Fig. 1.

Different functional configurations of sensori-motor circuitry in Aplysia. a The neuronal pathways underlying reflexive behavior are composed of sensory input neurons which directly, and/or indirectly via relay interneurons, connect to motor output neurons (upper schematic). Neural activity in these otherwise silent circuits requires triggering by peripheral sensory stimuli (lower schematic) and propagates uni-directionally (arrows in upper schematic) to the motor neurons to elicit a transient behavioral response. b The neuronal circuits generating autonomous behaviors, including feeding behavior, consist of reciprocally interconnected groups of neurons (boxes and large circle in upper schematic) which interact (arrows) to produce patterned motor output. This is generated spontaneously and repetitively due to the regenerative membrane properties and synaptic connections of neurons within a central pattern-generating (CPG) network (encircled). Although sensory inputs are not required for actual motor pattern genesis, they can trigger or regulate ongoing CPG operation (lower schematic)