The inner ear system is a highly complex and intricate network of structures working together to process sound and send it to the brain, the auditory system’s final destination, via the central pathway. Both the organ of hearing and the balance organ are located in the inner ear. The organ responsible for hearing is the cochlea and the organ responsible for balance is the vestibule. They share a connection and may, but don’t necessarily, directly influence one another. In our explanation of the inner ear structures we will begin from the oval window and end with the VIIIth cranial nerve.
The cochlea is known as the hearing organ. It is the major way station where sounds are processed and then passed on to the central pathway. To some it sounds like the cochlea has quite a simple task, but the cochlea is quite complex and consists of multiple structures that work together in order to accomplish their purpose. The cochlea has a bony shell that is considered to be one of the hardest bones in the body. It resembles a spiral cone, like that of a snails shell. In a healthy ear it consists of 2 ½ turns, but there are syndromic and non-syndromic disorders where a portion of the cochlea is missing and an individual is left with fewer than 2 ½ turns (i.e. mondini malformation and alexander aplasia; see causes of hearing loss for more info).
Inside the cochlea there is a central modiolus through which nerve fibers run. There is a bony shelf-like osseous spiral lamina that spirals around the central modiolus from top to bottom. The spiral lamina has three membranes that run towards the bony cochlear shell: reissner’s membrane, the tectorial membrane, and the basal membrane.
Reissner’s membrane and the basal membrane split the cochlea into three primary compartments: the scala vestibuli, the scala media, and the scala tympani.
The scala vestibuli forms the upper chamber and consists of perilymph, which has more sodium than potassium. It is essentially a big open tube in which fluid moves. Its lower border is reissners membrane, which also forms the upper border of the scala media. In some cases (i.e. Meniere’s Disease, Enlarged Vestibular Aqueduct Syndrome, or any disease that results in formation of cochlear hydrops), excess fluid production will fill up the scala vestibuli and tear reissner’s membrane causing the endolymphatic fluid to mix with perilymph from the scala media and destroy the sensory structures within the scala media.
The scala media is the central chamber and is home to several structures, including the primary sensory structure, the organ of corti. We will discuss the organ of corti in a moment. This space is filled with Endolymph, a fluid that differs in concentration from the perilymph found in both the scala vestibuli and the scala tympani. The two fluids are not designed to mix. When mixing of the solutions occurs damage to integral cochlear structures (i.e. outer hair cells and the organ of corti) may occur. As previously mentioned, the superior border of the scala media is called reissner’s membrane, but the lower border is called the basilar membrane. It is the basilar membrane that vibrates in response to the stapes footplate (see physiology below for more info).
The scala tympani is the lowest chamber of the inner ear system and mirrors the scala vestibuli anatomically speaking.
As stated previously in our article titled Middle Ear Anatomy & Physiology the oval window is the entry way into the cochlea. It is the location where the stapes is fixed to the cochlea. In some cases, otosclerosis of the stapes footplate can cause bony growth (lamella) to form and fix the stapes footplate to the cochlea, causing hearing loss. In the healthy ear, the stapes footplate vibrates back and forth sending sound waves into the fluid filled cochlea.
The round window is located at the end of the cochlea just inferior (below) to the oval window. Although it is located at the end of the cochlea, it is not an exit. It is connected to the scala tympani it is completely covered by a membrane that seals endolymphatic fluid within the scala tympani. Both the round window and the oval window are protected by a ridge called the cochlear promontory. This ridge sticks out over both windows and protects them from foreign objects that could find their way into the cochlea. In some instances, significant changes to middle ear pressure or foreign objects can rupture the round window’s membrane and create what is known as a perilymph fistula (see causes of hearing loss for more info).
Of the three membranes mentioned, the basilar membrane (along with the tectorial membrane) is one of the more important membranes. Its function will be discussed under physiology, but it is important to realize that the basilar membrane changes in mass and stiffness as it goes from base to apex. At the base, the basilar membrane has less mass and is slightly stiffer. At the apex, it has more mass and is more flaccid. This change in mass and stiffness is important because it affects the tonotopicity and frequency resonance of the basilar membrane.
The tectorial membrane is seated above the organ of corti. The tectorial membrane is connected to the stereocilia of the outer hair cells (OHCs) in the organ of corti. There is no direct connection between the inner hair cells (IHCs) and the tectorial membrane.
