Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 40

Fig. 3.7 Excitory neurotransmitter ions result in membrane triggering
Eventually a restored negative potential within the presynaptic boutons attracts
the free positive neurotransmitter ions and encourages them to “dock” near where
they came from. The return of excitatory neurotransmitters has given us the curious
word “reuptake.”
Inhibitory Ions
Figure 3.8 intuitively visualizes larger neurotransmitter ions that uniformly cover a
significant area of surface, including receptors of a dendritic segment. The insulating
influence of inhibitory neurotransmitter ions is sufficient to prevent the membrane
from regenerating a pulse, so pulses attempting to pass through die out rapidly.
When a segment of dendritic membrane is blocked, attenuation is significant
because of resistance R and conductance G, as suggested in the figure. A single
pulse that enters a passive segment will soon be too weak to trigger its own
regeneration; consequently propagation will stop.
Inhibitory ions must stick long enough to block propagating pulses. Attached
inhibitory ions tend to be repelled by dendritic pulses and also by thermal activity;
so they, or parts of them likely are attracted back home once their homes return to a
negative rest potential.
Circuit Elements for Synapses
The physical model delineated above is that a pulse burst arrives at a bouton and results
in a release of neurotransmitters. These in turn trigger pulses in a dendritic receptor.