The criterion is this: the boundaries of a cognitive system are given by the location of its transducers and its effectors. A transducer, in Pylyshyn’s terms (pp. 151–178) is a device that (1) maps inputs described in physical terms into outputs described in representational terms in a way that is (2) interrupt-driven and (3) primitive and nonsymbolic. Saying that transducers are interrupt-driven is just to say that their activation is mandatorily determined by the presence of their physical input conditions. Saying that they are primitive implies that they do not carry out their mapping function by any internal representational means; their operations do not involve cognitive processes, although they may obviously be physically complex. The most important condition on transducers, for our purposes, is that they have the function of turning physical stimuli into representational or computational states. The inputs to a transducer are not themselves representational; transducers respond only to physical properties and magnitudes. They take, for example, pressure, temperature, vibrations in the air, or ambient light in a region of space, and produce vehicles that represent something, most frequently some aspect of the environment that the stimulus typically carries information about. Transducers can thus be thought of as the place in where things in the external environment become input for the cognitive system.So, is a single neuron a cognitive system by this criterion? Sounds like yes to me.
The same can be said of effectors. Corresponding to the above definition of a transducer, an effector is a device that (1) maps inputs described in representational terms into outputs described in physical terms in a way that is (2) interrupt-driven and (3) primitive and nonsymbolic. That is, an effector does what a transducer does, but in reverse. It takes a representation and produces a physical event; for example, activation pattern in certain muscle groups. The input representation can be understood as something like a direct motor command, and this command acts immediately on the body. Both transducers and effectors are important for delimiting systems, but for brevity I will sometimes simply call this the transducer view of systems. (Weiskopf, 2010)