current in an axon

[Gerald Pollack]

In article <Pine.A32.3.91i.950422150520.15322A-100000 at homer09.u.washington.edu> "C. Wigren" <cwig at u.washington.edu> writes:
>From: "C. Wigren" <cwig at u.washington.edu>
>Subject: current in an axon
>Date: Sat, 22 Apr 1995 15:14:03 -0700

>Can anyone out there provide me (a high school teacher) with a simple 
>analogy or explanation on the propagation of current in an axon bringing 
>into play concepts such as resistance, voltage, current, and the way the 
>membrane (channels), intracellular and extracellular fluid come into 
>play.  The only one I can come up with is a line of dominoes in which the 
>effect is felt at the beginning and subsequently at the end but in which 
>other than the dominoes falling over, no other displacement has occurred

>Carl Wigren
 
I'm not sure whether you're interested in progation of current per se, or in 
propagation of an action potential. The two are different, but related. The 
dominoes analogy is not a bad one for propagation of an action potential (AP), 
in which the occurence of the AP at one site provides the stimulus for the AP 
at the next site. Another often used analogy is a fuse; the heat generated 
when one segment burns ignites the next. In the case of an AP, the current 
generated by the inflow of positive charges through ion channels provides the 
stimulus (increasing internal positivity of the axon relative to the outside)  
that opens the ion channels in the next segment of axon, thus allowing 
current to flow in there, etc., etc.
  The propagation of the AP thus depends on the spatial spread of current 
along the length of the axon, which is usually referred to as passive current 
spread. The best analogy I know of for this is a leaky garden hose, i.e. one 
with holes spaced along the length of the hose. As water flows through the 
hose, some of it leaks out through the holes, thus leaving less available to 
flow down the length of the hose. So, the water current within  the hose 
decreases with length.  For an axon, this means that the change in voltage 
across the axon membrane will decrease with distance from the site of current 
inflow. The parameters of the axon affect this analogously to the way the 
parameters of the garden hose would; the larger the axon diameter (i.e. the 
lower the internal resistance), the more current can flow; the higher the 
resistance across the axon membrane (the smaller, or fewer, the holes in the 
garden hose) the less rapidly the internal current will drop off with 
distance. These parameters are important for AP conduction because the greater 
the current flow  down the length of an axon, the more rapidly will channels 
in the next segment be stimulated to open, and thus the more rapidly will the 
AP propagate. 
  Hope this helps.