The specific gravity of the cupula is comparable to that of the surrounding endolymph. The membranous contains two fluids called perilymph and endolymph. The two outer are filled with perilymph and the inner with endolymph. The delayed movement of the endolymph bends and activates the cupula. This hammering causes the fluid within the cochlea (perilymph and endolymph) to be displaced. Inner ear fluid called endolymph receives the sound energy. The cilia of the hair cell are in the endolymph. Gap-junction channels recycle potassium ions back to the endolymph after mechanotransduction in hair cells. As sound waves propagate in the cochlea, the movement of endolymph fluid bends the stereocilia. Due to inertia, however, the endolymph continues on. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. The membranous labyrinth contains fluid called endolymph. When the head is tilted, the endolymph flows and moves the cupula. The endolymph is the fluid that surrounds the apical surfaces of hair cells. The endolymph surrounds the ampullary cupula which contains hair cells within the semicircular canals.