Physiological signaling pathways both diverge and converge-a single neurotransmitter can have multiple effects acid multiple transmitters can have the same effects-in the same target cell. Divergence couples the effects of a transmitter together in a relatively fixed ratio. Different physiological circumstances may require a different ratio, however, the coupling must be made modifiable. This can be achieved through convergence. If two transmitters couple the effects in different ratios, then combinations of the transmitters can yield all intermediate ratios of the effects, thus functionally uncoupling them. This mechanism is analyzed in a well-understood, simple invertebrate neuromuscular circuit.