  Many cells use calcium signalling to carry information from the extracellular
side of the plasma membrane to targets in their interior. Since virtually all
cells employ a network of biochemical reactions for Ca2+ signalling, much
effort has been devoted to understand the functional role of Ca2+ responses and
to decipher how their complex dynamics is regulated by the biochemical network
of Ca2+-related signal transduction pathways. Experimental observations show
that Ca2+ signals in response to external stimuli encode information via
frequency modulation or alternatively via amplitude modulation. Although
minimal models can capture separately both types of dynamics, they fail to
exhibit different and more advanced encoding modes. By arguments of bifurcation
theory, we propose instead that under some biophysical conditions more complex
modes of information encoding can also be manifested by minimal models. We
consider the minimal model of Li and Rinzel and show that information encoding
can occur by amplitude modulation (AM) of Ca2+ oscillations, by frequency
modulation (FM) or by both modes (AFM). Our work is motivated by calcium
signalling in astrocytes, the predominant type of cortical glial cells that is
nowadays recognized to play a crucial role in the regulation of neuronal
activity and information processing of the brain. We explain that our results
can be crucial for a better understanding of synaptic information transfer.
Furthermore, our results might also be important for better insight on other
examples of physiological processes regulated by Ca2+ signalling.
