Dowling in Neurons and Networks (1992, Belknap) indicates that habituation
is associated with fewer vesicles releasing transmitter from the
presynaptic terminal. His guess is that this is associated with reduced
numbers of Ca++ ions entering the presynaptic terminal and participating
in vesicle binding to the membrane. He connects this to a build-up of Ca++
in the presynaptic terminal, which either reduces the conductance of the
voltage-sensitive Ca++ channels (which open while the synaptic terminal is
depolarized to allow Ca++ into the terminal) or enhances the Ca++ pump
activity, reducing the availability of Ca++ for vesicle binding. He points
out elsewhere that reducing Ca++ in the extracellular space or raising the
extracellular concentration of Mg++ or Co++ will diminish or stop
transmitter release. The current flow in a neuron has positive ions
entering at the excitatory post-synaptic membranes, negative ions
entering at the inhibitory synapses and positive ions exiting at the axon
hillock. This leads to a current flow of positive ions away from the soma
and towards the dendrites. The implications appear to be:
1. Inhibition by axo-somatic synapses should not lead to habituation.
2. Failure to fire not caused by inhibition should reduce the positive ion
current, leading to habituation.
3. Since NMDA potentiation involves Mg++ ions being ejected into the
extracellular space, coupled with Ca++ ions entering the post-synaptic
membrane, I would suspect habituation is involved in potentiation.
Just some speculations,
Internet: erwin at trwacs.fp.trw.comherwin at cs.gmu.edu
Working on Freeman nets....