The conditioned reflex: detectors and command neurons.

The conditioned reflex is characterized by plasticity supporting bilateral selective connections between its input and output. In simple nervous systems, input stimuli are represented by selective detectors connected to command neurons by plastic synapses whose activity increases on learning and decreases on extinction. The process of associative learning occurs when excitation of the detector and the command neuron coincide. Short-term memory in a plastic synapse is associated with phosphorylation of postsynaptic receptor molecules and does not require protein synthesis. Long-term memory is associated with early gene expression, structural genes, and protein synthesis. The simple "detector-command neuron" association has increased in complexity during evolution. At the input, pre-detector interneurons activating a specific detector converge on the command neuron: the command neuron determines the selectivity of the mechanisms of conditioned reflexes for complex stimuli. The output mechanism has also become more complex: command neurons have become more specialized and premotor interneurons have appeared between them and motor neurons, excitation of premotor neurons being passed to groups of motor neurons responsible for the configuration of the behavioral act. Conditioned reflexes combining more complex signals at the input with more flexible results at the output allow a diversity of behavioral acts.