards. That is, it is the duration of the synaptic response that
is prolonged, not the delay. But it is certainly theoretically
possible for there to be a long delay.
