Response of Aplysia statocyst receptor cells to physiologic stimulation.

1. The electrical responses of Aplysia statocyst receptor cells were investigated using intracellular micro-electrodes. These ciliated mechanoreceptor cells were stimulated by downward tilting about a horizontal axis. 2. Tilting so that the receptor cell was excited produced a depolarizing receptor potential which, if large enough, could generate action potentials. 3. Large fluctuations in membrane potential were evident during depolarizing receptor potentials and were reduced or sometimes absent when a cell was tilted upward. Power-density spectra of the noise voltage revealed that most of the energy added by downward tilt is contained in frequency components below 3 Hz. 4. Removing synaptic input to the receptor cells by cutting the statocyst nerve or adding excess Mg2+ to the bath did not abolish the increase in fluctuations caused by downward, excitatory tilts. 5. The depolarizing receptor potential was often associated with a decrease in membrane resistance as measured with constant current pulses using a bridge circuit. 6. Replacing most of the Na+ in the bath with either Tris or Mg2+ abolished both potential and resistance changes caused by downward tilt. These results indicate that an increased permeability to Na+ underlies the receptor potential.