One important feature of a 
nerve impulse is its all-or-none 
quality. If the original 
depolarization is sufficient--if 
it exceeds some threshold 
value, (going from the resting 
level of 70 millivolts to 40 
millivolts, positive outside)--
the process becomes 
regenerative, and reversal 
occurs all the way to 40 
millivolts, negative outside. 
The magnitude of the reversed 
potential traveling down the 
nerve (that is, the impulse) is 
determined by the nerve itself, 
not by the intensity of the 
depolarization that originally 
sets it going. It is analogous to 
any explosive event. How fast 
the bullet travels has nothing 
to do with how hard you pull 
the trigger.

    For many brain functions 
the speed of the impulse seems 
to be very important, and the 
nervous system has evolved a 
special mechanism for 
increasing it. Glial cells wrap 
their plasma membrane around 
and around the axon like a jelly 
roll, forming a sheath that 
greatly increases the effective 
thickness of the nerve 
membrane. This added 
thickness reduces the 
membrane's capacitance, and 
hence the amount of charge 
required to depolarize the 
nerve. The layered substance, 
rich in fatty material, is called 
myelin. The sheath is 
interrupted every few 
millimeters, at nodes of 
Ranvier, to allow the currents 
associated with the impulse to 
enter or leave the axon. The 
result is that the nerve impulse 
in effect jumps from one node to 
the next rather than traveling 
continuously along the 
membrane, which produces a 
great increase in conduction 
velocity. The fibers making up 
most of the large, prominent 
cables in the brain are 
myelinated, giving them a 
glistening white appearance on 
freshly cut sections. White 
matter in the brain and spinal 
cord consists of myelinated 
axons but no nerve cell bodies, 
dendrites, or synapses. Grey 
matter is made up mainly of cell 
bodies, dendrites, axon 
terminals, and synapses, but 
may contain myelinated axons.

    The main gaps remaining in 
our understanding of the 
impulse, and also the main 
areas of present-day research 
on the subject, have to do with 
the structure and function of 
the protein channels.