Synonyms
Latin: Auris interna
Definition
The inner ear is located inside the petrous bone and contains the hearing and balance organs. It consists of a membranous labyrinth surrounded by a bony labyrinth of the same shape. The cochlea is the organ of hearing in the inner ear.
It consists of the cochlear labyrinth with a membranous cochlear duct. It contains the sensory epithelium with two different receptor cells, the so-called Corti organ. The tip of the cochlea points to the front and not upwards.
The bony cochlear duct (Canalis spiralis cochleae) in the inner ear is about 30-35 mm long. It winds about 2.5 times around the modiolus, its bony axis, which is riddled with several cavities and contains the spiral ganglion (nerves for receiving impulses of frequencies). The basal spiral from the inner ear is visible from the tympanic cavity (middle ear) as a promontory.
The membranous compartments are arranged in a floor-like cross-section. Above and below are compartments filled with perilymph (ultrafiltrate of blood plasma; similar to extracellular fluid): the scala vestibuli and the scala tympani. In the middle of the inner ear there is another space, the cochlear duct, which is filled with endolymph (similar in composition to intracellular fluid).
It ends blind towards the tip of the cochlea, while the scala vestibuli and scala tympani communicate with each other at the helicotrema at the tip of the cochlea in the inner ear. In cross-section, the cochlear duct appears triangular and is separated from the vestibule scala by the Reissner membrane and from the tympanic scala by the basilar membrane. On the lateral wall there is a particularly metabolically active area (Stria vascularis), which secretes the endolymph.
The basilar membrane originates at a bony protrusion and becomes wider and wider from the base of the snail to the tip of the snail. This is where the sensory apparatus is located with the inner and outer hair cells, which are in a 1:3 ratio. The hair cells carry Stereovilli of different lengths.
The smallest of them are connected to each other by protein threads. Here the transformation of an external stimulus into a physiological signal (transduction) takes place via certain ion channels. The Corti organ is covered by the tectorial membrane.
At rest, i.e. without external stimulus, only the outer hair cells in the inner ear touch the tectorial membrane. The inner hair cells are connected to fibers of the auditory nerve (cochlear nerve), which transmits information to the brain. The function of the hearing organ is to convert the incoming sound waves into electrical impulses.
The exact transduction processes and principle of sound conduction are described below. The sound arriving in the inner ear is conducted via the outer ear to the eardrum. There, the resulting vibrations are transmitted further to the ossicular chain via the hammer, anvil and stirrup in the middle ear to the oval window to the inner ear.
The oval window is adjacent to the scala vestibuli. The stirrup footplate sets the inner ear fluid and the membranes of the cochlea in motion through continuous inward and outward movements. This is where the signal transduction process begins, which can be divided into 3 stages:
- Formation of a travelling wave
- Excitation of the outer hair cells
- Excitation of the inner hair cells by amplification of the travelling wave by the outer hair cells
A travelling wave is created in the inner ear by undulating movements.
It begins at the oval window and then runs up the scala vestibuli to the tip of the snail. If the cochlear partition wall were a homogeneous structure, a synchronous oscillation would occur. But its stiffness decreases from the base of the snail to the tip of the snail.
It follows that the partition wall oscillates in the form of a travelling wave. In total, there is an amplitude (vibration) maximum for each frequency. So if the excitation frequency of the external sound stimulus is equal to the natural frequency of the basilar membrane, an amplitude maximum follows.
This principle of frequency dispersion (frequency-location imaging, location theory) allows a characteristic assignment of frequencies (tonotopy). High frequencies are found at the base of the cochlea in the inner ear, low frequencies are found at the tip of the cochlea in the inner ear. At the maximum of the wave movement, the stereovilli of the outer hair cells are most strongly bent.
A shearing motion occurs between the basilar and tectorial membrane.Up and down movements stretch or relax the tip-links. This opens or closes ion channels in the inner ear and changes the potential of the hair cells. They then actively change their length and amplify the travelling wave.
The frequency selectivity is thus improved. The inner hair cells in the inner ear are only excited by the amplification mechanism of the outer hair cells. Now they also partially come into contact with the tectorial membrane and the shearing of the stereovilli causes the release of a neurotransmitter at the base of the hair cell, which then excites the nerves of the auditory nerve (cochlear nerve).
From here, the information is passed on to the brain and processed. The vibrations in the inner ear lead to the emission of sound energy to the outside. The travelling wave continues from the scala vestibuli via the tip of the snail to the scala tympani, which ends at the round window. Sound coming from the ear can be measured as so-called evoked otoacoustic emissions. Emissions in the inner ear triggered by “clicks” can be recorded with a microphone and used for hearing screening, especially in newborns.
