Separate neural pathways respond to different noxious stimuli affecting respiratory pump frequency in Aplysia fasciata.

Neural circuits responsible for both conditioned and unconditioned respiratory pumping to three stimuli modulating respiratory pumping were examined. The stimuli used were: (i) reduction of pH; (ii) increase and (iii) decrease in seawater concentration. Ablation of the osphradium, but not of the rhinophores, abolished responses to all 3 stimuli. Cutting the pleural-abdominal connectives led to a decrease in responses to lowered pH, but did not affect responses to changes in seawater concentration. Further lesions showed that integrity of the cerebral-pleural ganglion is needed for animals to respond to a decrease in pH. Thus, neural circuitry entirely within the abdominal ganglion and the periphery innervated by the ganglion is sufficient for mediating responses to changes in seawater concentration, while the cerebral ganglion is needed to respond to lowered pH. Different transmitter mechanisms are also used by pathways responding to changes in seawater concentration and to decreased pH: 5,7-dihydroxytryptamine in concentrations which cause depletion of serotonin blocked the response to lowered pH, but not to altered seawater concentrations.