Auditory Cochlea: Structure, Function & Diseases

In order for us to hear sounds, a finely tuned interaction of different areas of the inner ear is necessary. In this process, the cochlea is the switching point to the brain.

What is the cochlea?

The cochlea is the actual hearing organ in the inner ear. It is made up of special hair sensory cells. When a sound hits these sensory cells, they start moving and the sensory cells convert the mechanical stimuli into electrical signals that are transmitted to the brain via the auditory nerve. Because it looks like a snail that has retreated into its home, it is called the “cochlea.”

Anatomy and structure

The cochlea has a snail-like structure in two and a half coils and lies surrounded by bone in the petrous bone. Within it are three tubular ducts, one above the other, filled with fluid:

  • Atrial staircase (Scala vestibuli).
  • Snail walk (Scala media)
  • Timpani staircase (Scala tympani)

These ducts are separated by fine membranes. The base of the cochlea is located just behind the middle ear with the ossicles and is separated from the middle ear by two membranes (oval and round window). The base of the stapes is movably connected to the oval window. Behind it lies the atrial staircase, which passes over the Reissner’s membrane into the cochlear canal, where the actual organ of hearing, the contiorgan (named after the Italian anatomist Alfredo Conti) with its fine hair cells, is located. The cochlear duct opens into the tympanic staircase via the basilar membrane. In the tip of the snail, the atrial staircase and the tympanic staircase are brought together in the snail hole. Both contain a clear fluid (perilymph), while the cochlear duct contains a different fluid (endolymph). The organ of Corti contains inner and outer hair cells with different tasks. The inner hair cells are responsible for transmitting sound signals to the brain.

Function and tasks

When we hear, sound waves are first transmitted through the ear canal to the eardrum, which begins to vibrate as a result of the sound waves. This sets the three ossicles in the middle ear (malleus, incus, stapes) in motion. Via a fluid-filled tube, the sound waves are transmitted into the bony housing of the inner ear into the actual hearing organ, the cochlea. This snail-shaped fluid-filled structure transmits the vibrations to the upper end of the fine sensory cells, where they are converted into nerve impulses and transmitted to the brain. We perceive these impulses as sounds. This makes the cochlea the most important interface to the brain. If just a few of these fine cells are damaged, an uncontrolled stream of nerve impulses is received and sent like ringing in the ears in tinnitus.

Diseases

There are several diseases of the inner ear in which the cochlea may be involved. One cause can be stress. We are exposed to a certain amount of noise every day and often have to work under psychological pressure. This stress is transferred to the inner ear and can lead to ringing in the ears (tinnitus) and even to a hearing loss. Affected persons then suddenly no longer hear properly in one ear and can perceive ringing in the ear and a pressure on the affected ear. Hearing loss is often thought of as a stress disorder, but experts disagree about whether other factors play a role. Vascular problems, inflammation, autoimmune reactions are seen as possible triggers, along with psychological reasons. A tumor on the auditory nerve may also be a rare cause. A major problem for the cochlea is noise exposure. It does not matter whether it is a one-time event such as violent noise trauma or prolonged noise exposure. The sensitive hair cells in the auditory organ protect themselves from high noise levels by reducing their activity, in effect “turning a deaf ear”. They can recover several times, but if loud noises keep hitting the ear, this can lead to chronic hearing loss. With sensorineural hearing loss, hearing deteriorates with age. Not everyone is affected, however; some people still hear well in old age. It is not clear whether this is due to circulatory disorders, deposits in the ear, altered connective tissue structures or aging processes of the brain, family predispositions or harmful influences in the course of life.However, age-related hearing loss is one of the typical problems of the auditory organ. Both areas of the hair cells are then affected. Both sound perception and sound conduction may be disturbed. Infectious diseases can also play a role. A middle ear infection can spread to the inner ear and cause permanent hearing damage there. Meningitis, measles, mumps, rubella and shingles can cause hearing loss. The inflammations attack the hearing cells in one or both ears and can cause permanent damage. Therefore, if inflammation of the inner ear is suspected, an ear, nose and throat doctor should be consulted quickly. A rarer disease in which the hearing organ can also be affected is Meniere’s disease, for which the causes are not clearly understood. Experts suspect fluid buildup in the hearing and vestibular organs, leading to an increase in pressure in both areas of the inner ear, affecting the sensory cells. It is also possible that a ruptured membrane causes the different fluids in the cochlea to mix. In Meniere’s disease, hearing loss and dizziness occur in equal measure, which can lead to social withdrawal for affected individuals.

Typical and common ear disorders

  • Ear flow (otorrhea)
  • Otitis media
  • Ear canal inflammation
  • Mastoiditis
  • Ear furuncle