Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 37

Dendritic pulses in STM neurons are triggered by excitatory neurotransmitters in
the usual way. But within the dendritic receivers and the dendrites themselves are
modeled with a shortfall of potassium and calcium, so that the dendritic voltage
does not immediately recover as it would otherwise. This results in a long dendritic
pulse, which may be visualized as in Fig. 3.5 as derived from Fig. 3.2 . To have an
extended pulse, membrane conductance must be low, insulated by myelin or some
other insulator. This results in a longer lasting internal charge that becomes an
extended trigger to the soma, which pulsates until the dendritic pulse's voltage
drops below a threshold for triggering.
Holding charge to a soma produces what has been termed a maximum rate of
pulsing. Maximum rate of pulsing occurs by applying a steady triggering voltage
above

55 mV to the soma so that it is continually retriggered as soon as possible.
This is expected to result in some reduction in the amplitude of the output pulses,
but not enough loss to render them ineffective in signaling other neurons.
Note that a shortfall of dendritic receiver potassium and calcium implies that
electrical current is unavailable to reduce in a timely way the dendritic pulse after it
reaches and holds at +40 mV. But over time, a trickle of charge will penetrate
through the membrane and reduce internal positive voltage as electrons return to the
interior. Thus STM fades in a controlled way.
Modeling as a shortfall of internal potassium is probably a simplification.
Experiments concerning implicit memory (reflex memory) with a sea snail called
Aplysia were conducted over a period of several years by Kandel [ 2 ] and others. It
was found that an interneuron can serve to release serotonin near the synapses of an
ordinary neuron, giving a memory effect.
Theory is, receptors on a neural membrane receive serotonin and convert it into
substance known as cyclic AMP inside a neuron. Cyclic AMP is what results in a
measurable memory effect. Cyclic AMP inside a neuron shuts down potassium
channels, with the end result there is an increase in the period and quantity of
glutamate (excitatory neurotransmitters) released from the neuron. Releases can
last from milliseconds to minutes. This work was about implicit memory, whereas
brain STM is explicit memory, but it may somehow apply to STM neurons.

Fig. 3.5 Extended trigger
produces an extended pulse
burst