  The Artificial Axon is a unique synthetic system, based on biomolecular
components, which supports action potentials. Here we examine, experimentally
and theoretically, the properties of the threshold for firing in this system.
As in real neurons, this threshold corresponds to the critical point of a
saddle-node bifurcation. We measure the delay time for firing as a function of
the distance to threshold, recovering the expected scaling exponent of $- 1/2$.
We introduce a minimal model of the Morris-Lecar type, validate it on the
experiments, and use it to extend analytical results obtained in the limit of
"fast" ion channel dynamics. In particular, we discuss the dependence of the
firing threshold on the number of channels. The Artificial Axon is a simplified
system, an Ur-neuron, relying on only one ion channel species for functioning.
Nonetheless, universal properties such as the action potential behavior near
threshold are the same as in real neurons. Thus we may think of the Artificial
Axon as a cell-free breadboard for electrophysiology research.
