Board index › Topical Discussion › Computers, Math, Science, and Technology

Stochastic Behavior of Calcium Ions in Nerve Terminals

Page 1 of 1 [ 5 posts ] 

Previous topic | Next topic

Joybob
Velociraptor
Velociraptor

User avatar

Joined: 22 Sep 2007
Gender: Male
Posts: 460

04 Nov 2007, 8:15 pm

I've always been intrigued by Henry P. Stapps hypothesis that quantum mechanics must be invoked to explain the behavior of Calcium Ions in neurons.

Quote:
The pertinent analogous situation in the brain involves the motion of a calcium ion from the exit of a microchannel of diameter 1 nanometer to a target trigger site for the release of a vesicle of neuro-transmitter into the synaptic cleft. The irreducible Heisenberg uncertainty in the velocity of the ion as it exits the microchannel is about 1.5 m/s which is smaller than its thermal velocity by a factor of about .0004. The distance to the target trigger site is about 50nanometers. So the spreading of the wave packet is of the order of .2 nanometers which is of the order of the size of the ion itself and of the target trigger site. Thus the decision as to whether the vesicle is released or not in an individual instance will have a large uncertainty due to the Heisenberg quantum uncertainty in the position of the calcium ion relative to the trigger site. The ion may hit the trigger site and release the vesicle or it may miss the trigger site and fail to release the vesicle.


The possibility that neurotransmitter release may not be a deterministic process is very appealing to me because it would imply that emergent brain processes are themselves non-deterministic and it'd give room for things like consciousness, free-will, etc. to be produced. I've found further studies that put the probability of neurotransmitter release at between 40% to 20% depending on the distance between the micro-channel and receptor protein.

I've tried to find better studies to substantiate this but it seems pretty scarce. Anyone know any good articles or studies I can use to get more background?



yesplease
Veteran
Veteran

User avatar

Joined: 28 Dec 2006
Age: 43
Gender: Male
Posts: 517

04 Nov 2007, 9:15 pm

I think you're reachin' just a bit. Just because something isn't deterministic from our POV doesn't mean it isn't deterministic. As to whether or not we have free will and all that, who knows...



Joybob
Velociraptor
Velociraptor

User avatar

Joined: 22 Sep 2007
Gender: Male
Posts: 460

04 Nov 2007, 9:25 pm

yesplease wrote:
I think you're reachin' just a bit. Just because something isn't deterministic from our POV doesn't mean it isn't deterministic. As to whether or not we have free will and all that, who knows...


Well that's only if you disagree with the Copenhagen Interpretation of quantum physics.



yesplease
Veteran
Veteran

User avatar

Joined: 28 Dec 2006
Age: 43
Gender: Male
Posts: 517

05 Nov 2007, 4:59 am

Not necessarily. I think of it more as "Shut up and calculate!" ;)
The assertion that what we've described as the influence of quantum mechanics on ourselves implying that we are somehow not deterministic (Or deterministic, or something else?) is as valid imo as stating that the universe was deterministic when classical physics was first being introduced because the force of our impact on the ground was equivalent to the product of our mass and acceleration. Capice?

Like I said, I think yer reachin'. Although... It may make for an interesting Sokal Affair part Deux. :D

Edited for
Image



Last edited by yesplease on 06 Nov 2007, 3:40 am, edited 1 time in total.

LKL
Veteran
Veteran

User avatar

Joined: 21 Jul 2007
Age: 48
Gender: Female
Posts: 7,402

05 Nov 2007, 9:37 pm

I've never heard of Heisenberg uncertianty being applied to particles at the ionic/atomic level; my understanding was that it applied to sub-atomic particles only. My training, however is in biology rather than physics.

That said, I do find this theory interesting and am glad that you posted it. Whether you invoke Heisenberg or not, Stochastic effects have to be important in the action potentials and synaptic transfers of neurons; when you think about it, the distances involved are huge at the level of ions or even molecultes. I suspect that the answer is partly in the sheer quantity of ions or neurotransmitters involved: the probability of one ion or molecule hitting its target receptor is quite small, but if you throw a couple million of them into the gap, the probabilities change significantly.

Do you know if the quantitiy of neurotransmitters released varies from synapse to synapse? I know that some pathways can be primed to respond more quickly and/or reliably; my impression was that the actual number of synapses increased, but maybe the volume of neurotransmitter released, or the rate of uptake, also varies.