cwilms at stud.uni-frankfurt.de
Wed Jan 16 17:55:09 EST 2002
yan king yin <y.k.y@(dontspam)lycos.com> wrote:
> There could be two types of diffusion. One is retrograde, from
> postsynaptic to presynaptic. Another type is from one postsynaptic
> site to another postsynaptic site nearby (inside the same dendrite).
Diffusion is a random process. A particle will as a rule diffuse in
_all_ directions, unless it is physically hindered from doing so (see
below). So a signaling molecule can not be directed to one location by
means of diffusion.
> The secondary messengers of metabotropic receptors should
> serve some purpose, otherwise they could be replaced entirely
> by electrical synapses. Thats why Im suggesting that there
> could be some downstream events after their activation, and
> 2nd messengers might diffuse to a nearby synapse.
Secondary messengers serve a purpose, as do the first messengers of any
metabotropic receptor. Usually a second messenger plays an important
role in signal amplification. Second messengers will normally work
downstream within the cell. Since an electric synapse can not regulate
metabolism or gene-expression, they never could replace a metabotropic
receptor. Signaling to nearby synapses will use different means, NO (see
other thread) being only one of the possibilities.
> Lets say there are 1-3 synapses per um (micrometer) of dendrite.
> That means synapses are about 1-0.3 um apart. "Fast" axonal
> transport is 2 to 5 um per second. Diffusion might even be faster
> than this.
I don't have much informationen on diffusion speed of different
particles in tissues or the synaptic cleft at hand, but you may want to
do a short PubMed search. Erwin Neher (or was it Bert Sakmann - dang,
getting confused here) recently published a paper in which he discusses
measurements of diffusion in the synaptic cleft. If I remember
correctly, Matt Jones has some stuff on diffusion of GABA to nearby
synapses. Boris Barbour has done a bit of work on similar topics.
Just to give you a pointer to more information - which is more accurate
than mine :-)
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