Neural control of circulation in Aplysia. I. Motoneurons

The properties of seven cardiovascular motoneurons in the abdominal ganglion of Aplysia identified are described. Two of these are heart excitatory motoneurons, RB(HE) and LD(HE). RB(HE) produces a long lasting increase in heart rate, and its spontaneous firing pattern results in both tonic and phasic heart excitatory tone that supplements the myogenic pacemaker activity of the heart. The second heart excitor, LD(HE) has a more phasic motor effect and no tonic effect on heart rate. There are two heart inhibitory motoneurons, LD(HI1) and LD(HI2). They appear identical to each other with regard to motor effects, synaptic input, and firing patterns. These two cells are normally silent and produce phasic inhibition of heart beat by brief, synaptically driven bursts of activity. There are three vasoconstrictor motoneurons [LB(VC1), LB(VC2) and LB(VC3)] that have similar synaptic inputs and firing patterns, but differ in their innervation patterns. All of these cells appear to mediate their motor effects by chemical transmission. LD(HE) and the LB(VC) cells produce discrete, fast rising, facilitating EJPs. RB(HE) produces slow, smoothly graded depolarizations of heart muscle fibers. The seven motor cells are not interconnected; each cell forms an independent parallel motor pathway capable of producing powerful motor effects. Together the seven motoneurons account for at least the major portion, perhaps all, of the spontaneously occurring changes in heart rate and blood pressure.