In this article, we will be exploring the anatomy and function of the organ of corti in the inner ear.
The bony shell of the cochlea encompasses a spiraling duct that spans 31-35 mm in length. This duct originates near the saccule, and is connected to it via a small channel, running through the entire bony structure of the snail-shaped cochlea until it terminates beneath its dome. The cochlea is divided into three distinct sections: the vestibular scale, which comprises the upper portion; the tympanic scale, which makes up the lower portion; and the cochlear duct, which lies between these scales lies and has a triangular shape. The vestibular and tympanic scales are directly connected in the uppermost part of the cochlea, at the level of the helicotrema.
Within the cochlear duct, there are three distinct walls. The upper wall, known as the vestibular wall, is formed by the Reissner membrane. The lateral wall is composed of the stria vascularis and the spiral ligament. The basal wall, also known as the tympanic wall, is formed by a membrane of the same name that houses the organ of Corti, which consists of both inner and outer hair cells (neurosensory cells). There are approximately 3,500 inner hair cells, which are arranged in a single row and have 20 cilia known as "hair cells" at their apical end. The outer hair cells, approximately 13,000, are arranged in multiple rows and have approximately 60-100 cilia at their distal end. At the basal end of each hair cell, there are nerve endings that are divided into two types, type I and type II. The neurosensory cells are covered by the tectorial membrane and serve as the main receptors for hearing.
The interaction between the tectorial membrane and the cilia of the hair cells plays a crucial role in stimulating the acoustic receptors. When the Reissner's membrane vibrates, these vibrations are transmitted to the endolymph and then to the tectorial membrane. This membrane is in contact with the cilia of the hair cells, which move in response to these vibrations, generating stimulation of the nerve fibers that are in contact with the excited ciliated elements. This stimulation leads to the release of chemical mediators at the cytoneural junction and the development of an action potential in the fibers of the cochlear nerve. After passing through some intermediate stations, this stimulus ultimately reaches the cortical acoustic areas (areas 41 and 42).
In conclusion, the sound waves that enter the auditory system are first transmitted by the tympanic membrane to the chain of ossicles, which then convey them to the organ of Corti. This organ converts the vibratory energy produced by the sound stimulus into nerve impulses that travel via the eighth cranial nerve to the cortical centers, where they are processed and translated into auditory sensations. The eighth cranial nerve is composed of two distinct parts: the vestibular nerve, which carries stimuli related to the sense of balance, and the cochlear nerve, which transmits acoustic stimuli collected by the neurons of the ganglion of Corti to the cortical centers.
Damage to the organ of Corti or to the inner ear can result in a reduction in hearing ability, which is known as perceptive or sensorineural hearing loss. This can occur due to various factors, such as noise exposure, inflammatory or vascular conditions, trauma, improper use of medications, chronic diseases, acute illnesses, smoking, alcohol consumption, and poor dietary habits. These are the most common causes of cochlear damage and subsequent hearing loss.