The number and size of neurons in the CNS of gastropod molluscs and their suitability for optical recording of activity.

Several factors that affect the suitability of opticaL methods for monitoring neuron activity were evaluated in several species of gastropod molluscs. The mean cell body diameter and the total number of cells in the central nervous system were determined In 6 preparations and qualitative evaluations were made for an additional 25 species. There was a factor of 10 difference in mean diameter between species with the smallest cells (prosobranchs) and those with the largest (certain opisthobranchs). Several opisthobranchs had about 5000 central neurons; we estimate that the prosobranchs and pulmonates had at least 5 times as many neurons. When the opacity to transmitted light was measured the percent transmissions of circumesophageal ganglia were between 4% and 40%. We attempted to measure optical signals associated with spike activity in 20 gastropod species; in most species signals were readily detected in single trials.