Primitive nervous systems: electrical activity in the nerve cords of the parasitic flatworm, Gyrocotyle fimbriata.

A large compound potential (LCP) could be evoked in the longitudinal nerve cords of Gyrocotyle fimbriata. This potential was graded and showed decremental conduction. Decrement was more pronounced in the posterior direction and was exponential with distance, dropping to 50% of its initial value in less than 5 mm. Conduction velocities in the anterior to posterior direction were greater than the reverse direction (means = 37.3 cm sec-1 vs. means = 30.3 cm sec-1). Single suprathreshold stimuli evoked long-lasting trains of spikes. Frequency of spiking varied linearly with stimulus intensity and averaged 0.7 spikes per volt stimulus. Mechanical stimulation of the acetabulum produced a short-lasting (10 sec) facilitatory effect to the second of a pair of electrical stimuli, whereas mechanical stimulation of the holdfast resulted in facilitation lasting several minutes.