TRANSDUCTION AND ADAPTATION IN SPIDER SLIT SENSE ORGAN MECHANORECEPTORS

1. Mechanoreceptor neurons in spider (Cupiennius salei) slit sense org an were examined by intracellular current- and voltage clamp recording s. Steps and pseudorandomly modulated displace ment stimuli were deliv ered to the mechanosensitive cuticular slits. The resulting responses were used to determine the response dynamics and signal-to-noise ratio (SNR) of mechanoelectrical transduction. 2. Neurons were separated in to two groups that, in terms of their afferent discharges, displayed d ifferent adaptations to displacement stimuli. Both responded at the on set of the step but then adapted fully, either immediately or within 1 0-200 ms. Voltage-clamp recordings showed only small differences in th e receptor currents of the two groups. 3. Displacement of the slit cau sed a large inward current that decayed in seconds to a steady level o f similar to 10-25% of the initial transient. When adapted to a steady displacement, the neurons responded to superimposed displacements in the same direction with additional transient currents, whose decay cou ld be fitted by two exponentials with time constants of similar to 10 and 100 ms. In contrast, displacement in the opposite direction caused small ''outward'' currents without obvious adaptation. This behavior persisted with increasing background displacements, suggesting a shift in the displacement-response curve along the displacement axis. 4. Wh ite noise stimulation supported the step data and confirmed that the r eceptor's sensitivity was independent of mean slit membrane displaceme nt. When the relative displacement of the stimulus (i.e., strain) was held constant at different maintained backgrounds, the SNR of the neur ons remained fairly constant at similar to 2-10 over the frequency ran ge from 4 to 450 Hz. The receptor current frequency responses showed h igh-pass characteristics, with a two- to sevenfold enhancement of the response amplitude and a phase lag relative to the stimulus of 90 degr ees at 300 Hz. Low coherence values in the frequency range of 0.5-125 Hz were explained by nonlinear adaptation. 5. We conclude that, by rap idly adapting to the mean displacement of the slit membrane, slit orga n mechanoreceptor neurons maintain a high sensitivity and SNR that all ow the detection of small and rapid changes in cuticular strain.