"Phase portrait of cell membrane passive depolarization"
Relatore: Gaetano Aiello- Dipartimento di Fisica & Tecnologie Relative - Palermo
23 Febbraio 2005 ore 15.30
When ions leak through the cell membrane the voltage
across slowly drifts towards depolarization. Very
little is known about the dynamics of this process
except that it is very slow. Sustained, prolonged
activity, however, would be impossible without the
action of the ionic pumps, which ultimately guarantees
the correct amplitude of the AP, and the return of the membrane voltage to the resting level.
The length of time a neuron could fire with its
metabolic pumps blocked (e.g., poisoned by oubain)
or lagging behind the rate of firing, has not been
determined yet. In the case of paroxistic firing
induced by a neurotoxicant this becomes an important issue. In neurons, organotins (especially TMT
reduceGABAergic inhibition, enhancing at the same
time the release of glutamate. The combined action
causes hyperexcitation, and therefore a quite prolonged firing. Even at full power, the pumps might
be unable to stand the demands of paroxistic
firing induced by a neurotoxicant. As a consequence, the baseline of the
spike train is expected to rise to a value no longer compatible with the generation of a typical action
potential, or even interfere with the mechanism
of generation of the AP itself. Sudden, complete
cessation of burst activity has actually been
reported in cultured neuronal networks exposed
to critical concentrations of TMT [Gramowski].
cute TMT intoxication did not apparently cause
permanent cellular damage, and a fully functional
recovery was claimed after washing out the medium.
Among the possible causes accounting for the
shut-off mechanism,a generic “suppression of
action potential generation”
as suggested. We do favor this hypothesis, hereby
providing a plausible shutoff mechanism within the
framework of the dynamics of passive depolarization.
o Membrane potential rivisited
o The cell as a nonlinear dynamical system
o Flow in the phase space
o Phase Portrait and Stability