Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 32

combine to charge a passive region and thus exceed a threshold for triggering in a
region that lies just beyond the passive region.
The axons of a neuron, in contrast to dendrites, have membranes that are chiefly
passive because they are covered with myelination, or white matter that insulates
them from the surrounding ions. With membranes blocked, axons are linear
conductors with distributed series resistance, and reduced membrane shunt conduc-
tance and capacitance. So losses are lower. The speed of pulses is higher.
Signals move faster in axons since capacitance is lower. Eventually there is some
loss of energy because of axon series resistance, but pulses in neurons are
reenforced every millimeter or so by the exposed regions at the nodes of Ranvier.
Each node consists of 1 or 2 μ m of exposed membrane that serves to restore

attenuated pulses.

What Membrane's Do

Neurons are enclosed in a membrane that in turn is surrounded inside and out by
thermally active ionic solutions. Generally one may imagine sodium ions (Na + )on
the outside, and potassium ions (K + ) on the inside, although other elements are
possible. All particles, ions included, undergo chaotic thermal activity; they
vigorously speed about, bouncing off each other and also neural membranes. This
chaotic activity represents thermal energy and relates to temperature for any
temperature above absolute zero (

274 C).
Consider a membrane in equilibrium with given type of ion, sodium, for
instance, such that there are different concentrations on one side compared to the
other. Assume ionic concentration (ions/cm 3 ) on the outside much higher than on
the inside. As sodium ions collide with the outside of the membrane, they result in a
transfer of electrically positive charge to the inside.
Positive ions have been visualized as penetrating into the interior by diffusion
through the membrane, or through ion channels, building up positive charge, and
ion population inside. Equally possible is that electrons from the interior are
tunneling into the membrane, and into ion channels, and being captured by external
Na + ions, so as to leave behind a positive charge. If there is a positive voltage built
up inside, lower speed positive ions from the outside will be reversed and may
block channels, thus terminating charge transfer. Overall the end result is a certain
voltage positive on the inside relative to the outside. When all ions types are
considered there will be a certain resting voltage inside relative to the outside.
Potassium ions on the inside have their own effect, which turns out to be a larger
effect than sodium, resulting in negative voltage on the inside. When all ions are
considered, the net resting voltage is about

70 mV inside, relative to outside of a
typical neuron. And since a membrane is conductive, there is current and energy
dissipation that has to be offset by a process of “metabolism”.
Since membranes are thin, perhaps as thin as 5 nm (5 10 9 m), there is an

electric field of roughly 140 kV/cm. This is roughly five times what it takes to cause