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Saltatory Conduction: single AP or not?

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somasimple
#109
Oct3-08, 12:09 AM
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Quote Quote by atyy View Post
I think I may finally understand somasimple's "discontinuity" objection - it makes sense to me if "discontinuous" means "non-analytic".
Not at all.
Continuity is a prerequisite for an electrical signal in a wire/cable.
There is discontinuities at internode/node junctions when the signal leaves the internode entering in the node and when it leaves the node entering to the next internode.
somasimple
#110
Oct3-08, 12:11 AM
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Quote Quote by granpa View Post
it says the internode is just modeled as a resistor.
That is the problem I'm pointing out.
Normally the nodes are connected to external milieu.
atyy
#111
Oct3-08, 01:06 AM
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Quote Quote by somasimple View Post
The above doesn't even model most nodes as active. HS discuss resistance and capacitance of the internode, and it is very important for them to come to the conclusion that the internode is passive, or at least much less active than the nodes (p328 bottom paragraph through p329).

Quote Quote by somasimple View Post
There is discontinuities at internode/node junctions when the signal leaves the internode entering in the node and when it leaves the node entering to the next internode.
In the data or in someone's model?
somasimple
#112
Oct3-08, 01:10 AM
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Quote Quote by atyy View Post
The above doesn't even model most nodes as active. HS discuss resistance and capacitance of the internode, and it is very important for them to come to the conclusion that the internode is passive, or at least much less active than the nodes (p328 bottom paragraph through p329).
Adding a capacitor doesn't change the passivity but it is missing (I added the table 2)
Quote Quote by atyy View Post
In the data or in someone's model?
Both.
Edit: In the model a node is connected to 2 internodes and must be at the same potential.
In data: the end of an internode is not at the same potential than the beginning of the next internode.
somasimple
#113
Oct3-08, 01:13 AM
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Quote Quote by atyy View Post
HS discuss resistance and capacitance of the internode, and it is very important for them to come to the conclusion that the internode is passive, or at least much less active than the nodes (p328 bottom paragraph through p329).
I agree.
Edit:
http://www.pubmedcentral.nih.gov/art...?artid=1473353
see figure 1 for a more appropriate electric model.
granpa
#114
Oct3-08, 01:58 AM
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" The conduction velocity also is relatively insensitive to the internodal length"

i like that.
somasimple
#115
Oct3-08, 01:59 AM
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Here is the problem:
And, active node or not, it does not change the passive internodes, does it?
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somasimple
#116
Oct3-08, 02:01 AM
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Quote Quote by granpa View Post
" The conduction velocity also is relatively insensitive to the internodal length"

i like that.
Me too. It is normal in a body that moves and thus stretches or shrinks nerves: The message must be delivered (safety factor) and insensitivity to internal motion.
granpa
#117
Oct3-08, 02:12 AM
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if the impulse does indeed move at or just below the speed of sound in water or is even just limited by the speed of sound in water then that would mean that significant amounts of water are being moved. the mass of the water would add an inductance to the equivalent circuit. or so it seems to me.
somasimple
#118
Oct3-08, 02:15 AM
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Quote Quote by granpa View Post
if the impulse does indeed move at or just below the speed of sound in water or is even just limited by the speed of sound in water then that would mean that significant amounts of water are being moved. the mass of the water would add an inductance to the equivalent circuit. or so it seems to me.
Why an inductance?
granpa
#119
Oct3-08, 02:17 AM
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because inductance is the electrical equivalent of mass.
granpa
#120
Oct3-08, 02:20 AM
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just think of a sound wave as passing through a series of masses connectedby springs. the mass effect becomes obvious.
atyy
#121
Oct3-08, 02:22 AM
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Quote Quote by somasimple View Post
Edit: In the model a node is connected to 2 internodes and must be at the same potential.
In data: the end of an internode is not at the same potential than the beginning of the next internode.
Quote Quote by somasimple View Post
Edit:
http://www.pubmedcentral.nih.gov/art...?artid=1473353
see figure 1 for a more appropriate electric model.
The data doesn't show a discontinuity, just a quick change in voltage over distance (HS Fig. 11). But I agree that if the node and internode are each modelled as a single compartment, it looks like there will be some discontinuity. I suppose more compartments can be added for both the node and internode, or it could also be taken care of by a partial differential equation in which parameters vary continuously over space.

