MECHANORECEPTION IN MARINE COPEPODS - ELECTROPHYSIOLOGICAL STUDIES ON THE 1ST ANTENNAE

Neural activity was recorded extracellularly at the base of the first antenna in 15 marine copepods. Controlled mechanical stimuli were delivered with a vibrator driven by a waveform generator. Many species exhibited responses characterized by a large number of small spikes, while others were characterized by the presence of a small number of large units. Two bay species, Labidocera madurae and Acartia fossae, exhibited large units that could be easily distinguished from the background activity of smaller units. In these species, the antennal receptors fired short latency (> 5 ms) trains of one to several impulses in response to a brief mechanical stimulus and sustained trains to a prolonged sinusoidal stimulus. They were extremely sensitive to small displacements and sensitivity increased with stimulus frequency. The receptors responded to stimuli between 40 and 1000 Hz and receptors required displacement velocities of 20-mu-m s-1 or more to fire. Displacements as small as 10 nm were capable of triggering spikes. With an increase in the amplitude of the displacement, a decrease in the latency and an increase in the number of units recruited and/or firing frequency was recorded. Phase-locking to oscillatory stimuli was observed over a frequency range of 80-500 Hz. Neural activity increased in response to bending of individual setae. Setae appear innervated and structurally constrained to movements in specific directions. These experiments suggest that (i) some copepod setal receptors may be more nearly velocity detectors than purely displacement sensors, (ii) they may be capable of sensing closely spaced stimuli, (iii) the patterns of response may code for intensity and duration of the stimulus, and (iv) receptors may be capable of supplying directional information.