3.3 Transmission from a myelinated nerve fiber into a
neuromuscular junction
An impulse, traveling in a myelinated nerve fiber,
faces an electrical "loading" problem when it encounters a nerve
terminal at a neuromuscular junction. The
terminal is a
demyelinated
region because it must synapse with the muscle's endplate and
release transmitter to activate an
impulse in the muscle. The problem
for the impluse to invade the terminal is identical with that
into a
region
demyuelinated by multiple sclerosis. There is too little reserve
energy in the nodes
to depolarize
the terminal sufficiently to cause the generation of
an impulse there. This figure shows that the bare axon presents,
to
the upstream node, a far larger capacitance than that of a node
which it normally has to discharge. This means that
depolarization of
the bare axon will be so much slower than in a node that it will
fail
to
provoke a regenerative spike in it even in the presence of an
abundance of Na channels there.
Nature solves this problem of heavy electrical
"loading"
This recurring problem of the
high electrical (capacitive) "load" of the terminal faced by
the last nodes which must generate an impulse there in
order for calcium current to enter and cause transmitter
release. Nature has arranged to make the last few nodes closer
together than the normal 1 - 2 mM. Under the light microscope, one can
see that, at the lizard endplate, the last node is no more than
50
microns from the
endplate. This length of a mere 50 microns has been has been used
in our simulation of this region.
Furthermore, although I lack specific data, I have
gradually increased the internodal length, over the course of the
next few myelinated internodes, up to the normal value of 1mM
because:
- there is "tight coupling" between neighboring nodes (that is
their potentials change in near synchrony) and
- in order for the impulse to invade the terminal,it is
necessary to have more than a single internodal length shorter
than normal.
(You can see that this is necessary by increasing the last
internodal length to 1 micron and seeing the failure of the
impulse to invade the terminal.)
Once the internodal lengths have been set so that invasion of
the terminal can occur, there is almost no attenuation in the
amplitude of the impulse as it moves throughout the terminal -
despite the presence of K channels and
the absence of Na channels! In the next section on
nerve terminals at nruromuscular
junctions, I show experimental records of the ionic currents at
several locations throughout the endplate and equivalent
simulations for comparison.