Functional Organization of the Neural Control of Circulation in Aplysia

The abdominal ganglion of Aplysia provides a useful model forstudying the functional organization of motor systems. Herewe review studies of the neural network controlling circulation,emphasizing the organizational features it may share with othermotor systems controlled by the abdominal ganglion. We identifiedseven motor neurons to the heart and vascular system. Motorneurons having similar motor effects (e.g. the two heart inhibitors,or the three vasoconstrictors), together with cells of unknownmotor function located near them, make up distinct homogeneouscell groups. The members of each group appear to be nearly identicalwith respect to biophysical and neurochemical properties, sizeand effectiveness of synaplie inputs, and firing patterns. Thereare no interconnections between the members of the groups, butfive interneurons innervate the homogeneous groups in variouscombinations, exciting some groups and inhibiting others. Twoof the interneurons, Interneuron I (cell I₁₀) and InterneuronII, are command cells which produce centrally generated motorprograms in the absence of sensory feedback. Eacli command apparentlycodes for a specific homeostatic function, such as increasedcardiac output. Coordination of the two commands is achievedby mutual inhibitory connections between them, ensuring thatthe motor neurons of the system receive only one command ata time. Some synaptic connections made by the command interneuronsappear to be functionally ineffective; the possible significanceof them is discussed. Available evidence suggests that manyfeatures of the network controlling circulation may be characteristicof other visceromotor systems of the abdominal ganglion.