Moore 1978 does look more appropriate. Some papers that cite their work are:

Hartline DK, Colman DR. Rapid conduction and the evolution of giant axons and myelinated fibers. Curr Biol. 2007 Jan 9;17(1):R29-35.
http://www.pbrc.hawaii.edu/~danh/PDF...olman_2007.pdf

Richardson AG, McIntyre CC, Grill WM.
Modelling the effects of electric fields on nerve fibres: influence of the myelin sheath. Med Biol Eng Comput. 2000 Jul;38(4):438-46.

McIntyre CC, Richardson AG, Grill WM.
Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle. J Neurophysiol. 2002 Feb;87(2):995-1006.
http://jn.physiology.org/cgi/content/full/87/2/995

Hartline's site: http://www.pbrc.hawaii.edu/~danh/
Grill's site: http://fds.duke.edu/db/pratt/BME/fac...l/publications
somasimple
#122
Oct3-08, 02:23 AM
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Quote Quote by granpa View Post
because inductance is the electrical equivalent of mass.
http://en.wikipedia.org/wiki/Inductance
A neutral thing (water) seems unable to create electric field or voltage by itself.
granpa
#123
Oct3-08, 02:24 AM
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this all reminds me so much of the equivalent circuits of microscopic straight wires in megahertz microprocessor design. I've been trying to find a diagram but I dont even know what to google.

and if its being driven close to its limit (the speed of sound in water) then that is also similar to microprocessor wires being driven close to the speed of light.

both are semi-dc.
granpa
#124
Oct3-08, 02:26 AM
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Quote Quote by somasimple View Post
http://en.wikipedia.org/wiki/Inductance
A neutral thing (water) seems unable to create electric field or voltage by itself.
it has mass and inductance is the electrical equivalent of mass. it is an 'equivalent circuit'.
somasimple
#125
Oct3-08, 04:28 AM
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Quote Quote by atyy View Post
I suppose more compartments can be added for both the node and internode, or it could also be taken care of by a partial differential equation in which parameters vary continuously over space.
No, because adding a compartment does not change anything: Discontinuity will be... propagated,
And No because a model may be tortured until it fits your though but it is better when it sticks facts.
ps: I'll take a closer look to papers.

Granpa: Water may be a perfect silent actor.
Edit: I received "Biophysics of computation" By C Koch (it will help.)
granpa
#126
Oct3-08, 05:34 AM
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http://www.edn.com/article/CA56702.html

http://www.ece.uci.edu/docs/hspice/h...001_2-2874.jpg

http://www.ece.uci.edu/docs/hspice/h...001_2-269.html

It is only during the initial surge of the voltage that a transmission line behaves as a constant impedance, with a value equal to its characteristic impedance. For this reason the characteristic impedance of a line is also called the surge impedance. The surge time during which the impedance is constant is the round trip time of flight, or twice the time delay. Reflections from the far end complicate the electrical behavior of the line after the surge time.
The instantaneous impedance measured at the front end of a transmission line is a complicated function of time. It depends on the nature of the terminations at the far end. When the line is shunted to ground with a resistor of value equal to the characteristic impedance of the line, there is no reflection back, and the front end of the line behaves as a resistive load. When the termination at the far end is open, the impedance at the front end starts out at the characteristic impedance and eventually, after multiple reflections, approaches an infinite impedance. During some periods the instantaneous impedance may be zero.

http://en.wikipedia.org/wiki/Impedance_mismatch

Impedance matching is the electronics design practice of setting the output impedance (ZS) of a signal source equal to the input impedance (ZL) of the load to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load. This only applies when both are linear devices.
The concept of impedance matching was originally developed for electrical power, but can be applied to any other field where a form of energy (not just electrical) is transferred between a source and a load.

To prevent all reflections of the signal back into the source, the load (which must be totally resistive) must be matched exactly to the source impedance (which again must be totally resistive)

http://www.physicsforums.com/showpos...31&postcount=2


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