Optical measurement of action potential activity in invertebrate ganglia.

Optical monitoring methods have reached the level of development where activity from a network of cells can be recorded in a minimally-dissected behaving animal. The spike activity in the buccal ganglion of Navanax was monitored during feeding and activity in the Aplysia abdominal ganglion was monitored during the gill-withdrawal reflex. Approximately 30 neurons in the Navanax buccal ganglion were active during feeding and between 250 and 400 neurons in the Aplysia abdominal ganglion were active during the gill-withdrawal reflex. A reasonably complete understanding of the neuronal basis of the gill withdrawal may not be possible with presently available scientific methods. Substantial improvements in signal-to-noise ratio in optical measurements will be necessary before the majority of synaptic potentials can be detected optically. Understanding circuits that involve more than a few neurons will be a challenge to neurobiologists.