Saltatory Conduction: single AP or not?

granpa

none of that makes any sense. axon elongation wouldnt have any effect on the ap duration. you have apparently misunderstood something i said. and what delay are you talking about. the only delay is the one that is apparent in the graph. about 0.1 ms at each node.

granpa

I dont contest the facts. those curves are the basis of everything i am saying. why would i contest them?

look at one node on the axon. first it receives the A signal then the B signal then then it fires then receives the C signal. i think the A signal is the ap from 2 nodes away. the B signal before the delay is the ap from 1 node away. the B signal after the delay is the node itself firing. the C signal is the backward propagating anti-action-potential which is resetting the previous internode to its resting state.

granpa

i'm not sure i understand you but you appear to be forgetting that each ap passes through 2 internodes before being stopped by a locked node. so a single node con receive 2 signals just as fast as before. there is no delay.

the first signal is decayed and doest cause the node to fire. it just unlocks it. the second one causes it to fire (after a short delay).

somasimple

Computations may distort the facts in that case. But I cant' how you compute your two signals => They must be added and there is a delay.

granpa

i already told you in post 87. the only delay is the one everyone already agrees on. there are 2 signals because the ap passes through the first node it reaches (which after a short delay fires) and continues on without delay to the next node which it unlocks.

i dont understand what it is that you dont understand. please ask more specific questions.

granpa

this is just an idea that I am presenting. it isnt established fact. the facts are here:
http://www.pubmedcentral.nih.gov/pag...92&pageindex=8

nodes 1234. nodes 3 and 4 and all further nodes are locked. meaning that they will not pass signals.
ap=action potential

t0 node 1 fires producing ap1 which moves at the speed of sound in water. 1500 m/s
t1 ap1 almost instantly reaches node 2 and passes through WITHOUT DELAY
t2 ap1 almost instantly reaches and ends at node 3 and unlocks node 3 (which takes some time)
t3 after 0.1 ms node 2 fires producing ap2 which moves at the speed of sound in water. 1500 m/s
t4 ap2 almost instantly reaches node 3 and passes through WITHOUT DELAY
t5 ap2 almost instantly reaches and ends at node 4 and unlocks node 4 (which takes some time)
t6 after 0.1 ms node3 fires producing ap3 which moves at the speed of sound in water. 1500 m/s

t0 node 1 fires producing ap1
t1 ap1 almost instantly reaches node 2
t1-t3 delay of 0.1 ms at node 2 before it fires
t3 node 2 fires producing ap2
t4 ap2 almost instantly reaches node 3
t4-t6 delay of 0.1 ms at node 3 before it fires

there is therefore only one delay and it is the 0.1 ms one that everyone already agrees on. so it takes 0.1 ms for an action potential at one node to create an action potential at the next node which is typically 1 or 2 mm away. that gives a net speed of 10-20 m/s. if there were no delay at each node then the signal would move at 1500 m/s. the speed of sound in water. (thats just a guess but its certainly at least a good fraction of that speed)

just before each node fires its ap it relocks itself so the ap can only go in one direction and the previous internode can immediately begin to return to its resting state. this is seen in curve C:
http://www.pubmedcentral.nih.gov/pag...92&pageindex=8

somasimple

http://www.physicsforums.com/showpos...4&postcount=66

finite => delay
+ delay to initiate the next AP since there is a decay in internode.

So I reject, one more time, your point of view. Sorry.

granpa

i thought we agreed that the speed of the ap through the internode was around 1000 m/s and all the delay came at the node as is suggested by the data here:
http://www.pubmedcentral.nih.gov/pag...92&pageindex=8

somasimple

It gives 1~2µs for the internode and since the decay is quite 1/3 => total delay around 20 µs.

granpa

according to this:
http://www.pubmedcentral.nih.gov/pag...92&pageindex=8
the delay at the node is about 0.1 ms or 100 microseconds. a 20 microsecond delay would be almost negligible

granpa

travelling at 1000 m/s an ap will travel the 1-2 mm from node to node in about 1 or 2 microseconds. so i guess we agree on that. whether decay results in a delay i dont know but i dont see what it matters. the inherent delay at the node swamps it out anyway.

i fail to see what any of this has to do with the idea that nodes lock and unlock

somasimple

A lot, since the threshold will be reached later at next node.

atyy

HH model for unmyelinated axon:
-Describes AP at a point and its propagation.
-Considers active and passive circuit components uniformly distributed along the axon.
-Is a wave equation with a well-defined propagation velocity which matches experiement.
-Reduces in a certain limit to the linear passive cable equation which does not have a well defined velocity.

HS model for myelinated axon
-Nodes active, internodes passive
-Internodes considered as resistor and capacitor in parallel (I think I know what they mean, but agree with somasimple it's not obvious), and apparently equivalently with an equation that resembles the linear passive cable equation.
-Expected backwards propagation from node into internode is apparently seen in the data and discussed.
-No explicit calculation of internode velocity, but heuristic and dimensional arguments are given for its form.

FH model data for myelinated axon
-FH model is standard reference for myelinated axon
-Only FH 1964 seems to be available to me, and does not describe propagation, but there may be other FH papers.

granpa

not much later. not enough to make any difference. the delay at the node is already 100 microseconds anyway. i dont see what internode delay has to do with anything at all much less whether nodes lock or unlock.

granpa

http://www.pubmedcentral.nih.gov/pag...92&pageindex=8
but the backward propagation that is seen (at least in the data i saw) isnt a backward propagating ap. its an anti-ap. it doesnt depolorize the axon. it returns it to its resting state.

but thats not the issue at the moment.

atyy

Capacity is not omitted - they are discussing resistor and capacitor in parallel as a model for the internode.

DaleSpam

Hi atyy, do you by any chance have a link for this? None of the variants of the HH models that I have seen have any spatial terms, but it has been years since I studied this stuff. I would be very interested to see a single model that includes the voltage-gated channels and spatial terms.