Neural control of a molluscan blood vessel, anterior aorta of aplysia.

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Abstract
The properties and innervation of the anterior aorta of the marine mollusk A. californica are described. This aorta is 1 of 3 major arterial trunks that carry blood from the heart. It is directly innervated by motoneurons distinct from those innervating the heart and other 2 arteries. Three new motoneurons, which make monosynaptic chemical connections to anterior aorta muscle [AAM] fibers, were identified in the abdominal ganglion. One motoneuron, RDAAE, produces excitatory junctional potentials (EJP) in muscle fibers. This cell accounts for .apprx. 1/2 of the excitatory input to the aorta. Other EJP recorded from muscle fibers often occur in rhythmic bursts. Two inhibitory motoneurons, designated RDAAI cells, are tonically active and produce strong inhibition and relaxation of AAM fibers. These 2 neurons account for all the inhibition of this vessel. They are the 1st identified inhibitory neurons of molluscan blood vessel muscle. The inhibitory junctional potentials produced by these neurons are caused by increases in conductance to Cl- and K+ ions and are blocked by curare but not atropine. Low concentrations of acetylcholine hyperpolarize and relax anterior aorta fibers. The 3 motoneurons, cell R14 and a few other neurons send axons to the anterior aorta via the vulvar nerve. The RD motoneurons do not innervate the other 2 major trunk vessels from the heart; motoneurons to these vessels do not influence the anterior aorta via axons of the pericardial nerve. The RD motoneurons probably represent a subset of the previously described RD neurons in the abdominal ganglion. Some of the interneurons in the abdominal ganglion that make connections with the motoneurons to the heart and the other 2 trunk arteries also affect the motoneurons innervating the anterior aorta. Interneuron II weakly excites RDAAE and profoundly excites the RDAAI cells, causing a marked relaxation of the anterior aorta in conjunction with its depressing actions on blood pressure and heart rate. Cell L10 (interneuron I) does not directly affect the RDAAI cells, but interneuron XI (cell L24) is tentatively identified as the source of an inhibitory postsynaptic potential (IPSP) in RDAAI cells. The described motoneurons advance the understanding of the neural control of circulation in Aplysia. Since cell R14 also influences the anterior aorta, the cardiovascular system is also regulated by neurons in the abdominal ganglion that are independent of the vascular motoneurons.

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