After processing at the LGN in a separate network of nerve cells, the activity then travels to the visual cortex. Below, photons activate a chemical called rhodopsin which binds to G-proteins. 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.