Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 31

Introducing ... The Active Neuron

The neuron or at least a model of one will now be introduced. The brain contains a
great many neurons, estimated to be 100 billion more or less (1 billion 10 9 ). With

this many, it is only natural that some neurons will differ from others not only in
shape but also in makeup and function.
What is known about neurons follows from a combination of measurements in
the laboratory and from computerized simulations using realistic physical
parameters. It is found that neurons can be triggered to give pulses between about

¼

70 and +40 mV; each pulse is roughly 2 ms wide; these propagate back and forth
along dendrites, and down the axon to boutons, also known as terminations or tips.
Subsequently in the presynaptic region of a synapse, these boutons release
neurotransmitters into the synaptic cleft. Neurotransmitters, like any larger mole-
cule, can be ionized, which makes them more effective in making their presence
felt. They strongly impact a postsynaptic region.
In order to activate a neural burst, there needs to be a force that triggers
molecules in the membrane. Excitatory neurotransmitter ions are one way to
accomplish triggering in a receptor, usually on a spine located on a (basal or apical)
dendrite. These in turn connect to the soma (or body) of a neuron. Note that spines
and synapses are very numerous but not all are necessarily employed at the same
time for neural operations.
Pulses may also be triggered by internal potentials. The membrane of the soma
itself, as well as that of the dendrites have an electrical threshold for activation.
Once the charge across the membrane capacitance exceeds this threshold, trigger-
ing occurs. Pulses continue to be produced as long as the membrane voltage is
above a triggering threshold, which amounts to about 15 mV above a rest level at

55 mV.
A membrane becomes active because it is bounded inside and out by ionic
solutions. It can be triggered by internal voltage to generate pulses across the
capacitance of the membrane, pulses that have a characteristic waveform (shape,
amplitude, and width). Once a pulse forms, it triggers adjacent areas of a nonlinear
sensitive membrane and appears to propagate without attenuation, albeit much
slower than a pulse propagates in linear media.
Technically each pulse propagating along exposed dendritic membrane is a
soliton. By definition, a soliton is an electrical pulse that propagates and is
reenforced as every step of the way by nonlinear media to maintain a given shape
even though there are significant losses in the conductor and the membrane.
Without this important regeneration effect, pulses in dendrites and also axons
would soon die out, and brains as we know them would be impossible.
Membranes can be deactivated and rendered passive by cutting off their access
to ions. Important categories of Boolean logic in dendrites depend on regions of
active membrane alternated with regions of passive membrane. For example, an
AND gate results when one signal cannot get through a passive region, but two
signals applied simultaneously can. That is, two signals merging at a branch can

70 mV, with triggering at roughly