The organ of Corti is located in the inner ear in the cochlea and consists of supporting cells and sensory cells responsible for hearing. When a sound wave excites the hair sensory cells, they trigger an electrical signal in the downstream neuron that travels to the brain via the auditory nerve. Diseases that can affect the organ of Corti include Menière’s disease or hydrops cochleae, age-related hearing loss (presbycusis), and others.
What is the organ of Corti?
The organ of Corti is part of the human sense of hearing. The complex of supporting and sensory cells is located in the inner ear, which lies behind the oval and round windows. Before sound reaches the oval window, it passes through the external auditory canal, the eardrum, and the middle ear behind it. The latter consists of the tympanic cavity, which contains the ossicles. When a sound wave reaches the eardrum, it transmits the vibration to the ossicles, which in turn nudge each other in a chain reaction and eventually cause the membrane of the oval window to vibrate. Behind the oval window, the cochlea begins. It winds in the inner ear and leads longitudinally through three ducts that run parallel to each other and are filled with lymph. First, sound enters the atrial meatus, which leads to the tip of the cochlea and merges seamlessly into the tympanic meatus, which leads back to the round window. Between the two lies the cochlear duct, which contains the organ of Corti. It lies above the basilar membrane, which forms the floor of the duct, and below a covering membrane, also known as the tectorial membrane. The structural and functional unit owes its name to the Italian anatomist Alfonso Corti, who was the first to describe it in 1851. It is also known in technical language as organon spirale cochleae.
Anatomy and structure
Three rows of outer hair cells extend along the course of the cochlear duct. Hair-like projections protrude from the cell body (soma) into the snail duct and are called stereovilli. A single hair sensory cell can have 30-150 stereovilli. In addition, they have a special extension, the kinocilia, of which each cell has at most one. All extensions of the outer hair sensory cells project into the cochlea where they abut the tectorial membrane; deflections of the membrane are transmitted to the sensory cells and bend the stereovilli and kinocilia. The stereovilli are in contact with each other via tip connections (tip left); the flexible connections are also significant for the opening of pores at the tip of the stereovilli. In addition to the three rows of outer hair cells, a single row of inner hair cells extends through the cochlear duct. The inner hair sensory cells have the same structure as the outer ones, but do not touch the tectorial membrane. The hair sensory cells of human hearing are secondary sensory cells that do not have their own nerve fiber. When stimulated, they therefore first transmit their signal to another cell (ganglion spirale cochleae), which transports the information via its nerve fiber. Taken together, these fibers form the auditory nerve. Supporting cells stabilize the actual sensory cells of the organ of Corti.
Function and tasks
The organ of Corti converts the stimulation of hearing by sound waves into a nerve signal; physiology refers to this process as transduction. The sound propagates in waves through the lymph of the atrial canal. Reissner’s membrane between the atrial and cochlear ducts nudges the tectorial membrane, which in turn transmits the motion to the stereovilli of the outer hair cells of the organ of Corti. In this way, the tectorial membrane directs the stereovilli either out toward, or away from, the kinocilia. In the resting state, the hair sensory cell produces a so-called resting potential: spontaneous activity that leads to the release of the neurotransmitter glutamate. The amount released is constant. A deflection of the stereovilli toward the kinocilia signals an auditory stimulus to the cell. The tip left dilates the pores of the stereovilli, allowing potassium ions to enter the interior of the cell and change its electrical charge. As a result, the hair cell releases more glutamate, thereby irritating the downstream neuron. However, when the stereovilli deflect away from the kinocilia rather than toward it, they narrow the pores and fewer potassium ions can enter the hair sensory cell.Accordingly, the cell releases less glutamate and thereby actively inhibits the downstream nerve cell. The perception of the rotational sense organ in the arcades, which also belong to the inner ear, works in the same way. However, the stimulus here is not a sound wave, but the rotational movement of the head.
Diseases
Numerous diseases can manifest in the organ of Corti; these include Menière’s disease (hydrops cochleae), age-related hearing loss (presbycusis), and others. Menière’s disease or hydrops cochleae is a condition in which the inner ear produces too much lymph. Typical symptoms include dizziness, hearing loss, tinnitus and a feeling of pressure on the ears. Often, the excess lymph stretches the ducts in the cochlea, making it difficult to perceive low-pitched sounds as the first thing. The additional pressure on the hair sensory cells may deflect the stereovilli even though no acoustic stimulus is present. Even with temporary hydrops cochleae, permanent damage to the organ of Corti is possible, resulting in the persistence of some or all symptoms. Age-related hearing loss (presbycusis) usually appears after the age of 50 and manifests itself in hearing loss up to the loss of hearing ability as well as tinnitus. In addition to natural aging, other factors such as circulatory disorders, diabetes mellitus, and elevated blood pressure may contribute to the development and severity of age-related hearing loss.