Organ of Corti
The organ of corti consists of inner hair cells, outer hair cells, the reticular lamina, stria vascularis, and several supporting cells. All of these cells work together to process the mechanical energy received from the ME and generate an electrical signal that can be transferred along the central pathway. The supporting cells are: phalangeal cells, Deiters’ cells, pillar cells, Hensen cells, Claudian cells, Bottcher’s cells, and inner and outer border cells.
Inner Hair Cells
Inner hair cells are arranged in a single row (see above photo; located on left-hand side)and there is a total of about 3,500 inner hair cells in the human ear. They have a different function than OHCs (see Physiology below for more info). They are oval-like in shape, have an electrical charge of -40 mV, and they have potassium and calcium ion channels.
Outer Hair Cells
Outer hair cells are arranged in about 3-5 hair cells per row for a total of about 12,000 hair cells. They have a different function than IHCs (see Physiology below for more info). They are cylindrical in shape, have an electrical charge of -60 mV, and have potassium ion channels. They also contain proteins such as actin, myosin, prestin, and tubulin. These proteins are necessary for expansion and contraction of the hair cells.
The stereocilia are situated on top of each hair cells and are believed to stimulate each hair cell. Each stereocilia contains actin filaments and has an electron dense base. The stereocilia are arranged in a particular order with the tallest stereocilia known as the kinocilium on the right hand side. The stereocilia are bathed in endolymph, which has a high charge (+80 mV) in comparison to the charge inside the hair cells. These differences allow for potassium ions to flow into the cell and create a charge (cell depolarization). There are cross-links that help the stereocilia to move together and tip-links that help open and close the potassium channels.
The primary purpose of the cochlea is to process mechanical energy received from the middle ear space, convert it to electrical energy, and transfer it to the afferent spiral ganglion cells of the auditory nerve. This information is then sent through the central auditory pathway where it is broken down according to its intensity, pitch, and timing; and decoded in the auditory cortex.
The Travelling Wave
The travelling wave is the term assigned to the sound wave that results from stapedial vibrations at the oval window. When the stapes vibrates the fluid in the endolymph filled cochlea it generates longitudinal sound waves that creates transverse vibrations along the basilar membrane (BM). The vibrations from the stapes create a series of compression and rarefaction waves. The compression waves drive the BM up and the rarefaction waves drive the BM down. This transverse wave travels down the entire duration of the BM. When the wave hits the place of resonant frequency (determined by a match in the frequency of the mechanical vibration and mass/stiffness of the BM) the hair cells at that location are activated and an electrical signal is generated. This electrical signal is then released to the brainstem via the afferent spiral ganglion cells of the inner hair cells.
However, there are some problems with this theory. The traveling wave must overcome the natural viscosity/resistance of the fluid. Using this model, we cannot explain frequency resolution at high intensities. Therefore, scientist have concluded that there is an active cochlear amplifier that compresses soft sounds and loud sounds. This theory helps explain how soft sounds can travel the duration of the cochlea and how certain resonance peaks can be sharper. It is widely believed that OHCs play a major role in the active cochlear amplifier.
Hair Cell Mechanics
When the travelling wave arrives at its resonance frequency the stereocilia shear against the tectorial membrane during compression waves. This shearing causes the stereocilia to open and for potassium ions from the endolymph in the surrounding fluid to flow down the stereocilia and into the OHCs. This sudden inflow of the OHCs causes the OHCs to hyperpolarize. When the action potential threshold is reached, the hair cell releases a neurotransmitter. It is uncertain which neurotransmitter is released by the OHCs, however, it is believed that the IHCs release an amino acid known as glutamate. This amino acid is released from the IHC, travels across the synaptic cleft and activates synapses on the postsynaptic membrane of afferent nerve fibers. This action sends action potentials that run through the central auditory pathway.
IHCs are believed to be the primary source for afferent information, whereas the OHCs are believed to play a bigger role in the cochlear amplifier. This conclusion has been drawn based on the number of type I and type II spiral ganglion nerve fibers that run between the hair cells and the brain. 95% of all spiral ganglion cells are Type I afferent nerve fibers that innervate the IHCs. While only 5% of all spiral ganglion cells are type II afferent nerve fibers that innervate the OHCs.