# Medical Action potential question

1. Jun 21, 2006

### somasimple

Hi All,

I'm lost with these comments because I learnt that small fibres have low speed and larger ones have a higher one but, in my opinion, since membrane thickness doesn't really vary in unmyelinated axons, capacity is enlarged!

That seems to contradict the wiki?

2. Jun 21, 2006

### quasi426

The smaller the cross-sectional area of the fiber the more resistance to current and thus the slower the conduction velocity. This is derived from the assumption that an axon is equivalent to a cylinderical wire or cable. Just think of the equation for resistance of a wire Resistance is proportional to (Resistivity Constant) * (Characteristic Length)*1/(Cross-sectional Area).

I'm not really sure what you mean when you say the capacitance is enlarged. Can you re-word it for me please.

3. Jun 21, 2006

### somasimple

Hi,

If membrane is taken as the insulator of your cable, it is assumed that the thickness of this insulator change the capacitance property of the cable.

With small fibres, the ratio axon diameter/membrane involves a low capacitance and thus a high speed of propagation (see text above).

In large fibres, axon diameter is larger but membrane is quite constant then capacitance is increased and normally as they stated, the speed may be lowered? It is a fact that large fibres are faster than small ones.

4. Jun 21, 2006

### quasi426

I'm not a neuroscience expert, but I can tell you somethings from what I've learned.

The resting potential that is negative inside with the outside as reference is only true for places very close to the membrane. By this I mean that if you count all the ions inside the cell (includes axon) and divided by the volume of the cell to get the concentration, you would find that if this is done for the same volume outside the cell you would get the same concentrations. The only places the concentrations are different are very close to the membrane due to the myriad of protein channels and pumps.
Using this information I would say that the capacitance shouldn't depend on the diameter of the axon, since the thickness of it is all that the ions in close proximity "care" about. Maybe someone else can help, but this is my take.

5. Jun 21, 2006

### somasimple

Hi,
Thanks again, but the small citations are made by neuroscientists and biologists.

Hmm, Could we reverse the statement like that?
Since the close proximity of ions in the axon, and its membrane thickness, I would say that diameter of the axon and capacitance are not linked to the speed of propagation?

6. Jun 29, 2006

### Taliesin

This may be the wrong way to think about an axon. The membrane of the axon contains ion channels that actively pump ions inwards and outwards across the membrane to change (or restore, or maintain) the potential. So it's not accurate to say that the membrane is an insulator since the "resting" membrane works pretty hard to shepherd ions around.

Although I don't know how cross-section affects speed of conduction, my guess would be that more cross-section means more surface area, which means more ion channels.

7. Jun 29, 2006

### FHLew

Solitons

somasimple wrote:

< I'm lost with these comments because I learnt that small fibres have low speed and larger ones have a higher one but, in my opinion, since membrane thickness doesn't really vary in unmyelinated axons, capacity is enlarged! >

Hopefully, this webpage may be of help.

Solitons- Solitary wave packets

With regards
Lew

8. Jul 3, 2006

### somasimple

Hi All,

Action potential propagation is not solitonic at all. It can't. I thought it was but I'm convinced that the solution goes with a similar but complex solution.
BTW, I agree with many points of the page you posted.

It has been tested. when two APs coming from opposite direction, "collide", they vanish. It seems to me very understandable.

BTW, I elaborated a theory based upon facts.
Perhaps are you able to bring your criticisms?