Neural correlation between the central and the peripheral neurons in the regulation of the digestive tract in Aplysia

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The enteric nervous system, containing a large number of peripheral neuronal somata, is intrinsic to the digestive organs in Aplysia. Two types of rhythmic bursting activities were recorded from the peripheral neurons in preparations in which the central nervous system had been removed. One of the bursting activities was shorter in duration and was recorded in synchrony from the neurons on the crop, the ring nerve and the gizzard. This activity was suggested to cause the neurogenic movement of the gizzard. The other bursting activity was longer in duration and was recorded synchronously from the neurons on the crop and the ring nerve, but not on the gizzard. When the crop, the organ in which food is temporarily stored, was distended by means of a small balloon, the frequencies of both types of bursting activities were increased. These results suggest that varoceptive peripheral reflexes are involved in the regulation of digestive movements in Aplysia, while the functions of the longer-duration bursting activity should be examined in future studies.The autonomic movements of the gizzard comprise both constrictions for trituration and the partial contractions for mixing its contents. It was shown that the constrictions are the neurogenic contractions originating in the synchronous bursting activities of peripheral neurons as described above and the partial contractions are the myogenic ones generated by the pacemaker potential in the musculature. Acetylcholine applied to the gizzard increased frequencies of both the synchronous bursting activities of the peripheral neurons and the pacemaker potentials in the muscle cells with a dose-dependent manner, while the synchrony among the peripheral bursting neurons was augmented. The results suggest that both the peripheral neurons and muscle cells on the gizzard have cholinergic receptors and are involved in the acceleration of digestive movements of the gizzard.