  This chapter revisits the concept of excitability, a basic system property of
neurons. The focus is on excitable systems regarded as behaviors rather than
dynamical systems. By this we mean open systems modulated by specific
interconnection properties rather than closed systems classified by their
parameter ranges. Modeling, analysis, and synthesis questions can be formulated
in the classical language of circuit theory. The input-output characterization
of excitability is in terms of the local sensitivity of the current-voltage
relationship. It suggests the formulation of novel questions for non-linear
system theory, inspired by questions from experimental neurophysiology.
