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To have a long dendritic pulse, and a long trigger at the soma, which implies a
long burst of regular pulses at the axon, conductance must be lower than what it is
for a membrane directly exposed to the ionic solutions. This suggests regions of
myelination or neurotransmitter involvement on the surface of the dendrite, to
insulate it, as is typical for axons.
Short-term memory neurons, by the way, are not just for consciousness; they
also help regulate the asynchronous system. For instance, a short-term memory
neuron conveniently serves to extend in time the duration of a regular pulse burst.
This occurs, for instance, when a long-term memory element must remain active
longer than its normal ten or so pulses, to ensure that it is read properly. This
application has been termed burst stretching.
Another type of application occurs when a short-term memory neuron is used as
a timer for a cue editor so that, after a given duration with no results, the timer
indicates that a memory search has failed to find matches.
Perspective on Synapses
Synapses are very numerous, averaging about 500 per neuron for adults, but they
are not all receiving signals concurrently from other neurons. They are available,
however, and easy to find by interneurons that need connections to facilitate a
phenomenon known as neural placidity. For modeling purposes, regular (fast)
excitatory synapses often can be represented by a transconductance amplifier giving
positive current pulses, while regular (fast) inhibitory synapses often can be
represented by a transconductance amplifier giving negative pulses. Transcon-
ductance amplifiers effectively provide a packet of charge when a given neural
voltage pulse is applied to the input.
Transconductance amplifiers are appropriate models because they tend to oper-
ate “one way”, as does a synapse, from presynapse to postsynapse; they result in a
trigger by charging membrane capacitance. They avoid unrealistic current spikes
and unworkable loading.
Synaptic receptors often occur on spines, although other locations are possible.
When a presynaptic vesicle releases its excitatory neurotransmitters, they interact
with the postsynaptic neural membrane so as to trigger it. A membrane with typical
properties generates a regular neural pulse, which is a certain waveform between
about
70 and +40 mV, composed of pulses about 1 ms wide. Synapses are not
weighted in this topic, as they generally are in the field of artificial neural networks;
their only role is to trigger standard pulses.
Neurotransmitters are released in the cleft. Intuitively, positively charged excit-
atory neurotransmitter ions can be imagined as being repelled away from the
receptor into the thin synaptic cleft by the first positively charged neural pulse.
But they are soon attracted back to trigger additional pulses. The resulting burst
is assumed terminated only when the presynaptic bouton returns to its negative
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