As a complex structure, the inner ear serves primarily for sound perception and orientation of humans in space. Hearing loss correlates in many cases with disorders of sound perception and/or transmission in the inner ear.
What is an inner ear?
Anatomical structure of the ear. The inner ear (labyrinth), which has a complex structure, functions primarily as the organ of hearing and balance in humans, through which sound perception and spatial orientation in particular are ensured. The inner ear, located in the petrous pyramid (pars petrosa ossis temporalis), consists of a bony labyrinth (labyrinthus osseus), which is lined by a membranous labyrinth (labyrinthus membranaceus) and separated from it by a cleft space filled with perilymph. The membranous labyrinth is composed of an atrium, three semicircular canals (canales semicirculares), and the cochlea, and is filled with so-called endolymph, a liquid-like fluid rich in potassium. While the sensory cells in the atrium and arcades of the inner ear serve as the vestibular organ (organ of balance), the sensory cells located in the cochlea control auditory perception.
Anatomy and structure
The hearing organ of the inner ear is formed by the cochlea (auditory cochlea), which is divided into three merging ducts separated by membranes. These include the membranous and endolymph-filled cochlear duct, which contains the organ of Corti (the seat of the sense of hearing) covered by the tectorial membrane and is located between the other two ducts, the scala vestibuli (atrial staircase) and the scala tympani (tympanic staircase). The cochlear duct is demarcated from the scala vestibuli by the vestibular membrane (also called Reissner’s membrane) and from the scala tympani by the basilar membrane. The vestibular organ of the inner ear, responsible for the sense of balance, is composed of two atrial sacs, the sacculus adjacent to the scala vestibuli, and the slightly larger utriculus localized in the posterior portion of the vestibulum labyrinthi (bony cavity in the petrous bone) adjacent to the cochlea anteriorly, as well as the arcuate ducts posterior to the vestibulum labyrinthi.
Functions and Tasks
The organ of Corti inside the cochlea serves as a receptor area composed of supporting cells, sensory cells, and nerve fibers for sound perception; the sensory cells responsible for this are also called hair cells. Sound signals arriving from outside cause the basilar and tectorial membranes to shift in opposite directions, so that the outer hair cells are stimulated to change length, which amplifies the basilar membrane vibration. As a result of the amplified vibration, the inner hair cells are stimulated, which send impulses to the central nervous system via the so-called vestibulocochlear nerve (auditory nerve or 8th cranial nerve). The vestibular organ regulates the sense of balance and is responsible for spatial orientation. Here, the sense of rotation is regulated by the arcuate ducts, which are perpendicular to each other and filled with endolymph. The rotational movement of the human in space is sensed as the endolymph moves through the arcuates in opposition to the actual rotational movement of the head, causing the hair cells there to bend. The hair cells are thus stimulated and send an electrical signal to the brain via the arcuate nerve. The two atrial sacs, which are perpendicular to each other, record the translational acceleration of the human in space, with the utriculus recording the horizontal acceleration and the sacculus recording the vertical acceleration. The information sent from the hair cells to the brainstem via the vestibulocochlear nerve is coupled and processed there with additional information coming from the eyes, spinal cord, and cerebellum. In addition, the eye muscles are interconnected with the organ of equilibrium in the inner ear, allowing for a stable image during head movement.
Diseases
The cochlea, whose sensitive hair cells are primarily responsible for sound perception, requires adequate nutrients as well as oxygen, as do the auditory nerve and corresponding pathways in the brain. An insufficient supply due to circulatory disorders can lead to corresponding functional losses.In addition, external stresses (inflammation, noise, pollutants such as drugs, nicotine, alcohol or toxins) can cause sometimes irreversible damage to sound perception (sensorineural hearing loss) and, in particular, functional disorders of the inner ear (cochlear hearing loss). The inner ear is frequently affected in the case of age-related hearing loss (presbycusis), which is attributed to circulatory disorders, deposits in the inner ear area as well as damaging external factors and a genetic predisposition, among other things. In addition, disturbed sound perception in the inner ear can cause ringing in the ears such as tinnitus. Stress as well as stressful life situations can trigger a hearing loss (acute, unilateral hearing loss). This form of inner ear disorder can also be caused by vascular problems (insufficient blood supply and circulation), infectious diseases, autoimmune reactions or benign tumors on the auditory nerve (including acoustic neuroma). Middle ear infection, in addition to other infectious diseases (meningitis, mumps, measles, herpes zoster), can spread to the inner ear if left untreated, causing labyrinthitis (inner ear inflammation). Rarely, the etiologically still unexplained Meniere’s disease is observed, which is characterized by the attack-like symptom triad of hearing loss, tinnitus and dizziness. Direct impairment of the vestibular organ in the inner ear further entails balance disorders and/or vertigo.
Typical and common ear disorders
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle