The standard model of neurons places the manipulation of gradients of metallic ions, potassium or sodium ions, for example, at the heart of their functioning. Manipulating these chemical gradients – keeping potassium at high concentrations inside the cell and at low concentrations outside of it – produces yet another gradient, one of electrical potential. The key to the conversion of one gradient into another is the selective permeability of a neuron’s membrane to metallic ions: as membrane permeability changes ions flow in and out of the cell creating variations in the gradient of electrical potential. Neurons use these smooth low-level variations to record the electrical signals acting as their input. Producing an output involves changing the electrical gradients from continuous to discrete, transforming them into a chain or train of practically identical electrical spikes. This spike train is used to produce a chemical signal that, when received by other neurons, increases the permeability of their membranes if the chemical substances are excitatory or decreases it if they are inhibitory. [Nota de rodapé 2: Ronald J. MacGregor and Edwin R. Lewis. Neural Modeling. (New York: Plenum Press, 1977). p. 21–30.] To provide a mechanism for habituation in simple creatures all we need to assume is that the supply of an excitatory substance in a sensory neuron becomes progressively depleted after successive stimulations, so that less and less of this substance crosses the gap to affect the permeability of a motor neuron’s membrane leading to a decrease in its response. A similar mechanism can account for sensitization if we add a second sensory neuron interacting with the first causing it to increase production of the excitatory substance arid therefore the amount of the latter that reaches the motor neuron. [Nota de rodapé 3: John T. Bonner. The Evolution of Culture in Animals. (Princeton: Princeton University Press, 1980). p. 113.] (Delanda 2011:76-7)
DELANDA, Manuel. 2011. Philosophy and Simulation: The emergence of Synthetic Reason. Londres: Bloomsbury Academic.