The accessory structures assist in the action of the sense organ, but do not actually enter directly into the sensory transduction process per se. In other words, the conductive and cochlear structures help the hair cell to do its job, but are not themselves receptors. The stiffness gradient of the basilar membrane causes it to act as a series of low-pass filters. Bending of the hair cell stereocilia toward and away from the modiolus involves a motion that is across the cochlear duct, that is, in the radial direction. Several electrical potentials may be recorded from various parts of the cochlea and its environs. References have already been made to the cochlear microphonic, which is one of these, and to the general conceptualization of the action of sensory receptors. Cochlear microphonics are produced by both the inner and outer hair cells, although there is evidence indicating that they reflect a greater contribution by the outer hair cells.