The receptor potential and adaptation in the cockroach tactile spine

The receptor potential in the cockroach tactile spine was measured during mechanotransduction by recording from the afferent axon about 0.5 mm from the sensory ending. The receptor potential was a linear function of spine position, but its amplitude and phase varied with the mean position of the spine. This can be related to the mechanical linkage from the socket of the spine to the sensory ending. The frequency response for mechanotransduction was flat over a frequency range of 0.1 to 100 Hz, after accounting for the cable properties of the axon, and there was no evidence of adaptation. The length constant and membrane time constant of the axon were estimated to be 130 microns and 1 msec, respectively. The threshold amplitude of movement required to produce action potentials was also measured as a function of frequency. It gave a power law relationship which was exactly the inverse of the frequency response of the receptor during strong stimulation and rapid firing. This suggests that adaptation of the sensory discharge in this receptor is produced by the action potential encoding mechanism.