  The origin of nonlinear responses in cells has been suggested to be crucial
for various cell functions including the propagation of the nervous impulse. In
physics nonlinear behavior often originates from phase transitions. Evidence
for such transitions on the single cell level, however, has so far not been
provided leaving the field unattended by the biological community. Here we
demonstrate that single cells of a human neuronal cell line, display all
optical features of a sharp, highly nonlinear phase transition within their
membrane. The transition is reversible and does not origin from protein
denaturation. Triggered by temperature and modified by pH here, a thermodynamic
approach, strongly suggests, that similar nonlinear state changes can be
induced by other variables such as calcium or mechanical stress. At least in
lipid membranes such state changes are accompanied by significant changes in
permeability, enzyme activity, elastic and electrical properties.
