Hair cells are columnar cells, each with a "hair bundle" of 100–200 specialized stereocilia at the top, for which they are named. There are two types of hair cells specific to the auditory system; "inner" and "outer" "hair" "cells". Inner hair cells are the mechanoreceptors for hearing: they transduce the vibration of sound into electrical activity in nerve fibers, which is transmitted to the brain. Outer hair cells are a motor structure. Sound energy causes changes in the shape of these cells, which serves to amplify sound vibrations in a frequency specific manner. Lightly resting atop the longest cilia of the inner hair cells is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia, which is what elicits the hair cells' electrical responses.