coclea

Coclea, or cochlea in English, is the spiral-shaped cavity of the inner ear responsible for hearing. It is part of the bony labyrinth and encloses the membranous cochlear duct. In humans it is about 35 millimeters long and completes roughly 2.5 turns around the modiolus.

Anatomy: The cochlea contains three fluid-filled compartments: scala vestibuli and scala tympani (perilymph) and scala media (endolymph) which houses the organ of Corti on the basilar membrane. The oval window at the base transmits vibrations from the middle ear; pressure waves travel through the scalae and are dissipated at the round window. The helicotrema at the apex connects scala vestibuli and scala tympani, allowing low-frequency waves to mix. The basilar membrane’s stiffness varies along its length, producing tonotopic organization: high frequencies stimulate the base, low frequencies the apex. Hair cells—inner and outer—within the organ of Corti transduce mechanical deflections into neural signals via the spiral ganglion to the cochlear nerve, a branch of the auditory portion of cranial nerve VIII.

Physiology and function: Endolymph in the scala media has high potassium, enabling mechanoelectrical transduction by hair cells. Deflection of hair cell stereocilia leads to neurotransmitter release and activation of auditory nerve fibers, conveying sound information to the brain.

Clinical significance: Cochlear function can be impaired by noise exposure, ototoxic drugs, aging, or disease, causing sensorineural hearing loss. Cochlear implants may restore a sense of sound by directly stimulating surviving auditory nerve fibers.

Evolution and variation: In humans the cochlea is highly coiled; in other vertebrates the degree of coiling and number of turns vary across species.