THE IMPULSE
The membrane of a glial cell is 
wrapped around and around an 
axon, shown in cross section in 
this enlarged electron 
microscopic view. The 
encircling membrane is 
myelin, which speeds nerve 
impulses by raising the 
resistance and lowering the 
capacitance between inside and 
outside. The axon contains a 
few organelles called 
microtubules.

    When the nerve is at rest, 
most but not all potassium 
channels are open, and most 
sodium channels are closed; 
the charge is consequently 
positive outside. During an 
impulse, a large number of 
sodium pores in a short length 
of the nerve fiber suddenly 
open, so that briefly the sodium 
ions dominate and that part of 
the nerve suddenly becomes 
negative outside, relative to 
inside. The sodium pores then 
reclose, and meanwhile even 
more potassium pores have 
opened than are open in the 
resting state. Both events--the 
sodium pores reclosing and 
additional potassium pores 
opening--lead to the rapid 
restoration of the positive-
outside resting state. The whole 
sequence lasts about one-
thousandth of a second.

    All this depends on the 
circumstances that influence 
pores to open and close. For 
both Na+ and K+ channels, the 
pores are sensitive to the 
charge across the membrane. 
Making the membrane less 
positive outside--depolarizing 
it from its resting state--results 
in the opening of the pores. The 
effects are not identical for the 
two kinds of pores: the sodium 
pores, once opened, close of 
their own accord, even though 
the depolarization is 
maintained, and are then 
incapable of reopening for a few 
thousandths of a second; the 
potassium pores stay open as 
long as the depolarization is 
kept up. For a given 
depolarization, the number of 
sodium ions entering is at first 
greater than the number of 
potassium ions leaving, and the 
membrane swings negative 
outside with respect to inside; 
later, potassium dominates and 
the resting potential is 
restored.