In this sequence of events 
constituting an impulse, in 
which pores open, ions cross, 
and the membrane potential 
changes and changes back, the 
number of ions that actually 
cross the membrane--sodium 
entering and potassium 
leaving--is miniscule, not 
nearly enough to produce a 
measurable change in the 
concentrations of ions inside 
or outside the cell. In several 
minutes a nerve might fire a 
thousand times, however, and 
that might be enough to change 
the concentrations, were it not 
that the pump is meanwhile 
continually ejecting sodium 
and bringing in potassium so as 
to keep the concentrations at 
their proper resting levels. The 
reason that during an impulse 
such small charge transfers 
result in such large potential 
swings is a simple matter of 
electricity: the capacitance of 
the membrane is low, and 
potential is equal to charge 
transferred divided by 
capacitance.

    A depolarization of the 
membrane--making it less 
positive-outside than it is at 
rest--is what starts up the 
impulse in the first place. If, 
for example, we suddenly insert 
some sodium ions into the 
resting fiber, causing a small 
initial depolarization, a few 
sodium pores open as a 
consequence of that 
depolarization but because 
many potassium pores are 
already open, enough potassium 
can flow out to compensate and 
quickly restore the membrane 
to its resting state. But suppose 
that the initial charge transfer 
is so large, and so many sodium 
pores open, that more charge is 
brought in by sodium than can 
be removed by potassium: the 
membrane will then depolarize 
still further. This will cause 
even more sodium pores to 
open, and still more 
depolarization, and so on, in a 
regenerative, explosive 
process. When all the sodium 
pores have opened that can 
open, the membrane potential 
is reversed in sign, relative to 
the resting potential: instead of 
being 70 millivolts, positive 
outside, it becomes 40 
millivolts, negative outside.

    The reduction in potential 
across the membrane, with 
ultimate reversal of potential, 
doesn't take place all at once 
along the fiber's length, 
because transfer of charge 
requires time. It starts in one 
place and spreads along the 
fiber at a rate of 0.1 to 10 or so 
meters per second. At any 
instant there will be one active 
region of charge reversal, 
perhaps several inches long, 
and this reversal will be 
traveling away from the cell 
body, with still unopened 
channels ahead of it and 
reclosed channels, temporarily 
incapable of reopening, behind.

    This event constitutes the 
impulse. You can see that the 
impulse is not at all like the 
current in a copper wire. No 
electricity or ions or anything 
tangible travels along the 
nerve, just as nothing travels 
from handle to point when a 
pair of scissors closes. (Ions do 
flow in and out, just as the 
blades of the scissors move up 
and down.) It is the event, the 
intersection of the blades of the 
scissors or the impulse in the 
nerve, that travels.

    Because it takes some time 
before sodium channels are 
ready for another opening and 
closing, the highest rate at 
which a cell or axon can fire 
impulses is about 800 per 
second. Such high rates are 
unusual, and the rate of firing 
of a very active nerve fiber is 
usually more like 100 or 200 
impulses per second.