The inner ear is a structure in the skull, specifically in the temporal bone on each side. It consists of the membranous and bony labyrinth, and is filled with a potassium-rich fluid called endolymph. The membranous labyrinth lies within the bony labyrinth and is surrounded by perilymph (a sodium-rich liquid). Our most important sensory organs are in the inner ear, accommodating the anatomical components of the auditory and vestibular function. The semicircular canals within the inner ear are part of our balance system. These canals are called the utricle and the saccule, and they are responsible for our sense of steadiness and balance.
The inner ear: the auditory organ
The cochlea is responsible for the brain’s ability to interpret and understand sounds. There are three components that make up the cochlea:
- the scala vestibuli
- the scala tympani
- the cochlear duct
These three components are separated by membranes. The scala vestibuli and the scala tympani are joined by an opening in the cochlea called helicotrema. Between the scala vestibular and the scala tympani is the basilar membrane. The Reissner membrane moves from the bony wall of the scala vestibuli to the center of the basilar membrane, separating the cochlear duct (or the scala media) of the scala vestibuli.
The organ of Corti, named after the Italian anatomist Alfonso Corti, rests in the basilar membrane in the inner ear. It is made up of sensory cells commonly referred to as hair cells. These hair cells (stereocilia) are in contact with the tectorial membrane, which is a gelatinous membrane.
If the basilar membrane is deflected by vibrations, the hair cells are bent and there is an electrical stimulus.
The hair cells in the ear
There are two types of hair cells in the ear: Inner hair cells and outer hair cells. The inner hair cells are responsible for sending afferent information to the brain through the eighth cranial nerve. The approximately 3,000 hair cells stimulate these cranial nerve fibers, carrying information to the brain. The outer hair cells, on the other hand, receive efferent information from the brain. There are approximately 12,000 outer hair cells. Each hair cell is made up stereocilia, which are mechanosensing organelles. The basic role of hair cells, whether inner or outer, is to change the vibrations caused by sound waves into electrical signals. These signals are sent to the brain, where hearing and distinguishing sounds actually occurs.
Inner ear: the vestibular system
The vestibular apparatus is responsible for your sense of balance. This system consists of two vestibular sacs – saccule and utricle – and three semicircular canals.
Saccule and utricle assess your linear acceleration or head tilt (gravity). The three semicircular canals are different in space and can be divided into anterior, posterior, and horizontal canal. The semicircular canal system detects rotational movements, and the semicircular canals are filled with endolymph.
As the head rotates in one direction, inertia of the fluid causes it to lag, and hence generate relative motion in the semicircular duct in the direction opposite that of the head movement. This moving fluid bends the broad vane of the cupula. The stereocilia of the hair cells are bent because they are embedded in the gelatinous cupula.
How we hear?
How does all that information fit in with hearing. The inner ear is a crucial part of our hearing process. When we are exposed to a sound, the sound wave goes into the ear canal until it gets to the eardrum. The vibration is then passed on from the eardrum through the middle ear bones to the inner ear. The tiny hair cells in the inner ear turn the vibrations into electrical signals that go to the brain through the hearing nerve. Then the brain signals to you that you are hearing something and also indicates what the sound is.
Diseases of the inner ear
Such a complex system is prone to health complications, which is why a variety of disorders within the different parts of the ear exist, such as the following:
- Due to excessive sound pressure, damage to the hair cells in the inner ear can occur, contributing to hearing loss. In such a case, hearing aids are used to counteract the effects and symptoms of hearing loss.
- Inflammation can cause damage to the middle and inner ear, and carry out the deterioration of important structures within the ear leading to deafness. This can also lead to sudden dizziness which is a symptom of vertigo.
- Benign and malignant tumors can cause hearing loss. A recommended treatment is hearing aids as they can help improve hearing.
Inner ear infections are common and the treatment is available. No matter what disease you think you might have related to the inner ear, it is important to consult a professional audiologist.
Inner ear and hearing aids
There are many health issues related to the damage of the inner ear as described above. Many of them can be easily resolved by using hearing aids. There are other treatment options too but hearing aids can help with most of the issues and ensure that the hearing of the person isn’t affected.