endolymph

Meaning: Endolymph fills the membranous labyrinth: the cochlear duct (scala media), the semicircular canals, the utricle and the saccule. Hair cells have stereocilia that bend in it, and bending changes the cell's firing onto sensory neurons.

In the cochlea, sound vibrations reach the oval window and set the cochlear fluids in motion. The basilar membrane vibrates, and hair cells of the organ of Corti bend against the tectorial membrane. The auditory nerve then carries the signal to the brain.

In the semicircular canals, head rotation makes the endolymph lag behind because of inertia. It pushes on the cupula and bends the hair cells, which signals rotational motion. The utricle and saccule use otoliths (small crystals) to sense linear acceleration and head position relative to gravity.

On the MCAT: Passages on sensation and perception ask which structure detects sound versus balance, or why spinning then stopping causes dizziness. The endolymph keeps moving after the head stops, so the vestibular system reports rotation that is no longer happening. This conflicts with vision.

Common confusion: Endolymph is not perilymph. Perilymph fills the scala vestibuli and scala tympani, which surround the membranous labyrinth. The endolymph is inside it. Also, endolymph is not the receptor. The hair cells are.