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Fig. 3.8
Inhibitory neurotransmitter ions tends to inactivate a membrane
This process can be approximated in a circuit model by viewing a general synapse as a
one-way transconductance amplifier that produces a packet of chargewhenever a pulse
arrives. Thus, if ten pulses arrive, there will be ten packets of charge in this model; this
may generate ten postsynaptic pulses, although there may be situations involving
highermembrane capacitance, as below, inwhich fewer dendritic pulses are generated.
This is because some of the packets of charge are employed in raising membrane
voltage to a threshold for triggering.
Signals do not usually flow backwards into a bouton. Partly for this reason a one-
way transconductance amplifier is a reasonable way to model a synaptic contact,a
symbol for which is shown in Fig.
3.9
. Transconductance G, high load resistance
R
L
, and low capacitance C
L
are available to optimize the contact. So a general
synapse is modeled as a transconductance amplifier that transmits one packet of
charge for each voltage pulse applied to its input. Under ideal conditions of ideal
low capacitive loading at the receptor, each pulse triggers a postsynaptic pulse and
is simply transmitted along the destination dendrite.
Weak Synapses, Single Pulses
As opposed to a general synapse, a weak synapse is occasionally required that
stimulates a single pulse in its dendritic receptor, here denoted by Pulse(1). A single
dendritic pulse is often necessary for precise timing purposes. Such contacts are
postulated to be accomplished physically by excitatory neurotransmitters drifting
and diffusing through a small area cleft to a postsynaptic receptor. Once repelled by
the first dendritic pulse, they escape from the cleft so that only one postsynaptic
pulse results.
Figure
3.10
proposes an intuitive structure that depends on geometry to encour-
age a single pulse. Neurotransmitters are emitted as usual, enough to trigger a
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