Distribution in the central nervous system of Aplysia of afferent fibers arising from cell bodies located in the periphery

Abstract

The present study used autoradiography to determine the location of the projections of presumptive peripheral afferent neurons into the central nervous system of Aplysia. Selected peripheral tissues (with an empliasis on structures involved in feeding behavior) were exposed to radioactive amino acids, and the distribution of macromolecules transported into the nervous system via afferent fibers was determined by autoradiography. Different regions of the body exhibited different patterns of projections, and within the neuropil of the cerebral ganglion, there was a loose topographical prganization of projections from the head. For some regions of the body, the projection was largely limited to the ganglion from which the nerve enters; for other regions, the projection was very widespread. In some cases (e.g., rhinophore to eye), there was evidence of projections from one peripheral structure to another. Experiments with all peripheral tissues that were studied resulted in extensive labeling of central ganglia, indicating that afferents with peripheral cell bodies may provide a major source of sensory input to the central nervous system and suggesting that many or all of the numerous ultrafine axons visualized via electron microscopy in the nerves of Aplysia may originate from first‐ or second‐order sensory afferents whose cell bodies are located in the periphery.