Ultimately, the differing intracellular and extracellular ionic concentrations induce an electrical potential across the cell membrane. Again, the stimulus here is light and the response is an electrical depolarization. Subsystems are networked into larger subsystems, i.e., the space scale is very relevant. Complexity arises from how the subsystems are networked. Stimuli and responses are measureable quantities.
Mathematical models exist for each of the subsystems previously described.
Glucose causes a cascade of chemical events which open and close ionic channels in the cell membrane. Calcium is extremely relevant in the secretion and production of insulin. Like the visual system, subsystems can be categorized by their size and connectivity. Depending on what is measured, the beta cell may have glucose as a stimulus and any of voltage, calcium concentration or insulin secretion rate as a response. For the last point note that duration of an electrical spike, calcium oscillation and blood insulin level occur on time scales of milliseconds, several seconds and hours, respectively.