Q: dendrites/gates

Matthew Kirkcaldie m.kirkcaldie at unsw.edu.au
Mon Mar 21 18:51:28 EST 2005


In article <07jt31h9rnp7pbmoeh98ktm8oupjuiic89 at 4ax.com>,
 r norman <rsn_ at _comcast.net> wrote:

> A change in the electric field is, indeed, a current.  Displacement
> current, to be precise.   Try explaining to students that the inward
> current flow in a current loop is Na+ moving into the cell at the site
> of the action potential, the longitudinal current flow inside the cell
> is almost certainly K+ moving, the longitudinal current flow outside
> is Na+ moving in the direction of the loop and Cl- moving the opposite
> direction, but the outward flow at the far end of the loop (the node
> of Ranvier, for example) is just the i = C dV/dt term.  At this last
> flow, nothing actually moves across the membrane; it is mostly
> displacement current.  OK, for a more thorough nit, behind the action
> potential the outward current is outward K+ in the recovery phase of
> the action potential.
> 
> Each participant in the wave (we avid football fans don't call it a
> "Mexican" wave, simply "The Stadium Wave") does move, but only short
> distance.  The wave, itself, moves a large distance.  It is like
> falling dominos; each domino falls a short way, the pattern of falling
> can travel across the room.  Without the short movement of each
> participant (without the local current loops) there would be no
> propagation.

I can see you've been doing this a bit longer than I!  Thanks for the 
sharpen-up - of course I do talk about local current circulation, in 
fact I talk about the movement of a person standing up and sitting down 
as being analogous to local passage of ions across the membrane, which 
ignores the small lateral spread I guess.  But analogies have to break 
down somewhere.

I do feel vaguely uncomfortable with the term "Mexican" wave - but I 
find nobody knows what I'm talking about unless I use it ...

      Cheers,

         MK.



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