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Postby lerrybobs » Fri Dec 02, 2005 12:40 pm

Why is it that Calcium ions enter the presynaptic neurone even though due to depolarisation the inside of the nerone is positive? Surely the 2+ charge on the calcium ion would repel the sodiums + charge.

Is it that the diffusion gradient effect is greater than any electrostatic repulsion?
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Postby sdekivit » Fri Dec 02, 2005 3:25 pm

let's consider a synaps at rest:

the membranepotential is negative due to K(+) inside the cell is higher than outside, causing a negative potential of -90 mV. Also Na(+) outside the cell is higher than inside cuasing a potential of +60 mV

--> at rest the membranepotential is about -70 mV because the permeabilty for K(+) is higher than for Na(+)

Also: outside the cell is a higher Ca(2+)-concentration causing a potential of +120 mV, but Ca(2+) is almost impermeable and thus has no influence on the membranepotential. But there is a Ca(2+)-concentrationgradient.

In the membrane,there are different channels: voltage gated Na(+), K(+) and Ca(2+)-channels.

A actionpotential reaches the synaps and due to depolarization of the membrane of the synaps, voltage gated Ca(2+)-channels open, entering the synaps and causing release of neurotransmitter release. This is all due to the Ca(2+)-electrochemical gradient of +120 mV, wehich is stronger than the Na-electrochemical gradient.

--. also the synaps has Ca-ATPases, Na/Ca-exchangers and so on to pump Ca out the synaps again restoring the rest membranepotential for Ca.

--> remember: it's only a few ions that cause neurotransmitter release.
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Postby lerrybobs » Fri Dec 02, 2005 4:19 pm

Thanks!

Do you have any links that offer more information on the subject?
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Postby Ilyaas » Sun Dec 04, 2005 11:44 am

when the Ca ions enter how do they bring about the release of neurotransmitter, from what I have heard the vesicles containing the neurotransmitter bind to the presynaptic membrane and release their contents but I dont understand the mechanism behind this, I mean how does the entry of Ca ions bring this about
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Postby MrMistery » Sun Dec 04, 2005 11:51 am

Well it is college level cellular biology. Do you know what the SNARE complex is? Then vesicle binds to the plasma membrane and releases the neurotransmitter by these proteins(v-SNARE and t-SNARE) that fuse together like a key and a keywhole. Usually there is a protein binded to the v-SNARE in the vesicle membrane called synaptotogmin that in it's active form doesn't allow the proteins to fuse. When Ca ions enter the cell, they inactivate synaptotogmin and the SNARE complex can be formed...
Is this what you meant? I can go into the biochemistry of syntaxin and VAMP but i don't understand those very well myself, i need to take more organic chemistry
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Postby Ilyaas » Sun Dec 04, 2005 8:24 pm

Is this what you meant? I can go into the biochemistry of syntaxin and VAMP but i don't understand those very well myself, i need to take more organic chemistry


Yes yes go ahead tell me whatever you know, tell me about the SNARE complex
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Postby MrMistery » Sun Dec 04, 2005 8:33 pm

Well i don't know much about the SNARE complex. a vesicle reaches it's destination when v-SNARE binds to t-SNARE. These proteins are only for finding of the target membrane, not for the fusion of the vesicle with the membrane. We don't know how fusion takes place, we think there might be some proteins involved there. After the vesicle has fused with the target membrane the SNARE complex is recicled. Two proteins named SNF and alpha-SNAP use GTP to hidrolise the SNARE complex. v-SNARE is recycled, while t-SNARE remains in the target membrane. What some people don't realise is that v-SNARE and t-SNARE are not proteins, they are PROTEIN FAMILIES. There are many types of v-SNARE, each with it's own type of t-SNARE. If there only one type, secretion vesicles might end in the endoplasmic reticulum.
An enzyme in the venom of the black widow spider, named latrodotoxin, acts as a hydrolase, breaking down v-SNARE....

What else do you want to know about the incredible activity of the cell?
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Postby Ilyaas » Sun Dec 04, 2005 9:00 pm

what role do the calcium ions have bringing about such events?
(that'll be all thanks a bunch)
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